ANTILOCK BRAKE SYSTEM
There are a lot of
ABS-equipped vehicles on the road today that will eventually need ABS
replacement parts as they age.
What kind of parts fail?
Wheel speed sensors are probably the most often replaced component. Most wheel
speed sensors (WSS) are located at the wheels, which makes them vulnerable to
road splash, corrosion and road hazards. The WSS wires and connectors are also
often a source of trouble.
On many vehicles, each
wheel has its own speed sensor. These are called "four channel" ABS
systems. On others, a common sensor is used for the rear wheels (which may be
mounted in the differential or transmission) but each front wheel still has its
own wheel speed sensor. These are called "three-channel" ABS systems.
Another variation is the
"single-channel" rear-wheel-only ABS system that is used on many
rear-wheel-drive pickups and vans. This includes Ford "Rear Antilock
Brakes" (RABS) and GM and Dodge "Rear-Wheel Anti-Lock" (RWAL).
The front wheels on these trucks have no speed sensors and only a single speed
sensor mounted in the differential or transmission for both rear wheels.
Rear-wheel antilock systems are typically used on applications where vehicle
loading can affect rear wheel traction, which is why it's used on pickup trucks
Most ABS systems use
magnetic wheel speed sensors that generate a frequency signal as the notches on
the sensor ring pass by the tip of the sensor. The sensor ring is usually
mounted on the back of the brake rotor or outer CV joint.
On some vehicles (many GM
models, for example), the wheel speed sensor is built into the sealed wheel hub
assembly - making it very expensive to replace if it fails because the entire
hub has to be changed. On most vehicles, though, the wheel speed sensor can be
replaced separately if it fails.
The WSS signal is sent to
the ABS control module, where the signal pulses are counted so the module can
monitor wheel speed. If the tip of the WSS becomes coated with debris, or
there's a poor wiring connection between the sensor and module, or the
"air gap" between the end of the sensor and the sensor ring is too
large, it may prevent the WSS from generating a good signal. An internal short
or open in the sensor itself can also render it useless. The ABS control module
will detect the problem and set a fault code that identifies the
"bad" WSS circuit (a number that corresponds to left front, for
example.) This does not mean the sensor itself is bad, only that there is a
signal problem in that WSS circuit.
The troublesome WSS circuit
can be diagnosed by measuring the resistance of the WSS with an ohmmeter, by
checking wiring continuity and inspecting the wiring and connectors, or by
spinning the wheel and checking the sensor's output signal.
If a WSS sensor is lost,
the ABS system can't monitor wheel speed and can't tell if ABS is needed or
not. So it will set a code and disable itself. As long as the warning light
remains on, the ABS system will stay "off line." Only when the fault
has been fixed and the code cleared will the warning light go out, allowing the
ABS system to become active again.
ABS systems are designed to
be as "fail-safe" as possible. All ABS systems run their own
self-diagnostics and will set codes and deactivate themselves when a serious
fault is detected. An illuminated ABS warning light may seem ominous, but most
vehicles should still be safe to drive and have normal braking. The only
exceptions are some older vehicles with "integral" ABS systems that
use the ABS pump and accumulator to provide power-assisted braking. On these
vehicles, the brakes should still work, but without the benefit of any power
assist if the ABS system goes down.
If the brake warning light
is also on, however, it may indicate a more serious problem such as loss of
brake fluid or a low fluid level. So if both warning lights are on (ABS and
brakes), the vehicle should not be driven until the problem can be
Another ABS component that
often fails is the hydraulic modulator assembly that contains the ABS solenoid
valves, or in the case of GM ABS-IV systems, the motor pack and valve assembly.
This is the heart of the ABS system that controls the cycling of hydraulic
pressure to the wheels. On some systems, the ABS solenoid valves can be
replaced separately if one fails.
But on many systems, the
entire modulator must be replaced if anything inside fails. Failures may be
caused by electrical faults (opens or shorts in solenoids), by internal
corrosion or by debris in the brake fluid that jams a solenoid valve or
prevents it from fully closing.
Modulators can be tricky to
replace because they must be bled after they have been installed to remove all
air from the circuits. On some vehicles, they may require using a scan tool to
cycle the ABS solenoids. On others, manual bleeder screws may be provided for
this purpose. Either way, your customer will also need brake fluid to refill
the system. The old fluid should also be flushed out to get rid of any sediment
or contamination that may cause future problems.
On systems that use a
high-pressure pump and accumulator to provide power-assisted braking or to
reapply pressure during an ABS stop, a defective pump or relay can cause a loss
of pressure. So can a leaky accumulator. The ABS system monitors these
components and will set a code if it detects a problem.
The accumulator can be
dangerous to replace because it may contain up to 1,500 or more pounds of
pressure. Your customers should be warned to never open up an ABS system that
may still be under pressure. The accumulator must first be discharged by
pumping the brake pedal 40 times with the ignition key off. Once all pressure
has been relieved, it can be safety removed and replaced.
ABS control modules can
also fail, though they don't fail that often. Most ABS problems are electrical
(bad WSS, pump relay, pump motor, wiring or solenoid) or mechanical (sticking,
jammed or leaking ABS valve.) If a module does go bad, it may cause erratic or
abnormal operation of the system (such as ABS braking during a normal stop.)
ABS should only come into play when traction conditions are marginal or during
sudden "panic" stops. The rest of the time, it should have no effect
on normal driving or braking.
When ABS comes into play,
the module energizes the solenoid valves in the hydraulic modulator to hold,
release and reapply hydraulic pressure to the brakes. Sealing off the affected
brake circuit prevents any additional pressure from being applied to the brake.
The module then energizes a vent solenoid to release pressure from the line.
This allows the brake to release momentarily so the tire can regain its grip.
The vent valve is then closed and the pressure valve reopened to reapply the
brake. This rapid cycling produces a pulsating effect that can usually be felt
in the brake pedal during an ABS stop. The driver may also hear a buzzing or
chattering noise from the ABS hydraulic unit - which is normal and tells the
driver that the ABS system is active.
The brake system includes the master cylinder, power
booster, disc brake calipers and rotors, wheel cylinders and brake drums, brake
hardware, brake hoses and lines, various valves, disc brake pads and drum
shoes, and brake fluid. On vehicles equipped with anti-lock brakes (ABS),
additional parts include up to four wheel speed sensors, the ABS hydraulic
modulator and control module, and a pump motor and accumulator (not used on
When the driver steps on the brake pedal, it moves a rod in the master cylinder
forward to push a pair of pistons against fluid in the primary and secondary
chambers. Brake systems are split into two separate hydraulic circuits. Each
circuit operates two of the four brakes (both fronts, both rears or a diagonal
pair). This is a safety requirement, so if one circuit fails, at least two
brakes will continue to operate so the vehicle can still be stopped.
Brake fluid carries the hydraulic pressure created in the master cylinder
through the brake lines to the front calipers and rear calipers or drums to
apply the brakes. Most older vehicles have front disc brakes and drums in the
rear, but many newer cars, SUVs and trucks have disc brakes in the front and
rear. When pressure reaches the brakes, the pads are squeezed against the
rotors, and if the vehicle has drums in the rear, the shoes are pushed out
against the drums to generate friction and stop the vehicle.
On vehicles equipped with power brakes, a brake booster located behind the
master cylinder on the firewall multiplies the force of the brake pedal input
using engine vacuum and a large diaphragm. This reduces the pedal effort needed
to stop the vehicle. On some older vehicles with integral ABS systems, the ABS
pump and accumulator provide power assist.
Additional parts involved in the braking process include a "pressure
differential valve" (a safety switch that turns on the brake warning lamp if
there's a loss of pressure in either brake circuit), a "proportioning valve" to
reduce pressure to the rear brakes for more balanced braking (not used on all
vehicles), and on some, a "load-sensing proportioning valve" to increase or
decrease hydraulic pressure to the rear brakes based on vehicle loading.
The major wear components in the brake system are the disc brake pads and drum
shoes. Every time the brakes are applied these parts are subjected to friction
and wear. Lining life depends on how the vehicle is driven. Stop-and-go city
driving, mountain driving and aggressive driving all involve more frequent
braking, harder braking and higher brake temperatures - all of which add up to
more wear and shorter lining life. Highway driving and gradual, light braking
produce less lining wear and longer lining life.
On most vehicles, the front linings wear out twice as fast as the rear linings,
so when the linings need to be replaced for the first time it's usually only
the front pads that need to be changed. Replacement linings should be the same
or better than the original linings. Semi-metallic linings are often used in
high-heat applications because they can withstand high operating temperatures
without fading or wearing excessively. Other high-temperature friction
materials include linings with ceramic content. Pads and shoes with
non-asbestos organic (NAO) linings are typically used for lower heat
applications such as rear brakes and front brakes on rear-wheel drive cars and
When linings are replaced, the rotors and drums may need to be resurfaced or
replaced depending on their condition. If a rotor is worn to minimum thickness
or a drum is worn to maximum diameter, replacement is necessary. Most rotors
are made of cast iron, but "composite" rotors have a thin, stamped steel center
hat attached to a cast iron rotor ring. Composite rotors are more difficult to
resurface and more prone to pedal pulsation problems than one-piece cast
rotors. They are also more expensive than cast. Composite rotors can be
replaced with cast rotors as long as both rotors are replaced at the same time
(don't intermix different kinds of rotors side-to-side).
Most front rotors are vented, while most rear rotors are not because more
cooling is usually needed for the harder-working front brakes. Most rotors are
interchangeable left to right, but some are directional, so pay close attention
to the catalog listings when looking up part numbers. New rotors are ready to
install and do not require additional resurfacing (turning rotors unnecessarily
shortens the service life).
