Most people are familiar with the term Antilock Brakes (ABS), but many do not know much about antilock braking, how it works, what if any special maintenance is required, or what parts can be replaced in antilock brake systems.
Antilock Brakes are essentially an enhanced or improved version of ordinary brakes. Simply put, the antilock brake system is designed to prevent the brakes from locking up and skidding when braking hard or when braking on wet or slick surfaces. This adds a significant margin of safety for everyday driving by preventing dangerous skids and allowing the driver to maintain steering control while trying to stop.
Antilock brakes do not necessarily reduce the stopping distance, and in fact may actually increase stopping slightly on dry pavement. But on wet or slick pavement, antilock brakes may reduce the stopping distance up to 25% or more, which could be the difference between a safe stop and an accident.
There are quite a few different antilock brake systems in use today, but one thing they all share in common is the ability to control wheel lockup during hard braking. A tire that is just on the verge of slipping (10 to 20% slippage) produces more friction with respect to the road than one which is locked and skidding (100% slippage). Once traction is lost, friction is reduced, the tire skids and the vehicle takes longer to stop.
The only exception to this rule is when a tire is on loose snow. A locked tire allows a small wedge of snow to build up ahead of it which allows it to stop in a somewhat shorter distance than a rolling tire. That is why some vehicles have an on/off switch for deactivating the antilock system when driving on snow.
Directional stability also depends on traction. As long as a tire does not slip, it will roll only in the direction it turns. But once it skids, it has about as much directional stability as a hockey puck on ice. By minimizing the loss of traction, antilock braking helps maintain directional stability and steering control.
Another point to keep in mind about antilock brakes is that it is essentially an "add-on" to the existing brake system. It only comes into play when traction conditions are marginal or during sudden "panic" stops. The rest of the time, it has no effect on normal driving or braking.
Antilock brake systems are also designed to be as "failsafe" as possible. Should a failure occur in the ABS control electronics, most systems will deactivate themselves. The ABS warning light will come on, but the vehicle should still have normal braking. This does not necessarily make the vehicle unsafe to drive, but it does mean the ABS system will not be there if needed in an emergency.
An ABS warning light should never be ignored, especially if the brake warning light is also on. This could indicate a potentially dangerous loss of hydraulic pressure or a low fluid level!). If both warning lights are on, the vehicle should not be driven until the brakes can be inspected.
All antilock brake systems control tire slip by monitoring the relative deceleration rates of the wheels during braking. If one wheel starts to slow at a faster rate than the others, or at a faster rate than that which is programmed into the antilock control module, it indicates the wheel is starting to slip and is in danger of breaking traction and locking up. The ABS system responds by momentarily reducing hydraulic pressure to the brake on the affected wheel or wheels.
Electrically operated solenoid valves are used to hold, release and reapply hydraulic pressure to the brakes. This produces a pulsating effect, which can usually be felt in the brake pedal during hard braking. The driver may also hear a buzzing or chattering noise from the ABS hydraulic unit.
The rapid modulation of brake pressure in the brake circuit reduces the braking load on the slipping wheel and allows it to regain traction, thus preventing lockup. It is the same as pumping the brakes, except that the ABS system does it automatically for each brake circuit, and at speeds that would be humanly impossible, up to dozens of times per second depending on the system (some are faster than others).
Once the rate of deceleration for the affected wheel comes back in line with the others, normal braking function and pressure resume, and antilock reverts to a passive mode.
The major OEM suppliers of antilock brakes are:
Bendix, Acquired from Allied Signal by Bosch, used primarily on Chrysler and Jeep products.
Bosch, Main ABS supplier for most imports and assorted domestic vehicles.
Delco, Now known as Delphi, is used exclusively on GM applications.
Continental Teves, found on various Ford, GM, Chrysler and import applications.
Kelsey-Hayes, supplier of rear-wheel ABS and four-wheel ABS systems on Ford, Chevy and Dodge trucks.
Nippondenso, used on Infiniti and Lexus
Sumitomo, found on certain Mazda and Honda applications, as well as Ford Escort.
Toyota, rear wheel only ABS systems on Toyota pickups.
Regardless of who makes them, all ABS systems keep track of wheel deceleration rates with wheel speed sensors. On some applications, each wheel is equipped with its own speed sensor. This type of arrangement would be called a "four wheel, four channel" system since each wheel speed sensor would give its input into a separate control circuit (the word "channel" here actually refers to each individual electronic circuit rather than the individual hydraulic brake circuits).
On other applications, fewer sensors are used. Many four-wheel ABS systems have a separate wheel speed sensor for each front wheel but use a common speed sensor for both rear wheels. These are called "three channel" systems. The rear wheel speed sensor is mounted in either the differential or the transmission. The sensor reads the combined or average speed of both rear wheels. This type of setup saves the cost of an additional sensor and reduces the complexity of the system by allowing both rear wheels to be controlled simultaneously.
Another variation is the "single channel" rear-wheel only ABS system that is used on many rear-wheel drive pickups and vans. Fords version is called "Rear Antilock Brakes" (RABS) while GM and Chrysler call theirs "Rear Wheel Anti-Lock" (RWAL). The front wheels have no speed sensors and only a single speed sensor mounted in the differential or transmission is used 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 is used on pickup trucks and vans. Because the rear-wheel antilock systems have only a single channel, they are much less complex and costly than their three- and four-channel, four-wheel counterparts.
Another difference in ABS systems is that some are "integral" and others are "nonintegral."
