WHAT YOU NEED TO KNOW ABOUT ABS
It's the Nineties and safety is in. That's why antilock brake systems (ABS) have become standard or optional equipment on
almost all new cars and light trucks. When ABS first appeared on vehicles as an expensive safety option back in the late
1980s, no one thought it would catch on as quickly as it did. But by 1990 25% of new cars and light trucks offered ABS as
either standard or optional equipment. By 1994, the figure had grown to well over 50%, and now it's over 70%!
WHAT ABS DOES
Antilock brakes improve ordinary braking by rapidly pumping brake pressure to prevent the brakes from locking during a hard
panic stop or when braking on wet or slick surfaces. This, in turn, prevents the tires from skidding and allows you to maintain
steering control and directional stability.
ABS works its magic by monitoring and comparing the relative speed of the wheels to one another when the brakes are
applied. If one or more wheels start to slow at a faster rate than their companions, ABS takes over and begins to modulate
brake pressure so the wheels don't lock up. Depending on the vehicle, you'll typically hear some clicking or buzzing noises and
feel a rapid
pulsing in the brake pedal. Don't worry. It's just the ABS system doing its job. As soon as traction is regained or the
vehicles comes to a halt, ABS reverts back to its standby mode until the next time it might be needed.
Contrary to what some people believe, ABS does not decrease a vehicle's stopping distance under all driving conditions. In
fact, on smooth, dry pavement that provides good traction, a highly skilled driver may be able to stop in a somewhat shorter
distance than a vehicle equipped with antilock brakes by keeping the tires just on the verge of slipping. But on wet or slick
surfaces where traction is not so good, antilock brakes will almost always provide superior braking and stop the vehicle in a
shorter distance for the average driver.
In some situations, ABS may actually increase a vehicle's stopping distance slightly. Loose gravel or dirt as well as deep, fluffy
snow create a unique situation with respect to traction. When braking on these kinds of surfaces, a locked wheel will actually
stop a vehicle in a shorter distance than an unlocked wheel because loose debris forms a wedge ahead of the wheel. That's
why some vehicles have an on/off switch to deactivate the ABS system when driving on snow or loose gravel.
TWO TYPES OF ABS
There are actually two basic types of ABS systems: four-wheel and rear-wheel only. Four-wheel ABS systems monitor the
wheel speeds of all four wheels and apply ABS braking to the front
and/or rear wheels as needed. On many four-wheel ABS
systems, a separate speed sensor is used for each wheel. These are called "four channel" systems because there are four
wheel speed sensor input channels and four ABS control circuits. On other four- wheel ABS systems, there are only three
wheel speed sensors. A common sensor located in the differential or transmission is used to monitor the speed of both rear
wheels. Both rear brake hydraulic circuits may also be controlled with a single solenoid. These are called "three channel"
Rear-wheel antilock (RWAL) is found on many pickup trucks and vans. Only the rear wheels are controlled in these
applications because they are the ones most likely to lock up when braking hard or on slick surfaces. This is because the rear
wheels on pickups and vans carry little weight unless the vehicle has a load in the pickup bed or cargo area. RWAL systems,
which are manufactured by Kelsey-Hayes, use a single ABS valve to control the hydraulic circuit to both rear wheels. A single
vehicle speed sensor in the transmission is also used to monitor rear wheel speed.
Which type of ABS is best? Four-wheel, because it controls all four wheels. This means the front wheels won't lockup (which
can still happen with RWAL systems) so you can maintain steering control.
TRACTION CONTROL, TOO
On many vehicles, the ABS system also provides "traction control" capability. The same sensors that monitor wheel speed
while braking to prevent lockup are used to monitor wheel speed during acceleration to prevent wheel spin. If a drive wheel
starts to lose its grip, the traction control system kicks in. What happens next depends on the system and the control strategies
it uses to limit wheel spin.
On most vehicles, the ABS/traction control system will apply brake pressure to a drive wheel if it starts to spin. Braking shifts
torque through the differential to the opposite wheel which has better traction. The effect is much like that of a limited slip or
Positraction differential. If both wheels are losing their grip, then both wheels are braked to keep things under control. Most
systems are designed to discontinue traction control braking above certain speeds (35 to 50 mph) because of the adverse
effects it can have on handling.