Other parts that wear out over time include hydraulic components such as the
calipers, wheel cylinders and master cylinder. Most calipers have one or two
pistons, but some have up to four pistons mounted in a rigid housing. Most
calipers are cast iron (though some are aluminum) with steel or molded phenolic
(plastic) pistons. Most calipers are a "floating" design with slides or
bushings that allow the caliper to move sideways and center itself over the
rotor when the brakes are applied.
Others are a "fixed" design with rigid mounts and do not move. If the slides or
bushings on a floating caliper become badly corroded or worn, it may prevent
the caliper from sliding, causing uneven pad wear. The inside pad will wear
faster than the outside pad. If a piston in either type of caliper sticks, the
caliper may not release. This causes the brake to drag, puts rapid pad wear on
one side, and an uneven braking and a pull to one side when the brakes are
Leaky piston seals will allow brake fluid to contaminate the brake linings.
Leaky calipers must be rebuilt or replaced. Loaded calipers come ready to
install with new pads. Bare calipers do not include pads. On vehicles with
four-wheel disc brakes, the rear calipers may also include some type of parking
brake mechanism. This makes the calipers more complicated and expensive to
The wheel cylinders inside drum brakes have two opposing pistons that move
outward when pressure is applied. The wheel cylinder is mounted on the brake
backing plate and has dust seals over the pistons to keep out dust and water.
Each piston has a cup-shaped seal for the fluid inside. Common problems with
wheel cylinders include fluid leaks and sticking. Wheel cylinders can be
rebuilt or replaced. Leaking fluid can contaminate the brake shoes, requiring
their replacement as well.
Wear in the master cylinder may allow fluid to leak past the piston or shaft
seals. One symptom that might indicate a bad master cylinder is a brake pedal
that slowly sinks to the floor when braking at a stop light. Leaks or failure
to hold pressure require rebuilding or replacing the master cylinder.
Rebuilding aluminum master cylinders is not recommended. On some older vehicles
with ABS, the master cylinder is part of the ABS modulator and is very
expensive to replace.
Rubber brake hoses can also deteriorate with age and leak. Any hose that is
cracked, bulging, leaking or damaged should be replaced without delay because
of the danger of brake failure should the hose leak. Steel brake lines can
corrode internally or externally. Replacement brake lines must be steel with
double-flared or ISO end fittings.
Brake fluid also wears out over time and should be replaced when the brakes are
serviced. The main issue here is moisture contamination that causes a breakdown
of corrosion inhibitors in the fluid and lowers the fluid's boiling temperature
(which increases the risk of fluid boil and pedal fade under hard use). DOT 3
and DOT 4 brake fluid are the two main types, and both are glycol-based
hydraulic fluids. DOT 5 fluid is a silicone-based fluid and is used only for
special applications (like older vehicles that sit for long periods of time or
are operated in extremely wet environments). DOT 4 has a higher temperature
rating than DOT 3 and is used in many European vehicles. Use the type of fluid
specified by the vehicle manufacturer.
Related items that may also need to be replaced when servicing the brakes
include the wheel bearings and seals. On older vehicles with serviceable wheel
bearings, the grease seals should always be replaced when the bearings are
cleaned and repacked with grease. Special high-temperature wheel bearing grease
is required (never ordinary chassis grease).
Disc and drum brake hardware should also be replaced when the brakes are
serviced. Drum hardware includes the return springs, hold-down springs,
self-adjusters and other cables, clips or springs used in the brake assembly.
Return springs that pull the shoes back away from the drum when the brakes are
released may become weak with age, allowing the brakes to drag.
Self-adjusters can become corroded and stick, which can cause increased pedal
travel as the shoes wear. On disc brakes, the hardware includes slides and
bushings that can become worn and corroded, and anti-rattle clips and springs
that reduce noise. A high-temperature, moly-based brake grease should be used
to lubricate slides, bushings and shoe pads on drum-brake backing plates
The cooling system includes the radiator, radiator cap,
coolant reservoir, fan, water pump, thermostat, hoses, belts and antifreeze.
Related parts include the coolant sensor and fan relay.
The radiators in most late-model vehicles are aluminum with plastic end tanks.
Most older vehicles have copper/brass radiators. Both types are vulnerable to
internal corrosion caused by coolant neglect. Replacement radiators should have
the same cooling capacity (or better) than the original, and the same hose
connections. Cooling capacity is determined by the thickness of the radiator,
the number of fins and tubes and/or the design of the fins and tubes. Increased
cooling capacity is recommended for towing and performance applications.
The water pump is the heart of the cooling system. It is belt-driven and
consists of an impeller mounted on a shaft inside a cast or stamped steel
housing. The pump is usually mounted on the front of the engine. On some
overhead cam (OHC) engines, the pump is mounted under the timing belt and
requires considerable labor to replace. For this reason, you should recommend
replacing the pump if the timing belt is being replaced for scheduled
maintenance (recommended every 60,000 to 100,000 miles, depending on the
The service life of the water pump and timing belt is about the same, so
changing both at the same time can save the vehicle owner money on future
repairs. Water pumps don't last forever, and leaks around the pump shaft seal
and bearing can quickly lead to overheating. Any water pump that is leaking,
making noise or has excessive shaft play should be replaced. Replacement
options include remanufactured and new pumps.
The water pump may also be driven by a V-belt or a flat serpentine belt. The
same belt may also drive other engine accessories. Belts deteriorate with age
and should be replaced if frayed, cracked, glazed or oil-soaked. Replacement
belt length and width must be the same as the original. On vehicles with
serpentine belts, the automatic tensioner may also need to be replaced if it is
sticking, making noise or cannot maintain proper belt tension. Belt idler
pulleys should also be replaced if noisy, worn or sticking.
For temperature control, the cooling system requires a thermostat. It is usually
located in a housing where the upper radiator hose connects to the engine. The
thermostat does two things: 1. It allows the engine to warm up quickly (which
reduces cold emissions and fuel consumption) and 2. Maintains a consistent
operating temperature (also important for low emissions, good fuel economy and
performance). The thermostat has a temperature-sensitive valve that remains
closed and blocks the flow of coolant until the engine reaches 195 to 210
degrees. It then opens and allows coolant to flow to the radiator. The
thermostat continues to cycle open and shut so the engine can run within a
certain temperature range. This is very important on late-model vehicles with
computer engine controls because engine temperature affects the fuel feedback control
loop, emissions, fuel economy and performance.
If the thermostat sticks shut, the engine will overheat. If it fails to close,
the engine will be slow to warm-up and the heater may not put out much heat
when the weather turns cold. Fuel economy, emissions and engine wear will also
suffer. Under no circumstances should an engine run without a thermostat.
Replacement thermostats should have the same rating as the original. A slightly
hotter thermostat may be used during cold weather for increased heater output,
but a colder thermostat should not be used on engines with computer controls.
Other items that may be needed when changing a thermostat include a new
thermostat housing and gasket or sealer.
To improve cooling, a fan is needed to pull air through the radiator when the
vehicle is stopped or traveling at low speed. Older rear-wheel-drive vehicles
may have a belt-driven fan with or without a clutch. The clutch allows some
slippage and is used to reduce fan noise at high RPM and to improve fuel economy
by reducing drag. Excessive slippage in the clutch, however, may reduce airflow
and cause the engine to overheat at low speed. Most newer vehicles have one or
two electric cooling fans mounted behind the radiator, and a few have hydraulic
fans driven by power steering fluid. Electric fans are powered through a relay
and controlled by a coolant temperature switch or the engine computer. A
failure of the fan motor, fan relay, coolant temperature switch or a wiring
problem that prevents the fan from coming on can cause the engine to overheat.
The major hoses in the cooling system include the upper and lower radiator
hoses (the lower one usually attaches to the water pump and the upper usually
attaches to the thermostat housing), plus a pair of heater hoses (one inlet and
one outlet) and various connecting hoses and bypass hoses depending on the
application. Replacement hoses must be the same length and diameter as the
original. New clamps should also be installed when hoses are replaced. Hoses
should be replaced if leaking, cracked or bulging. Electro-chemical degradation
due to coolant neglect can cause hoses to fail from the inside out.
The coolant is a mixture of ethylene glycol antifreeze and water (typically a
50/50 mix). This combination provides freezing protection down to -34 degrees
and boilover protection up to 265 degrees with a 15 psi radiator cap. The
condition of the coolant is just as important as the strength because corrosion
can attack the system from within if the coolant is neglected. The recommended
replacement interval for traditional "green" antifreeze is two years or 30,000
miles. For the newer extended life coolants, the change interval can be as long
as five years or 150,000 miles. Most long-life coolants use organic acid
technology (OAT) additives that are different from those used in standard
antifreeze. Long-life coolants may contain special dyes to distinguish them
from ordinary antifreeze, such as the orange dye in General Motor's Dex-Cool
coolant. Other manufacturers use different colors. Even so, the type of coolant
can't always be determined by its color, so color is not a reliable indicator
of the type of coolant in a vehicle.The cooling system should be topped off or
refilled with the same type of antifreeze as the original.
Have you ever seen the inside of an aluminum water pump that was not
adequately protected by the corrosion inhibitors in the coolant? Or a radiator
or heater core that failed from the inside out because of internal corrosion?
These kinds of parts failures are all too common. Yet they can be easily
prevented by using the "right" coolant and changing the coolant
before trouble starts to heat up.
All types of antifreeze contain corrosion inhibiting chemicals to protect bare
metal surfaces from electrolytic attack. Though auto makers disagree on which
chemical additives work best in their vehicles, essentially any kind of
antifreeze will work in any vehicle. But how well will it protect the cooling
system? And for how long? And will it void the OEM warranty? These are
important questions that need to be answered before choosing an antifreeze for
a particular vehicle application.
There are essentially three basic types of antifreeze corrosion additives for
passenger cars and light trucks:
- "IAT" (Inorganic
Acid Technology) is the traditional "green" formula antifreeze.
This is the stuff General Motors used until 1996, Chrysler used until 2001
and Ford used until 2002 in its trucks, and 2003 in its passenger cars.
The green additive package contains phosphate and silicates, and provides
good protection for cast iron and aluminum engine parts, as well as
copper/brass radiators in older vehicles and aluminum radiators in newer
vehicles. The corrosion-fighting chemicals are fast-acting but wear out
after two to three years or 36,000 miles of average use, so green coolant
needs to be changed periodically to minimize the risk of corrosion damage.
- "OAT" (Organic Acid
Technology) is usually dyed orange to distinguish it from other types of
antifreeze. In 1996, GM began using a new extended-life antifreeze, called
"Dex-Cool." The coolant contains a totally different kind of
additive package called Organic Acid Technology (OAT). The OAT corrosion
inhibitors are slower acting and provide protection over a longer period
of time. OAT coolants typically have a service life of up to five years or
150,000 miles, making coolant changes less frequent - but still necessary
(a fact that many motorists seem to forget).
Other applications that currently use OAT antifreeze as the factory-fill
coolant include 1996-and-newer Audi, Jaguar, Porsche, Volkswagen and Land
Rover, 2001-and-newer Saab, and 1996-and-newer Toyota, Nissan, Honda, Mazda and other
Though OAT provides good protection for aluminum, it may not be the best
choice for older vehicles with copper/brass radiators because of the
lead-based solder used in the radiator. Some say OAT-based coolants may
also provide little protection against cavitation erosion in water pumps
with aluminum housings (unless the pump impeller is carefully designed to
- "HOAT" (Hybrid
Organic Acid Technology) antifreeze is usually dyed yellow but may also be
dyed orange or green. HOAT coolants are currently used by Ford, Chrysler,
Mercedes, BMW and Volvo. The additive package in a HOAT formula coolant
also contains silicates for added aluminum protection. Most of the
antifreezes in this category also meet the European "G-O5"
specification for hybrid extended life coolant. The service life for HOAT
is also five years or 150,000 miles.
Why not just have one "universal" coolant for all makes and models?
Some antifreeze suppliers do sell a universal product. They claim it is fully
compatible with all types of coolants and is safe to use in virtually any
American, Asian or European vehicle application. Universal coolants are
typically OAT or HOAT formulas that can go up to five years or 150,000 miles
between changes when the coolant is used to replace another coolant or is added
to a cooling system that already contains an OAT or HOAT coolant. But if used
to top off a cooling system that already contains an IAT green formula coolant,
the service life is the same as the original IAT green coolant (two to three
years or 36,000 miles).
The issue with universal coolants is that a single formula cannot meet the
conflicting OEM specifications for IAT, OAT and HOAT coolants. If a universal
coolant contains silicates, it does not meet the OEM OAT specification. If it
contains no silicates, it can't meet the OEM HOAT specification. And if it
contains phosphates or inorganic acid technology ingredients, it can't meet the
OEM OAT or HOAT specifications. Consequently, some antifreeze suppliers argue
there is no such thing as a universal coolant because one formula cannot meet
all the conflicting OEM specifications. This means distributors must offer
three different coolants to meet the IAT, OAT and HOAT specifications -
otherwise the coolant may not satisfy the OEM warranty requirements. That's why
the safest recommendation is to use the type of coolant specified by the
Of course, once a vehicle is out of warranty, motorists can use any type of
coolant they want - and many do. Many people still prefer the traditional green
formula coolant because it's the least expensive, even if it requires a little
more maintenance. Others may want to switch from a green coolant to a
longer-life OAT or HOAT coolant to reduce the need for maintenance. The
aftermarket gives motorists a choice so they can choose a product that best
suits their needs.
According to market research, there are approximately 224 million registered
cars and light trucks on the road today. Of these, 56 percent or 125 million
were originally equipped with IAT (green formula) antifreeze, 34 percent or 76
million were factory filled with OAT (orange) antifreeze and 10 percent or 23
million were factory filled with HOAT (yellow or G-05) antifreeze.
If a customer chooses a different type of coolant than that which was
originally in the vehicle's cooling system, the cooling system should be
flushed to remove all of the old coolant. This will avoid any potential
incompatibility issues between IAT, OAT and HOAT coolants. We have not heard of
any horror stories of bad things happening inside a cooling system when
different types of coolants are intermixed. But antifreeze suppliers caution
against mixing different types. Their advice is to use "same with
Regardless of which type of coolant is used, there are several points worth
noting. If the cooling system contains visible sediment or has had a part fail
due to corrosion, it needs to be thoroughly cleaned and flushed before the
system is refilled. This can be done with a coolant flushing-cleaning machine,
or by adding a can of cooling system cleaner to the radiator, driving the
vehicle for several days (follow the directions on the product) and then
draining and flushing the cooling system. Accumulated rust and scale can reduce
heat transfer and may make the engine run hot or overheat. Any crud inside the
cooling system will also react with the corrosion inhibitors in the fresh
antifreeze, reducing the potential service life of the coolant.
Unless a customer is buying premixed antifreeze that requires no water (because
it already contains water), antifreeze should always be mixed in equal parts
with clean, distilled or deionized water when the cooling system is refilled.
Avoid using ordinary tap water or even bottled drinking water because it
usually contains dissolved minerals and salts that can shorten the life of the
corrosion inhibitors in the antifreeze.
A 50/50 mixture of water and ethylene glycol antifreeze (any type) will protect
against freezing down to -34 degrees F and boilover up to 263 degrees F with a
14 psi radiator cap. Never use straight water or straight antifreeze in the
cooling system! Straight water provides no corrosion protection whatsoever. It
will also freeze at 32 degrees F and boil at 212 degrees F (unpressurized).
Straight antifreeze is also a poor choice because it freezes at 10 degrees F
and does not conduct heat as efficiently as water (which may cause the engine
to run hot and overheat). The best way to change the coolant is with a coolant
exchange machine that replaces all of the cold coolant with new coolant. Just
draining the radiator may leave up to a gallon of old coolant inside the engine
The water pump is the heart of the cooling system because it circulates the
coolant between the engine and radiator. The radiator is a heat exchanger and
allows air flow to carry heat away from the coolant so the engine doesn't run
too hot. A pump failure or a plugged radiator will usually make the engine
Water pumps work hard, typically pumping several hundred gallons of coolant per
hour at highway speeds. Because of the continuous load, pump failures are not
uncommon. The first symptom is usually leakage at the pump weep hole. Other
symptoms include bearing noise (rumbling, chirping or growling), loss of
coolant (through the leaky shaft seal), overheating (from coolant loss or
separation of the impeller from its shaft) and fan wobble.
One way to spot a water pump with bad shaft bearings is to check pulley or fan
play with the engine off. The pulley or fan should not wobble or show any
visible play when it is tugged by hand. Suspected seal leaks can also be
diagnosed by pressure testing the cooling system.
If a water pump needs to be replaced, the choices are a new pump or
remanufactured pump. Reman pumps reuse the housing, impeller and shaft, and
typically include a new bearing and seal assembly. Because of the variety in
OEM pump configurations, make sure the replacement pump has the same mounting
configuration, bolt locations and hose connections as the original. Also
compare pump heights as these may also vary depending on the dimensions of the
timing cover or other belt-driven engine accessories.
When a water pump is replaced, the cooling system should always be drained,
flushed and refilled with a fresh mixture of antifreeze and water to restore
proper cooling performance and corrosion protection. Belts and hoses should
also be carefully inspected, and replaced if any are found to be in
less-than-perfect condition. Hoses that are brittle, aged cracked, bulging or
chaffed must be replaced. New clamps should also be recommended. Belts that are
frayed, cracked or glazed also need to be replaced.
On overhead cam (OHC) engines that use the timing belt to drive the water pump,
check the mileage on the original timing belt. If the belt has been in service
more than the OEM recommended replacement interval (60,000 miles on older
vehicles, and up to 100,000 miles on newer ones), the timing belt should also
be replaced. This will save the labor of having to do the job twice because the
timing belt usually has to be removed to install the pump.
If the engine has overheated because of a pump failure or loss of coolant, the
thermostat should be replaced, too.
Another item that may also need replacing is the fan clutch (if the vehicle has
one). The lifespan of the fan clutch and water pump are about the same. Because
of this, many experts recommend replacing both components at the same time to
reduce the risk of future cooling problems.
A radiator may have to be replaced if it is leaking or is clogged internally
and cannot be cleaned. Leaks can be caused by external or internal corrosion,
vibration damage (hairline fatigue cracks) or by punctures (rocks or collision
damage). A radiator can also be damaged if the coolant freezes during cold
weather (not enough antifreeze in the coolant), or if the engine overheats or
runs hot creating steam erosion inside plastic end tanks.
A can of cooling system sealer can often provide a temporary fix for a leaky
radiator, and there are special epoxy glues available for patching small leaks
in aluminum radiators. But the only permanent fix for leak is to repair or
replace the radiator. A new radiator costs about the same as having an old
radiator repaired, and it usually comes with a better warranty. Plus, there's
no waiting for the radiator shop to fix the old radiator - which may take
several days depending on the work load.
Radiators on most late-model vehicles are aluminum with either aluminum or plastic
end tanks. Most radiators on older vehicles have a copper/brass core with metal
end tanks. Aluminum is lighter weight and lasts longer because it has no lead
soldered joints. Either type of radiator can be used in a vehicle, but the best
advice is to replace same with same. For high-performance applications or
hard-working vehicles, a stock copper-brass radiator or aluminum radiator can
be replaced with a larger, thicker or more efficient aluminum radiator to
improve cooling performance.