On some applications, additional methods are employed to control traction. The ABS control module may communicate with
the engine computer and tell it to reduce engine power until traction can be restored. On some cars, up to half of the engine's
fuel injectors may be momentarily shut off to reduce power. Some have a "throttle relaxer" mechanism on the throttle linkage
that reduces the throttle opening and pushes back against the gas pedal. On some new cars that have "fly-by-wire" throttles
(no direct mechanical connection between the gas pedal and throttle), the engine computer may reduce the throttle opening as
needed to limit power. Some systems also retard spark timing to reduce power. Once traction is restored and the wheels are
no longer in danger of spinning, braking and/or whatever control strategy that was employed is discontinued and everything
returns to normal.
While all of this is happening, a "traction control" indicator light may flash or come on to let you know what's going on so
hopefully you'll ease up on the gas a bit.
ABS IS ONLY AN ADD-ON
An important point to keep in mind with respect to antilock brakes is that ABS is only an "add-on" to your vehicle's existing
brake system. ABS only comes into play when traction conditions are marginal or during sudden panic stops when the tires
lose their grip and begin to slip excessively. The rest of the time ABS has no effect on normal driving or braking, and is just
along for the ride.
What happens if your ABS system fails? Nothing, except that it won't provide the benefits of ABS braking. ABS systems are
designed to be failsafe so if they quit working you still have normal braking. If the ABS system's self-diagnostics detects a fault,
it will illuminate the ABS warning light, log a code that corresponds to the fault in the ABS control module's memory and
deactivate the system. The system will remain deactivated as long as the ABS warning light is on.
How do you know if the ABS system is working or not? The ABS warning light should come on momentarily for a bulb check
when your vehicle is first started. If the the bulb goes out, one is supposed to assume the system is working normally. If the
bulb remains on, however, it means a fault has been detected and the ABS system is deactivated.
Under normal braking, there should be no ABS feedback (no clicking noises or pedal pulsation). When braking hard on a wet
or slick surface, however, you should hear and feel these cues that the system is working. If you get ABS feedback during
normal braking or if you experience wheel lockup and skidding, you should have the ABS system checked for a possible
BRAKE SERVICE ON ABS-EQUIPPED VEHICLES
The fact that your vehicle is equipped with ABS should make no difference with respect to normal brake system service and
repair -- with a couple of notable exceptions. One is with respect to preventative maintenance.
Most vehicle owner manuals today do not contain a specified or even a recommended interval for replacing the brake fluid.
This is because the auto makers have chosen to downplay the need for preventative maintenance except for things (like oil
and filter changes) where regular changes are absolutely necessary. But that doesn't mean your brake system never needs to
have its fluid changed. Quite the contrary.
DOT 3 and 4 brake fluid is glycol-based and absorbs moisture over time. Moisture contamination is a bad thing to have in your
brake system because it lowers the boiling point of the fluid significantly. This, in turn, increases the danger of fluid boil and
brake failure if the brakes get too hot (as when driving down a mountain, towing a trailer, etc.). Moisture also promotes internal
corrosion, which can rust and damage metal parts in the master cylinder, calipers and wheel cylinders, not to mention the
very-expensive-to-replace ABS hydraulic modulator assembly which contains the ABS control valves.
The auto makers recognize this and say in their shop manuals to always change the brake fluid when the brakes are relined.
But many brake experts say this isn't often enough. Changing the brake fluid every two years (or even yearly) for preventative
maintenance would be far better, especially on ABS-equipped vehicles, and would go a long ways towards preventing the type
of corrosion that eventually causes many of the hydraulic parts in your brake system to fail.
Changing the brake fluid means you'll have to "bleed" the brake lines. Refer to a shop manual for the procedure and wheel
sequence to use. On some ABS-equipped vehicles, special bleeding procedures are required so refer to a shop manual for
WARNING: If you plan to do any brake work yourself on vehicle equipped with an "integral" ABS-system (where the master
cylinder and modulator are one assembly and boost pressure is provided by the ABS pump and accumulator), The
accumulator must be completely depressurized before working on the brakes or bleeding the brakes. The accumulators in
these systems can store up to 2700 psi of hydraulic pressure!