To consolidate applications, many replacement radiators are designed to fit as
many vehicles as possible. The radiator may have extra fittings (which can be
plugged if they are not needed). But as long as the radiator fits the engine
compartment and has the necessary hose connections, it should work as long as
it has a cooling capacity that is equal to or greater than the original
Before installing a new radiator, the cooling system should be cleaned and
flushed. Other items that should also be replaced include the radiator cap
(make sure the pressure rating is the same as the original), radiator hoses
(upper and lower), heater hoses and hose clamps. The cooling system should then
be refilled with fresh antifreeze and clean distilled or deionized water (not tap
Electrical system parts include large rotating electrical
components such as alternators and starters, solenoids, relays, batteries,
battery cables, wiring, fuses, bulbs and small electrical motors for things
such as power windows and seats.
Of all these parts, batteries are probably the most often replaced component.
The average battery life is about four to five years in most areas of the
country, and only about three years in really hot climates. Heat is hard on
batteries because it increases evaporation of water from the cells.
Maintenance-free batteries normally do not require make-up water, but in hot
climates water loss can be a problem. Most people don't check their batteries
anymore, and on sealed-top batteries there are no caps to remove. Consequently,
battery life suffers.
If a customer needs a new battery, you might recommend upgrading to a gel-type
battery that contains no liquid water. The electrolyte is a gel-like substance
sandwiched between absorbent fiberglass mats in the cells. This makes the
batteries spill-proof and much more resistant to heat damage. High-density,
spiral-wound batteries also use this same type of gel electrolyte.
Each cell inside a battery produces a little over two volts of electricity.
Automotive batteries contain a total of six cells, so the total voltage is
about 12 volts. A fully charged battery will actually read about 12.65 volts if
you place a voltmeter across the terminals.
Post configurations will vary depending on the application. Most General Motors
batteries have flat terminals on the side to which the positive and negative
cables are attached with bolts. Everybody else uses batteries with two top
posts (one negative and one positive). Some aftermarket "universal" replacement
batteries have both types of posts (top and side) to reduce the number of
different batteries needed to cover vehicle applications.
Batteries come in different lengths, heights, widths and post configurations,
which are classified according to group sizes. A replacement battery must be a
compatible group size to physically fit the battery tray and cable locations on
Another difference in batteries is their power ratings. The most commonly used
number is the "Cold Cranking Amp" (CCA) capacity, which is the maximum number
of amps the battery can deliver when cranking the engine. The higher the
number, the more amps the battery can provide for reliable cold starting.
Another power rating number that's important, but may be less familiar, is the
battery "Reserve Capacity" (RC) rating. This is how many amp hours of current
the battery can provide should the charging system fail. A replacement battery
should have CCA and RC that meet or exceed the OEM battery requirements.
Battery date codes are also important.
Batteries age on the shelf, so the oldest ones should always be sold first to
keep the stock fresh. The number/letter date code on the battery reveals when
it was manufactured. The number indicates the year, and the letter corresponds
to the month (A = January, B = February, C = March, etc.)
To prevent comebacks, new and used batteries should be tested to confirm their
state of charge and condition. A low battery that still has good cell plates
can be recharged and returned to service. But if a battery fails a "load test"
or will no longer accept a charge, it must be replaced. Batteries contain lead
and should always be recycled. Handle all batteries with care because they
The alternator is part of the charging system and generates voltage to keep the
battery charged and to operate the ignition system, computer and other
electrical accessories on the vehicle. The alternator is belt driven and
produces an alternating current (AC) that is converted into 12 volts of direct
current (DC) by diodes (the rectifier assembly) on the back of the alternator.
The output voltage is controlled by the engine computer or an internal or
external voltage regulator according to demand. The higher the load on the
electrical system and battery, the higher the charging output of the
alternator. Most charging systems that are working properly produce a charging
voltage of about 13.8 to 14.2 volts at idle with the lights and accessories
Alternator output can be tested with a voltmeter, ammeter or special test
equipment. Noisy shaft bearings may also require alternator replacement.
Alternators often fail because of excessive heat and electrical overloads. The
higher electrical loads that are common in many vehicles today can tax many OEM
alternators to the limit, especially those that are equipped with high-output
aftermarket audio systems, auxiliary lighting and other electrical accessories
that increase the amp load on the charging system.
Replacement alternators should always have the same or higher amp rating as the
original. Some of these premium-priced units have twice the output of a stock
alternator and can greatly extend alternator life in demanding applications.
Most are bolt-on replacements for the stock alternator, but heavier gauge
cables are also required to handle the higher output.
Starters are replaced less often than alternators because they are only used to
start the engine. Fuel-injected engines usually require little cranking to
start, so the starter doesn't have to work very hard, except during cold
weather when the oil in the engine thickens and makes it harder to crank.
Prolonged cranking is what kills many starters because heavy cranking causes
the starter to overheat.
The starter is mounted on the engine or transmission bellhousing and engages
teeth on the flywheel to crank the engine. A one-way, over-running clutch is
used to protect most starters against damage should the starter remain engaged
after the engine starts.
There are several different types: direct-drive starter motors, gear-reduction
starter motors and permanent-magnet starter motors (reduced-size starters with
permanent magnets inside instead of wire coils). It's important to handle
permanent-magnet starters with care because banging them on the counter or
floor may break the magnets inside.
Because of the high load on the starter, good electrical connections are
extremely important. Loose, corroded or undersized battery cables may not
deliver enough amperage to crank the engine at normal speed, causing hard
starting. Starter drives (which can be replaced separately on many starters)
can also fail, preventing the motor from engaging the flywheel. A bad solenoid
or relay will prevent the starter motor from cranking at all. Accurate
diagnosis of a starter problem is important to prevent unnecessary parts
replacements and returns.
Battery cables should be replaced if loose,
damaged or too small for the application. Engine ground straps are equally
important and are an often-overlooked cause of charging and starting problems.
Fuses protect against electrical overloads and are designed to blow if there's
too much current in a circuit. Overloads should not normally occur, so if a
fuse has failed, there may be a short in the circuit. Replacement fuses must
have the same amp rating as the originals. Relays are switching devices used to
route power to other components such as the fuel pump, ABS system, lights and
so on. Relays may be located in the engine compartment or almost anywhere in the
vehicle (locating a particular relay often requires referring to the vehicle's
Lamps and bulbs for interior and exterior illumination come in various sizes
and styles. Replacement lamps and bulbs must have the same mounting and
electrical connections as the original (compare the old and new bulbs or refer
to an application chart), but headlamps can often be upgraded for more light
output with higher output bulbs.
EMISSION CONTROL SYSTEM
All 1996-and-newer vehicles have Onboard Diagnostics II (OBD
II) as part of their programming to detect and identify emission faults. OBD II
monitors ignition misfires, the efficiency of the catalytic converter, the
operation of the engine's sensors and feedback fuel control loop and many other
One way the engine control system minimizes pollution is by carefully
controlling the air/fuel mixture. The engine computer does this by monitoring
the amount of unburned oxygen (O2) in the exhaust with an oxygen sensor mounted
in the exhaust manifold. A voltage signal produced by the O2 sensor tells the
computer if the mixture is rich or lean. The computer then compensates by
decreasing or increasing the on-time of the injectors.
In addition to the Powertrain Control Module (PCM), key engine sensors and
various subsystems are used to control specific pollutants. These include the
Positive Crankcase Ventilation (PCV) system, the Exhaust Gas Recirculation
(EGR) system, the Evaporative Emission Control System (EVAP), the Air Injection
Reaction (AIR) system and the catalytic converter.
The PCV system recirculates blowby vapors from the crankcase back into the
intake manifold so the vapors can be reburned inside the engine. This prevents
the escape of blowby vapors into the atmosphere, and also helps remove moisture
from the crankcase to prolong the life of the oil and prevent the formation of
engine-damaging sludge. The PCV valve is usually located in a valve cover, and
is attached to the intake manifold by a hose.
The EGR system reduces the formation of oxides of nitrogen (NOx) in the engine
by reducing peak combustion temperatures when the engine is under load. When
the engine is under load, intake vacuum drops. This causes the EGR valve on the
intake manifold to open a port between the intake and exhaust manifolds. Loss
of EGR can result in increased emissions and engine-damaging detonation. The
EGR valve is calibrated to the engine application, so a replacement valve must
have the same flow characteristics. Some replacement EGR valves have various
adapters to modify the flow rate.
The EVAP system controls evaporative emissions from the fuel system and fuel
tank. Fuel vapors are trapped in a charcoal-filled canister, then vented into
the engine through a purge valve to be reburned. A leaky fuel filler cap can
allow fuel vapors to escape into the atmosphere. Replacement gas caps must be
the same type as the original (caps for some older vehicles may be vented, but
newer ones are sealed). Fuel vapor leaks are monitored by the OBD II system.
The Air Injection Reaction (AIR) system on older vehicles uses an air pump to
pump extra oxygen into the exhaust system to reduce pollution. A diverter valve
assembly on the air pump controls the flow of air into the exhaust manifold. On
some vehicles, air is also routed to the catalytic converter. An anti-backfire
valve prevents hot exhaust gases from flowing backwards into the system and
damaging the diverter valve or air pump. Problems in this system can cause an
increase in emissions.
The catalytic converter is an "afterburner" in the exhaust system that helps
reburn unburned pollutants. The catalyst inside the converter can be
contaminated by lead (leaded gasoline), silicon (coolant leaks) or phosphorus
(burning oil), reducing its efficiency. On 1996-and-newer vehicles with OBD II,
a second oxygen sensor is mounted behind the converter to monitor its
operation. If converter efficiency drops below a certain level, it will turn on
the "Check Engine" light.
Converters can also be damaged by overheating if an engine is misfiring,
leaking compression or burning oil. If the converter gets too hot, the ceramic
honeycomb inside may melt causing an obstruction in the exhaust. A plugged
converter will create backpressure that reduces engine performance and may even
cause the engine to stall. There is no way to clean a plugged or contaminated
converter, so replacement is the only option. Replacement converters must be
the same as the original.
Electronic fuel injection systems come in several versions.
Throttle Body Injection (TBI) was used in the 1980s on many vehicles as an
intermediate step from electronic carburetion to multiport fuel injection. TBI
uses one or two fuel injectors mounted in a throttle body to fuel the engine.
Multiport fuel injection (MFI), which is used on almost all late-model engines,
has a separate fuel injector for each cylinder. The injectors are usually
mounted in the intake manifold and spray fuel into the intake ports, but on
some of VW's "direct injection" applications, the injectors spray directly into
the combustion chamber. First-generation MFI systems fire all the injectors
simultaneously or each bank separately, but most of the newer MFI systems are
"sequential" fuel injection (SFI) systems that fire each injector separately
just as the intake valve is about to open.
Another variation is General Motor's "Central Point Injection" (CPI) system.
Here, a centrally-located "Maxi" injector routes fuel to mechanical poppet
valve injectors at each cylinder. The CPI system works much like an old Bosch
K-Jetronic fuel injection system, except that the electronic Maxi injector
replaces the complicated mechanical K-Jetronic fuel distributor.
Most injectors are electronic and have a solenoid valve at the top to open the
nozzle. The Powertrain Control Module (PCM) determines the on-time of each
injector pulse to regulate fuel delivery (a longer on-time means more fuel and
a richer mixture). The PCM uses inputs from the oxygen sensor in the exhaust,
as well as throttle position, engine speed, load and airflow to control the
Dirty injectors are a common problem. Injector nozzles can become clogged with
fuel varnish over time, causing a loss of engine performance and misfiring.
Injectors can also leak fuel, causing an increase in fuel consumption and
emissions. An injector failure will result in a dead cylinder and power loss. A
shorted injector may rob voltage from the other injectors and cause the engine
Fuel pump pressure ratings vary depending on the application, but typically
range from 35 to 85 psi. Pump designs also vary and include single- or
double-vane, roller vane, turbine or gerotor style pumps. Most have a one-way
check valve to maintain pressure in the fuel system when the engine is shut
Fuel pumps can fail for a variety of reasons: old age, loss of voltage, ground
at the power relay, wiring connections and pump motor or bearing damage.
Running the fuel tank empty may damage the pump because it relies on fuel for
Replacement fuel pumps must have the same pressure rating and flow
characteristics of the original, but do not have to be the same type as the
original. The pump is usually part of the fuel sending unit and may be replaced
separately or as a complete assembly. The fuel inlet strainer sock should also
be replaced when the pump is changed.
When fuel reaches the engine, it enters a fuel rail and goes to the injectors.
A "fuel pressure regulator" on the fuel rail maintains a certain operating
pressure. Inside is a spring-loaded diaphragm attached to a source of intake
vacuum. As engine load (vacuum) changes, pressure is adjusted as needed to
maintain proper fuel delivery. Excess fuel is routed back to the fuel tank
through a return line. Many newer vehicles have "returnless" systems that do
not have a regulator on the engine fuel rail. The regulator is in the fuel tank
with the pump. Regulator problems that alter fuel pressure can hurt engine
performance and emissions.
Airflow into the engine is regulated by a "throttle body" attached to the
intake manifold. Air first flows through an air filter, then through the
throttle body before passing through the manifold and into the engine. The PCM
must monitor the amount of air entering the engine, so some fuel injected
systems have a vane or mass airflow sensor ahead of the throttle body. Other
systems estimate airflow based on throttle position, RPM, temperature and
HIGH- TECH SUSPENSION AND RIDE CONTROL
performance shock market includes many different segments: trucks and SUVs,
sport compact cars (Honda, Dodge Neon, Mitsubishi, etc.), sports sedans (BMW,
Audi, etc.), sports cars (Porsche, Corvette, Mazda Miata, etc.), street
performance cars (late-model Mustang, Camaro), vintage muscle cars (older
Mustang, Camaro, Dodge Chargers, etc.), hot rods ('32 Fords, etc.) and kit cars
(replica Cobras and others).
Nobody knows for sure how many performance shocks and struts are sold annually
in the aftermarket, but the overall replacement aftermarket for all kinds of
shocks (stock and performance) is estimated to be about 22 million units a year
in the U.S.
The problem with stock replacement shocks and struts is that customers don't
buy them until their originals have worn out. Even then, they may not realize
how much the ride control characteristics of their original equipment dampers
have faded over the years. The piston seals inside shocks and struts wear as
the miles accumulate, and eventually they lose the ability to maintain a
leak-free seal. The valves that control compression and rebound inside a shock
also wear and provide less resistance than when they were new. So the question
is, at what point are the original equipment shocks and struts no longer
providing adequate ride control?
At least one of the leading shock manufacturers now recommends replacing shocks
and struts every 50,000 miles. Its reasoning is that the average shock loses a
significant portion of its original dampening ability by the time it has
experienced 50,000 miles of everyday driving. Shocks contribute to driving
safety by maximizing wheel contact with the road. They also minimize body roll,
which reduces the risk of rollover in SUVs with a high center of gravity.
With performance shocks, it's a different story. You don't have to wait for the
original equipment units to wear out. You can make a sale at virtually any
point in a vehicle's life, whether it is 20 years old or brand new off the showroom
floor. People buy performance shocks when they want to upgrade their vehicle's
suspension. So the key to selling performance shocks is (1) knowing what's
available from the various shock suppliers and (2) being able to recommend
specific types of shocks for specific types of vehicle applications and driving
For example, a customer walks into your store and asks if you have
"coil-overs" to fit his Honda. If you don't know what a coil-over is,
you're going to have a hard time looking up a product to fit his car let alone
make the sale. You need to be product savvy and speak the same lingo as your
Coil-overs are one of the hottest upgrade options today for sport compact cars.
Essentially, a coil-over is nothing more than an adjustable bolt-in MacPherson
strut. It's a shock absorber with a coil spring wrapped around it, and it has
an adjustable spring seat plate that can be turned to raise or lower the
position of the seat. This changes the spring rate of the coil-over for a
softer or stiffer ride. It can also be used to raise or lower ride height
depending on the application. There are also "air spring" versions of
coil-overs that use a rubber air spring instead of a coil spring to alter
spring rate and/or ride height. The shock in a coil-over may be a conventional
gas-charged twin-tube design, or a high-pressure monotube design with or
without adjustable valving. Some suppliers even sell electronic versions of
their coil-overs that allow the driver to adjust the suspension on the fly from
a control box mounted inside the car.
IT'S A GAS
To sell performance shocks, you also have to know something about gas
pressurizing. The basic idea behind pressurizing a shock is to keep the
hydraulic fluid under pressure so it won't cavitate and foam, which leads to
"shock fade" (a loss of ride control caused by uneven or decreased
fluid resistance). In a nonpressurized shock, the hydraulic fluid can't keep
pace with the rapid up and down motions of the piston and is quickly churned into
foam. This reduces resistance and the shock's ability to dampen suspension
motions. As a result, dampening control lags behind suspension movement and
both handling and traction suffer.
By pressurizing the fluid reservoir with nitrogen gas, the formation of bubbles
is minimized. The dampening characteristics of the fluid remain constant and
shock fade is eliminated. Pressurization also helps to extend the shock more
quickly after it's been compressed and at the same time increases its
resistance to compression (making it slightly stiffer). Gas charging provides
more consistent control, reduces lag, lessens body roll and improves traction
recovery on irregular surfaces. That's why gas shocks and struts should always
be recommended for upgrading suspension performance.
Most new vehicles come factory-equipped with gas shocks or struts, but these
are not necessarily performance dampers. The gas helps, but by itself doesn't
create a true performance shock. Generally speaking, the higher the pressure,
the greater the resistance to foaming and cavitation. But you can't really
compare shocks by pressure ratings alone because performance depends on the
design of the shock and its internal valving. There are a lot of differences
between shocks, and pressure ratings is only one of them.
Gas shocks come in one of two basic varieties: single tube (monotube) and
double or twin tube. The single tube variety has all the major components
contained within a single large tube (thus the name) and typically uses a very
high-pressure charge (280 to 360 psi). The gas charge is separated from the
hydraulic fluid by means of a floating piston in the top or bottom of the tube.
This type of shock must be manufactured with a heavier gauge cylinder and a
highly polished internal surface (some are Teflon lined).
A less expensive alternative for upgrading ride control performance is the
double or twin tube gas shock. Available from many of the same suppliers of
single tube shocks, the double tube design is essentially a gas-pressurized
conventional shock with lower pressure. Some are in the 70-130 psi range, while
others are 112 to 130 psi or higher.
Many OEM dampers are valved more for ride comfort than ride control. Soft
valving provides a nice boulevard ride, but the trade-off is often reduced body
control and more roll - exactly what you don't want on a sport compact car,
sports sedan, sports car or SUV with a high center of gravity.Valving is what
makes a shock a performance shock or a standard shock. It defines the
compression and rebound characteristics of the damper as well as its ride
control envelope. Some performance shocks are designed primarily for the track
and are probably too harsh for a daily driver.
Likewise, some street performance shocks provide significantly better handling
than stock shocks, but are not the best choice for an all-out racing
application. As we mentioned earlier, you have to match the product to the
application - and your best sources of guidance on this subject are your shock
suppliers. They design and build the shocks, and they know which products work
best in which kinds of driving situations. If they recommend a particular shock
for the street or the track, follow their advice.
One thing to keep in mind about performance shocks and struts is that stiffer
isn't always better. What works great on the race track may be too harsh on the
kidneys for normal driving on the street. That's where adjustable shocks and
struts come in handy.