To depressurize the accumulator, turn the ignition off and depress the brake pedal 25 to 50 times until the pedal becomes
firmer. Do not turn the ignition back on until you've completed your repairs and the lines have been reconnected. Always wear
safety goggles and never apply pedal pressure to bleed a brake line unless a hose has been attached to the open bleeder
WHEN THERE'S TROUBLE
Whether or not your vehicle is ABS-equipped should make no difference when it comes to ordinary brake repairs, provided you
heed the above caution. A normal brake job on an ABS-equipped vehicle should be no different or cost no more than one on a
vehicle without ABS brakes because basic brake service on both types of vehicles is essentially the same. The only time any
work on the ABS portion of the system should be necessary is if the ABS warning light is on.
As stated earlier, ABS systems are designed to failsafe so an ABS failure should not prevent your vehicle from stopping. The
brakes will continue to work, but may lockup during a hard panic stop or if the road is slick because the ABS system isn't there
to prevent it.
During normal operation, the ABS warning light should comes on momentarily for a bulb check when the vehicle is started,
then go out and remain out when the vehicle is driven. But if your ABS warning light comes on and remains on while you're
driving, proceed with caution because you're probably driving without the benefit of ABS braking.
Generally speaking, the ABS warning light will come on and remain on when a serious problem has been detected. All ABS
systems have a built-in self-diagnostic capability that may include a brief self-test when the vehicle is first driven. The detection
of a problem sets one or more "trouble codes" (numeric codes that correspond to particular faults) in the ABS control module.
Some problems are serious enough to disable the system. Others are not and allow the system to continue functioning even
though the ABS light may be on or has flashed momentarily. Even so, don't ignore an ABS warning light.
The codes that are stored in the ABS module can be retrieved using special diagnostic procedures or a "scan tool" that plugs
into the system (refer to a shop manual for the particulars for your vehicle). Each code refers to a specific diagnostic chart in a
service manual, which must then be followed to pinpoint the faulty component. Troubleshooting an ABS system can be tricky.
You have to know how to read a wiring diagram and how to use a digital volt-ohm meter (DVOM) to check resistances,
voltages and continuity. It can be a time-consuming job, especially if the problem lurks somewhere in the wiring rather than a
bad wheel speed sensor or other easily isolated component. So if in doubt, let a professional handle it.
If your ABS warning light and red brake warning light are both on, it probably means low brake fluid due to a leak somewhere
in the system. In such instances, there's usually nothing wrong with the ABS system. But don't assume so without first
inspecting the brake system and having a technician check out the ABS system.
Most ABS problems tend to be electrical: broken, corroded or loose wiring or wiring connectors to the wheel speed sensors, or
sometimes a bad electrical component such as a pump, solenoid or relay. But there's no way to know for sure without
performing a careful diagnosis. This is usually done by plugging a "scan tool" into the ABS system to check for fault codes. The
code tells a technician which part of the ABS system is malfunctioning, but may require additional diagnostic work to isolate the
fault to a particular component.
Be warned: major ABS components are not cheap to replace. Major components such as the control module or hydraulic
modulator assembly can cost hundreds of dollars.
WHEEL SPEED SENSORS
The wheel speed sensors may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tail shaft
or differential housing. On some applications, the sensor is an integral part of the wheel bearing and hub assembly. The sensor
produces an alternating current (AC) signal as teeth on the sensor ring rotate past it. The sensor ring may be mounted on the
axle hub behind the brake rotor, CV joint housing, brake rotor, drum, transmission tail shaft or differential pinion shaft.
Anything that interferes with a wheel speed sensor's ability to produce a reliable signal can set a wheel speed sensor fault
code. Though most such problems can be traced to opens or shorts in the sensor's wiring harness, or loose or corroded
connectors, a bad speed signal can sometimes be caused by broken or damaged teeth on the sensor ring. Sensors are
magnetic, so metallic debris can sometimes build up on the sensor's tip and cause bad readings. Removing and cleaning the
sensors may be all that's needed to clear up this type of problem. Internal shorts and opens are rare, but can be easily
diagnosed by checking the sensor's resistance with an ohmmeter.
CAUTION: If you attempt to do this yourself, make sure the ignition is off before you disconnect any wires. Also, refer to the
exact specs for your vehicle, and note that front and rear sensors on the same vehicle often have different values.
If a wheel speed sensor is removed or replaced, the distance or "air gap" between the end of the sensor and its ring must be
set to the specified gap if the sensor is adjustable (many are not). The proper gap is necessary to produce a strong, reliable
Information contained in this website is for reference purposes and gives a general outline of the different parts and functions of the chassis. Please see your mechanic or service provider for follow up work on your vehicle.