Adjustable shocks are a good choice for "dual-purpose" vehicles that
are driven mostly on the street but used occasionally off-road or for racing or
towing. Adjustable shocks allow the driver to dial-in the desired amount of
stiffness. By turning an adjustment screw or dial, or rotating the position of
the piston rod in the damper, the internal valving is changed to increase or
decrease the resistance offered by the shock. The shocks can be set at
"normal" for everyday driving, or "firm" or "extra
firm" when things get serious. Some aftermarket shocks offer up to eight
or more different settings. And some are available with electric motors or
solenoids that allow the driver to change settings from the driver's seat.
TRUCKS & SUVS
Owners of trucks and SUVs are typically interested in a different kind of
performance shock. They are usually interested in better handling stability
rather than cornering agility. When their vehicle is used to pull a trailer, it
should maintain a sure-footed track without tail wagging or whipping. The
steering feel shouldn't change drastically as the load increases, and the
headlights should remain well focused on the road ahead. There should be no
bumper dragging, no suspension bottoming, and no steering wander or
Firming up the suspension with a set of performance shocks will help keep the
body flat and reduce roll and sway. Excessive body roll when cornering is bad
because it unloads the wheels on the inside while causing excessive camber
changes on both sides of the suspension (camber is the inward or outward tilt
of the wheel that affects tread contact with the road). A more stable vehicle
is also a safer vehicle because it allows the driver to make sudden lane
changes or take evasive maneuvers with greater confidence.
For those who love to take their vehicles off-road, shocks with increased
suspension travel may be required. Off-road vehicles are often raised to
increase ground clearance and tire clearance. If the original shocks aren't
long enough, they may bottom out and limit suspension travel.
All gasoline-fueled engines have a spark ignition system to
ignite the air/fuel mixture in the cylinders. The spark is created by a
high-voltage surge from an ignition coil. The coil is triggered by an ignition
module and/or the PCM using a signal from a distributor pickup or crankshaft
position sensor. High-voltage from the coil travels though a thickly insulated
cable to the spark plug where it jumps across the plug's electrodes, creating a
If the engine has a distributor, a single coil is used on most engines to
supply high-voltage to all the spark plugs (a few Japanese applications use two
coils). If the engine has a "distributorless ignition system " (DIS), each
spark plug has its own separate coil. On General Motors "waste spark" DIS
systems, two spark plugs share the same coil. Many newer vehicles have coils
that are mounted directly over the spark plug and use no plug wires. These are
called "coil-on-plug" (COP) ignition systems. Another variation is
"coil-near-plug" (CNP) systems that mount individual coils near the spark plugs
and connect the coils to the plugs with short wires.
The most commonly replaced ignition parts are the spark plugs and plug wires.
On older vehicles with distributors, the distributor cap and rotor are also
service items. Ignition coils, modules and sensors are only replaced if they
Ignition coils come in various shapes and sizes, but all do essentially the
same thing: They are step-up transformers that convert 12 volts DC into 7,000
to 40,000 or more volts DC. The actual voltage required to fire a spark plug
will vary depending on engine speed, load, temperature, resistance in the plug
and wires, and the distance across the spark plug electrodes.
Inside the coil are two sets of copper wire windings, one inside the other. The
primary windings are made up of several hundred loops of heavy wire around the
iron core of the coil. The secondary windings consist of several thousand turns
of very fine wire inside the primary windings. When the primary current is
switched on, the coil charges up and creates a powerful magnetic field. When
the primary current is switched off, the collapse of the magnetic field induces
a high-voltage surge in the secondary windings that creates the spark.
If the coil windings short out or break, the coil may not produce enough
voltage to fire the spark plugs causing the engine to run rough or die.
Hairline cracks in the coil housing or insulation can also weaken or kill the
spark. Coils can be tested by measuring their primary and secondary resistance
with an ohmmeter or a spark tester. Replacement coils must be the same type as
the original to match the engine's voltage requirements.
Electronic ignition systems all use some type of transistorized switching
module to turn the coil(s) on and off. On some vehicles (GM and Ford), the
module may be mounted on or in the distributor. On DIS systems, it is often
part of the coil pack assembly. Modules can be damaged by heat and vibration. A
module failure will usually cause a no-spark, no-start condition. GM High
Energy Ignition (HEI) modules in older vehicles require a thin layer of
dielectric grease underneath to conduct heat away from the module. If your
customer forgets the grease, the module may not live very long.
Ignition modules may receive a trigger signal directly from a distributor
pickup (magnetic, Hall effect or optical), a crankshaft position sensor or the
PCM. A fault in any of these other components or the wiring can prevent the
ignition system from firing. Accurate diagnosis is essential to prevent
unnecessary parts replacements and returns.
If a vehicle has a distributor, the cap and rotor may develop carbon tracks and
cracks over time. This can lead to ignition misfire and hard starting.
Replacing the cap and rotor when the spark plugs are changed is often necessary
to restore "like-new" ignition performance.
Plug wires connect the distributor or individual coils to the spark plugs. Also
called ignition cables, they come in various types (suppression and solid core
- also called "mag" wires) and with various types and grades of insulation and
jacketing (silicone, EPDM and other materials). The higher the temperature
resistance of the insulation and jacketing, the better. Cable diameters are
usually 7 or 8 mm, and each cable is a different length to fit specific spark
plugs. Replacement cables must be the same size and length as the original.
Plug wires may be replaced individually or in complete sets (wires should be
changed one at a time to avoid mixing up the firing order). Replacement is
needed if internal resistance in the wires exceeds specifications, the wiring
is damaged, or the plug boots or terminals fit poorly or are loose.
Finally, we come to the business end of the ignition system: the spark plugs.
Spark plugs come in different sizes, lengths, threads and electrode
configurations, but all have some type of center electrode surrounded by a
ceramic insulator in a threaded steel shell. The "heat range" (operating
temperature) of a spark plug depends on the length and shape of the ceramic
insulator. The spark plug has to run hot enough so fuel deposits don't build up
on the tip, foul the electrode and cause it to misfire. But it also has to
conduct enough heat away from the tip so the tip doesn't get too hot when the
engine is under load and cause pre-ignition. Many spark plugs have a "copper
core" center electrode that improves heat conduction and gives the plug a
broader operating range.
Spark plugs are designed for specific engines. The diameter, length and pitch
of the threads that screw into the cylinder head must match the application.
How far the tip of the spark plug extends into the combustion chamber (called
"reach") must also be correct for the application, otherwise the tip of the
plug may hit the piston or valves. Always follow the spark plug listings in
your plug supplier's catalog.
The distance across the electrode gap at the end of the spark plug must also be
set to certain specifications for the engine to run properly. If the gap is too
narrow, the spark may not be long enough to ignite the fuel mixture reliably,
resulting in ignition misfire. If the gap is too wide, there may not be enough
available voltage to create a spark, also causing ignition misfire.
STEERING AND SUSPENSION
The steering system gives the driver control over the
vehicle's direction while the suspension allows the tires to roll over bumps
and dips in the road without losing their grip. Except for periodic checks of
the power steering fluid level, the steering and suspension systems on most
vehicles today do not require any maintenance. Parts are not replaced until
they wear out or are damaged - unless the vehicle's owner is upgrading the
suspension for a specific purpose such as increased load carrying capacity,
towing or handling.
The major components in the steering system include the steering box or rack,
inner and outer tie rod ends, idler arms, center links, power steering pump and
hoses.Most vehicles today have "rack and pinion" steering that uses a pinion
gear on the end of the steering input shaft to move a horizontal bar (rack)
sideways. The rack is connected to the tie rods with sockets, which are
enclosed in rubber bellows. The linkage has outer tie rod ends only. On some GM
applications, a "center-mount" rack is used where the tie rods bolt to the
center of the rack rather than the ends. Worn inner sockets can cause steering
looseness and tire wear. Wear in a power rack control valve housing can
increase steering effort. Fluid inside the bellows indicates leaky seals and a
need to replace the rack.
The other type of steering system is the "recirculating ball" steering gear in
which ball bearings turn against the worn gear to move the steering linkage.
The steering box is connected to the steering linkage with a pitman arm. An
idler arm supports the other side of the linkage, which includes a center link,
inner and outer tie rod ends and tie rods. Steering wander and looseness can
result if the idler arm bushing is worn.
Most vehicles have power steering and use a belt-driven pump to reduce the
effort required to steer the wheels. Some power steering pumps also provide
hydraulic assist for power brakes (Hydroboost systems). A worn pump will
usually make noise and/or leak fluid. If a replacement pump does not come with
a pulley, the pulley from the old pump will have to be removed and installed on
the new pump. Changing the power steering fluid is also recommended (use the
type specified by the vehicle manufacturer).
Power steering systems have two hoses, a high-pressure hose to carry pressure
from the pump to the steering gear or power cylinder, and a return hose back to
the pump reservoir. Leaks can cause steering problems. Replacement hoses may be
preformed or made up using various end fittings and a crimping press.
There are two basic types of front suspensions: short-long arm (SLA) and strut. An SLA
suspension uses upper and lower "control arms" of unequal length to support the
steering knuckle. Each arm is connected to the knuckle by a "ball joint" (one
upper, one lower). A strut suspension typically uses a MacPherson strut in
place of the upper control arm and ball joint. The strut combines a shock
absorber and spring into one assembly, and serves as the steering pivot for the
knuckle. A bearing plate at the top of the strut supports the weight of the
On some vehicles, a modified strut configuration is used when the spring is not
around the strut, but is located between the lower control arm and subframe. On
"wishbone" strut suspensions, the strut supports the weight, but an upper
control arm is also used to locate the steering knuckle. Almost all front-wheel
drive vehicles, as well as many rear-wheel drive vehicles, have strut suspensions.
SLA suspensions are used on most rear-wheel
drive cars and light trucks.
Suspension parts that may need to be replaced include ball joints, shocks,
struts, control arm bushings and springs. Most people don't think springs ever
wear out, but over time the constant force of gravity can cause springs to
weaken and sag. This changes ride height and alignment, which can increase tire
wear and cause handling and ride problems.
Shocks and struts are the most commonly replaced items because they suffer the
most wear. Inside is a piston that pumps back and forth through an oil-filled
tube. This creates friction that dampens the motions of the suspension and
keeps the vehicle stable and the tires in contact with the road. Control valves
in the piston and the bottom of the shock or strut vary the resistance by
venting fluid as the velocity of the piston changes. The piston rod has a seal
that keeps the oil inside and prevents outside contaminants from entering the
shock or strut. Over time, this seal wears out and allows the precious fluid
inside to leak out. As a result, the shock or strut loses its ability to
control the suspension, ride quality goes out the window and the suspension
becomes bouncy and rough.
There are two basic types of shocks and struts: twin-tube and monotube.
Twin-tube shocks have an oil reservoir around the outside of the piston
chamber. Oil moves back and forth from the chamber through the valves in the
end of the shock. Monotube shocks are only a single tube with no outer chamber.
One end of the shock is filled with pressurized gas with a floating piston seal
separating the gas charge from the oil. Twin-tube shocks may also be
pressurized with nitrogen gas because gas-charging reduces cavitation, foaming
and shock fade. Monotube shocks are typically charged at a much higher pressure
(up to 360 psi) and are used more on performance applications because of their
quick-acting and firmer ride characteristics.
Shocks and struts are usually replaced in pairs or sets. Replacement shocks and
struts with larger piston bores, increased gas pressure or other special
features like adjustable valving can be installed to upgrade ride control
performance. Likewise, monotube shocks are often used to replace twin-tube
shocks to increase suspension stiffness and cornering agility.
One item that may also need to be replaced when replacing struts are the
"bearing plates" atop the strut that allow the strut housing to pivot when the
wheels are steered. Looseness in the bearing plate can cause steering noise.
Binding may increase steering effort and prevent the wheels from recentering
following a turn.
MacPherson strut assemblies usually require a spring compressor to disassemble
and reassemble, but preassembled replacement struts with new springs and bearing
plates are also available to make installation easier. Wheel alignment is
usually necessary after replacing struts.
Ball joints are another part that may need to be replaced at some point in the
vehicle's life. The joints connect the steering knuckle to the control arms.
They may also be used on rear control arms. Joints that carry weight are called
"loaded joints" while those that do not are called "follower" joints. SLA suspensions have two upper and two lower ball joints.
MacPherson strut suspensions have only two lower ball joints.
A ball joint is so named because of its ball-and-socket construction. The ball
stud may ride against a metal gusher bearing, or it may be highly polished to
reduce friction and be enclosed in a polymer bearing. Most low-friction ball
joints are sealed and do not have a grease fitting for lubrication. Older,
gusher style joints have grease fittings so they can be lubricated
When ball joints become worn, they can make suspension noise and upset wheel
alignment. There is also a danger the joint may separate, allowing the
suspension to collapse. Replacing a ball joint requires a separator tool or
fork to separate the stud from the knuckle once the stud nut has been removed.
Some joints are bolted or riveted to the control arm, others are screwed in and
others are press-fit into the arm.
TECH FORUM: ALTERNATORS
QUESTION: Why do alternators have such a high return
ANSWER: Alternators have such a high return rate because they are often
misdiagnosed as the cause of a charging problem or low battery.
The alternator is the heart of the charging system. It generates the amps
needed to keep the battery fully charged and to supply the demands of the
ignition system, electrical system and onboard electronics. The alternator is
so named because it converts alternating current (AC) into direct current (DC).
An alternator produces current by rotating a magnetic field inside a stationary
conductor. The rotor inside the alternator produces the magnetic field. As the
magnetic poles of the rotor pass beneath the three stationary stator windings in
the alternator housing, a three-phased alternating current (AC) is induced in
the stator windings. The AC current is then "rectified" (converted) to direct
current (DC) by a diode trio (three sets of paired diodes) on the back of the
The alternator's output is controlled by switching its field current on and
off. A "voltage regulator," located inside the alternator, on the back of the
alternator or mounted elsewhere in the engine compartment, switches the field
current on and off to control current output. The higher the load on the
electrical system, the higher the current output of the alternator, if
everything is working right.
On many newer vehicles, the powertrain control module (PCM) controls the duty
cycle of the alternator to regulate voltage output. There is no voltage
regulator on the alternator.
When a vehicle has a charging problem, the fault might be in the alternator,
the voltage regulator (if it has one), the PCM or something else such as a
wiring problem, loose or corroded battery cables or even a bad battery.
The days of DIY alternator rebuilding are history, so if anything inside the
alternator has gone bad (stator or rotor windings, brushes, diodes or internal
regulator), the whole unit has to be replaced with a new or remanufactured
If the fault is not diagnosed correctly, and the real problem is not the
alternator, then obviously replacing the alternator won't help. The problem
will still be there, and the alternator will likely comeback as being "faulty."
Most alternators that are returned under warranty have nothing wrong with them.
If a new or reman alternator does not seem to be working correctly after it is
installed, chances are the real problem is something else (such as a bad
connector, wiring problem, bad battery cables, missing or loose ground strap,
bad battery, etc.).
When an alternator fails, the first indication of trouble may be a glowing
alternator warning light, a low voltage gauge, dim headlights or a battery that
keeps running down. Sometimes, there may be only a partial failure. The
alternator will still produce current, but not enough current to keep the
battery fully charged. This kind of failure can be caused by one or more bad
diodes in the diode trio (rectifier). The leading cause of alternator failure
is overheating - which, in turn, is often due to overworking the charging
system (extended idling with lights, air conditioning, sound system and other
accessories all on, or trying to keep a bad battery charged up).
One of the best ways to make sure an alternator has really failed and needs to
be replaced is to bench test the customer's old unit in the store. This
requires an alternator tester and some know-how about hooking up the test
connections and running the test.
The tester will spin the alternator and measure its voltage and current output.
As a rule, a good alternator will read within the specified voltage range
(usually 14 to 16.2 volts), and 90 percent of its rated amp output. If the
output is less than specifications, the alternator has reached the end of the
road and needs to be replaced.
If an externally regulated alternator tests well but does not work when
installed on the vehicle, the regulator may be faulty or have a poor ground
connection. Or, there may be a wiring problem.
It's also a good idea to test the new and/or remanufactured alternator the
customer is buying before it leaves the store. Why? To verify the unit is
working - so if it doesn't work after he installs it, he'll know the problem
wasn't the alternator, but something else.
Replacement alternators should have the same or higher amp rating as the
TECH FORUM: CERAMIC BRAKE PADS
QUESTION: What's so hot about ceramic brake pads?
ANSWER: In a nutshell, ceramic pads are quieter than semi-metallic pads.
Ceramic pads are also less harsh on rotors than their semi-metallic cousins,
tend to wear better, and unlike many European formula non-asbestos organic
(NAO) pads, don't produce a lot of ugly black brake dust that sticks to the
"Ceramic" has been a buzzword for a number of years in the brake pad business.
Ceramic brake pads were first installed as original equipment on a few import
cars back more than two decades ago.
Over the years, the use of ceramic linings has steadily grown, and today nearly
75 percent of all new vehicles come factory-equipped with some type of ceramic
linings. This has created a growing demand for ceramic replacement linings in
the aftermarket, to which brake suppliers have responded by introducing their
own ceramic brake pad product lines.
The trouble is that "ceramic" is a very vague word that some say has been
misused and abused. There is no standard industry definition of what a ceramic
brake pad is supposed to be. It's a lot like trying to describe pizza.
Everybody has his or her own recipe, and some are better than others.
The only thing we can say about ceramic brake pads, in general, is that they do
contain some type of ceramic fibers or particles. But how much ceramic and what
type of ceramic is anybody's guess. As a rule, most ceramic pads do not contain
any chopped iron, although some do. As for the ceramics themselves, the fibers
or particles may range in size from 0.4 to as much as 80 microns. Smaller is
supposed to be better, but suppliers disagree on that point as well.
The ceramic content and size of the fibers or particles is obviously an
important factor, but is only part of the total friction package. The wear and
friction characteristics of any given ceramic lining material also depend on
other things, such as the fillers, binders, resins and other ingredients that
go into the mix. In fact, up to 20 different ingredients may be used to achieve
the desired properties. Increasing the ceramic content improves overall braking
performance up to a point, but beyond a certain level it may actually reduce
braking effectiveness. The trick is to find the best balance of ingredients
that deliver the best results. Because of this, friction formulas are a closely
guarded secret among brake suppliers. It takes a lot of money to engineer, test
and perfect new friction products, so the last thing a brake supplier want to see
is a competitor copying its formula.
Some of the companies that sell ceramic brake pads actually use a number of
different ceramic formulas within their product lines to broaden their
coverage. Other companies may use the same friction material for almost
Compared to other types of friction materials, pads with a high ceramic content
generally perform best at brake temperatures of less than 500 degrees F, which
is well within the temperature range of most normal driving. On these kinds of application,
high ceramic content pads will generally show significantly less wear (50
percent or better) than most non-asbestos organic or semi-metallic linings, and
be noticeably quieter and produce almost no visible dust (which is a problem on
many European cars). Because of these advantages, ceramic linings are typically
marketed as a premium upgrade for cars that were originally equipped with NAO
or semi-metallic linings.
Does that mean ceramic linings are the best for all applications? No. For
higher temperature applications such as larger, heavier trucks and SUVs,
semi-metallic linings generally perform better at temperatures above 500
degrees F. Because of this, some brake suppliers do not offer ceramic linings
for certain trucks and SUVs that are originally equipped with semi-metallic
Q: What qualities are most important in replacement brake linings?
A: That's a difficult question to answer since different customers want
different performance characteristics, and different applications have
different requirements. In order to provide an answer, we looked to a recent JD
Powers survey, which revealed the most important things brake customers want.
1. Stopping power;
2. Good pedal feel (no soft or mushy pedal);
3. Quiet operation (no squeals or other objectionable noise);
4. No brake pulsation (which is a function of rotor wear and runout);
To satisfy these expectations, brake suppliers use a variety of friction
materials ranging from high-content ceramics to low-metallics to semi-metallics
to non-asbestos organics.
Ceramics provide a consistent pedal feel that is the same whether the pads are
hot or cold because the coefficient of friction doesn't drop off as quickly as
semi-metallics. NVH (noise, vibration and harshness) is also less with
ceramics, so that's another reason to recommend ceramic linings.
The survey didn't ask motorists about brake dust, but if it had it's almost
certain that owners of most BMWs and other European luxury cars would say
they're not overjoyed with the sticky black dust that their OEM brake pads
leave all over their wheels. The low dust characteristics of ceramic linings
would certainly appeal to this type of customer, so it's important to let them
know that low dust is one of the selling points of ceramic linings.
The survey also didn't ask about cost, which can be a concern with some
customers. Many brake suppliers offer value-line products as well as standard
and premium products to appeal to the budgets of a wide range of customers. All
are safe, but as a rule, value-priced linings use lower-grade materials that
won't wear as well as standard or premium linings. The pads may also lack
design features such as slots and chamfers that help reduce noise.
Q: Are there any tricks for quieting brake squeal?
A: Brake noise is usually the result of high-frequency vibrations
between the pads and rotors when the brakes are applied.
There are many causes of noisy pads. The underlying cause may be pads that are
inherently noisy and fail to dampen noise, rough worn rotors, missing shims,
springs or anti-rattle clips, excessive clearances between the pads and
calipers, or the calipers and their mounts.
The most common fix is to resurface the rotors and replace the pads. A smooth
finish on the rotors allows the pads to glide over the surface rather than bump
Most new OEM and aftermarket rotors come with a surface finish between 30 and
60 inches RA (roughness average), with many falling in the 40 to 50 RA range.
Some OEM specifications say any finish less than 80 RA is acceptable.
A brake lathe with sharp tool bits should produce a rotor finish that meets
these specifications. The most common mistakes that are made when turning
rotors is using dull bits, not mounting the rotors squarely on the lathe arbor
(too much runout) and turning the rotors at too high a feed rate. Some
technicians and machinists will also sand the rotors after they have been
turned to smooth the surface even more, though this really shouldn't be
As for replacement pads, recommend premium pads that have the same
noise-suppressing slots and chamfers as many OEM pads. High content ceramic
pads are generally the quietest material (semi-mets are the noisiest).
The pads should be installed with the noise-dampening shims on the back, unless
the shims are integrally molded into the pads. Another trick is to apply a thin
layer of high-temperature brake lubricant to the back of the pads so the pads
won't vibrate against the calipers or caliper pistons.
There are also aerosol products that can be sprayed on the rotors to dampen
noise during the initial pad break-in period. These products help fill voids on
the surface and act like a lubricant to help the pads glide quietly across the
rotors when the brakes are applied. The coating eventually wears off, but
usually after the critical break-in period has passed.
TECH FORUM: ENGINE MANAGEMENT
QUESTION: Why does the "Check Engine" Light Come On?
ANSWER: That's a question that many people ask. Most people think the
Check Engine Light comes on if an engine has any kind of problem whatsoever.
But guess what? They're wrong!
The Check Engine Light only comes on if a fault is emissions-related. The fault
does not actually have to cause a measurable increase in emissions, but if the
computer thinks the fault might cause emissions to increase, it will log a
diagnostic trouble code (DTC) and turn on the light.
The Check Engine Light, which is officially called the Malfunction Indicator
Lamp (MIL), is essentially an emissions warning light and nothing more. It does
not come on when the oil needs to be changed, if the oil level is low or if the
engine is overheating. There are other warning lights for these kind of
Many problems that trigger the light are relatively minor and have little or no
effect on engine performance. But because these problems might cause an
increase in emissions, the onboard diagnostic (OBD II) system is required by
law to monitor the fault, log a code and turn on the MIL anyway. The reason for
doing this is because a fault has the potential for increasing emissions beyond
federal limits. On late-model cars, the tailpipe and evaporative emission
standards are very strict. Consequently, it doesn't take much to push emissions
over the limit.
A loose gas cap can trigger the light. Why? Because fuel systems on late-model
vehicles are sealed to prevent the escape of fuel vapors into the atmosphere.
If the gas cap isn't tightened all the way, or the cap leaks because of a
damaged seal, OBD II will catch the fault, set a code and turn on the MIL.
Misfires can cause a huge increase in hydrocarbon emissions as well a loss of
performance, rough idle and other problems such as converter damage if the
misfire is steady rather than intermittent. Misfires can be caused by a lot of
things including worn or fouled spark plugs, bad plug wires or ignition coils,
dirty fuel injectors, air leaks or other conditions that cause the air/fuel
mixture to run leaner than normal, or even a loss of compression in one or more
cylinders because of a burned valve or blown head gasket. All engines will
experience an occasional misfire, but if the misfire rate exceeds a certain
threshold, OBD II sees it as a problem, sets a code and turns on the light.
Sometimes, the MIL will flash while the vehicle is being driven. This is a
warning to the driver that the engine is misfiring under load.
Sensor failures or electronic faults within the powertrain control module (PCM)
itself can also turn on the MIL. If a sensor is not generating a good signal,
or the signal does not make sense for the operating conditions at the time (a
wide open signal from the throttle position sensor while the engine is idling,
for example), it is recognized as a fault.
In some cases, loss of a key sensor signal or bogus information from the sensor
will wreak havoc on the operating logic of the engine management system. On
some systems, the PCM can substitute a "simulated" value for a missing sensor
signal, but it depends on the type of fault and the sensor that is involved.
A coolant sensor that always reads cold, for example, will prevent the engine
management system from going into a "closed loop" mode of operation. This means
the PCM ignores the signal from the oxygen sensor and keeps the fuel mixture
rich because that's what a cold engine likes. If the engine is warm, though,
staying in open loop too long increases emissions and fuel consumption.
A dirty mass airflow sensor, on the other hand, may mislead the PCM into
thinking the engine is using less air than it actually is. This can cause a
lean fuel condition, but the PCM may be able to compensate by using other
sensor inputs to check the accuracy of the mass airflow sensor's input.
One very important point to keep in mind with respect to the Check Engine Light
on OBD II vehicles is that you must use a code reader, scan tool or scanner
software to read and clear codes. There are no manual flash codes on OBD II
vehicles. Disconnecting the battery in an attempt to erase a code so the MIL
will go off is also a very bad idea because it can cause the PCM to forget
learned settings. This may cause additional driveability problems, and may even
require using a scan tool to "reset" lost information.
TECH FORUM: FUEL INJECTION
QUESTION: What happens when fuel injectors get dirty?
ANSWER: What will happen is that the engine may experience a variety of
driveability and performance problems such as hesitation or stumble when
accelerating, loss of power, rough idle, reduced fuel economy and increased
With electronic fuel injection systems, the air/fuel ratio is controlled by the
powertrain control module (PCM). The PCM monitors engine speed, load and other
operating conditions through its various sensors. Using these inputs, the PCM
calculates how much fuel the engine needs and commands the injectors to spray a
certain amount of fuel into the engine. The PCM does this by grounding each
injector circuit for a predetermined length of time. The longer the length or
duration of each injector pulse, the more fuel that injector sprays into the
The injector pulses are also timed to coincide with the rotation of the
crankshaft. Each injector may squirt once every revolution of the crank, or every
other revolution of the crank depending on the application. On older EFI
systems, all the injectors are usually triggered simultaneously. But on most
newer "sequential" EFI systems, each injector is pulsed individually just as
the intake valve is about to open. The duration of each injector pulse can also
be varied between cylinder firings to adjust the air/fuel mixture.
All this, of course, is based on the assumption that all the injectors are
clean and deliver their normal doses of fuel with each pulse.
The actual volume of fuel that each injector delivers depends on four things:
1) fuel pressure (which is maintained within a certain range by the fuel
pressure regulator), 2) intake vacuum (which changes with engine load), 3) the
on-time of each injector pulse (which the PCM varies depending on engine speed
and load), and 4) the size of the orifice in the injector spray nozzle (which
When dirt or fuel varnish deposits build up in the injector orifice, it creates
a restriction that reduces the amount of fuel delivered with each squirt. Dirty
injectors, therefore, run lean and don't deliver as much fuel as the engine
needs. This creates the lean fuel condition that can cause misfires, loss of
performance and increased emissions.
What's more, the deposits in the nozzle disrupt the injector's normal spray
pattern, which is very important for proper fuel atomization and clean
If the situation is bad enough to cause a significant number of misfires or
loss of power, the onboard diagnostics in the PCM may set one or more
diagnostic trouble codes and turn on the Check Engine Light (Malfunction
Indicator Lamp or MIL). Misfires may also cause the MIL to flash or come on
momentarily while the vehicle is being driven.
A P0300 random misfire code is typically an indication that the engine is
running lean. A P0171 lean code is another telltale sign that the engine is not
getting enough fuel. The cause may be dirty fuel injectors, or something else
such as a vacuum leak, low fuel pressure or a dirty mass airflow sensor. In any
event, the problem needs to be diagnosed and repaired to restore normal
performance and fuel economy. And if the vehicle has to take an emissions test,
it won't pass if there are any codes present regardless of cause or effect.
It doesn't take much of a restriction in an injector to lean out the fuel
mixture. Only an 8 to 10 percent restriction in a single fuel injector can be
enough to upset the air/fuel mixture and cause misfires.
In turbocharged engines, dirty injectors can have a dangerous leaning effect
that may lead to engine-damaging detonation and/or preignition.