Integral systems, which are found mostly on older full-size passenger car applications, combine the master brake cylinder and ABS hydraulic modulator, pump and accumulator into one assembly. Integral systems do not have a vacuum booster for power assist and rely instead on pressure generated by the electric pump for this purpose. The accumulators in these systems can contain over 2700 psi. The accumulator must be depressurized prior to doing any type of brake repair work by pumping the brake pedal 40 times while the key is off.
Nonintegral ABS systems, which are sometimes refereed to as "add-on" systems, are used on most of the newer vehicles. Nonintegral ABS systems use a conventional master brake cylinder and vacuum power booster with a separate hydraulic modulator unit. Some also have an electric pump for ABS braking (to reapply pressure during the ABS hold-release-reapply cycle), but do not use the pumps for normal power assist.
The wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor ring (sometimes called a "tone" ring). The sensor(s) may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tailshaft or differential housing. On some applications, the sensor is an integral part of the wheel bearing and hub assembly. The sensor ring(s) may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, inside the brake drum, on the transmission tailshaft or inside the differential on the pinion shaft.
The wheel speed sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup magnetic field. This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the pickup windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up.
The WSS signal is sent to the antilock brake control module, where the AC signal is converted into a digital signal and then processed. The control module then counts pulses to monitor changes in wheel speed.
On applications where the wheel speed sensor is not part of the hub or wheel bearing assembly, it can be replaced if defective. Sensor problems can be caused by an accumulation of debris on the end (they are magnetic), incorrect air gap or faults in the wiring or connectors.
The ABS electronic control module (which may be referred to as an EBCM "Electronic Brake Control Module" or EBM "Electronic Brake Module") is a microprocessor that functions like the engine control computer. It uses input from its sensors to regulate hydraulic pressure during braking to prevent wheel lockup. The ABS module may be located in the trunk, passenger compartment or under the hood. It may be a separate module or integrated with other electronics such as the body control or suspension computer. On the newer ABS systems (Delphi DBC-7, Teves Mark 20, etc.), it is mounted on the hydraulic modulator.
The key inputs for the ABS control module come from the wheel speed sensors and a brake pedal switch. The switch signals the control module when the brakes are being applied, which causes it to go from a "standby" mode to an active mode.
When ABS braking is needed, the control module kicks into action and orders the hydraulic unit to modulate brake pressure as needed. On systems that have a pump, it also energizes the pump and relay.
Like any other electronic control module, the ABS module is vulnerable to damage caused by electrical overloads, impacts and extreme temperatures. The module can usually be replaced if defective, except on some of the newest systems where the module is part of the hydraulic modulator assembly.
The hydraulic modulator or actuator unit contains the ABS solenoid valves for each brake circuit. The exact number of valves per circuit depends on the ABS system and application. Some have a pair of on-off solenoid valves for each brake circuit while others use a single valve that can operate in more than one position. On Delco VI ABS systems, small electric motors are used in place of solenoids to drive pistons up and down to modulate brake pressure.
On some systems, the individual ABS solenoids can be replaced if defective, but on most applications the modulator is considered a sealed assembly and must be replaced as a unit if defective.
A high pressure electric pump is used in some ABS systems to generate power assist for normal braking as well as the reapplication of brake pressure during ABS braking. In some systems, it is used only for the reapplication of pressure during ABS braking.
The pump motor is energized via a relay that is switched on and off by the ABS control module. The fluid pressure that is generated by the pump is stored in the "accumulator." The accumulator on ABS systems where the hydraulic modulator is part of the master cylinder assembly consists of a pressure storage chamber filled with nitrogen gas.
Should the pump fail (a warning light comes on when reserve pressure drops too low), there is usually enough reserve pressure in the accumulator for 10 to 20 power-assisted stops. After that, there is no power assist. The brakes still work, but with increased effort.
On ABS systems that have a conventional master cylinder and vacuum booster for power assist, a small accumulator or pair of accumulators may be used as temporary holding reservoirs for brake fluid during the hold-release-reapply cycle. This type of accumulator typically uses a spring loaded diaphragm rather than a nitrogen charged chamber to store pressure.
Most vehicles with ABS require no special maintenance according to the vehicle manufacturers. But considering how expensive the hydraulic modulators are on many vehicles, many brake experts say changing the brake fluid every year or two for preventative maintenance can save consumers a bundle in brake repairs. Brake fluid absorbs moisture over time, which promotes internal corrosion in the system. So changing your brake fluid periodically can prolong the life of your ABS hydraulic components and minimize the risk of failure.
Basic brake service is essentially the same on a vehicle equipped with ABS as that which does not have ABS. On some vehicles, special bleeding procedures are required. Replacement linings should have similar friction characteristics to the OEM linings. On applications where the wheel speed sensor rings are part of the rotors or drums, replacement rotors or drums must be the same (count the teeth to make sure!).
If a vehicle has an integral ABS system where the ABS hydraulic modulator is part of the master cylinder, if either component fails the whole assembly must usually be replaced (which is very expensive). On nonintegral systems, the master cylinder may or may not be the same as the same vehicle application without ABS, so check your catalog carefully to make sure. Some antilock brake applications may have extra ports or connections on the master cylinder.
Antilock brake systems (ABS) are used on millions of vehicles, but many drivers still don't know the "right way" to use ABS in an emergency. So here the "do's and don'ts" of antilock braking:
DO keep your foot on the brake pedal. Maintain firm and continuous pressure on the brake pedal to let four-wheel ABS work properly. Avoid pumping the brakes, even if the brake pedal is pulsating. If you own a light trucks with rear-wheel antilock (RWAL) brakes, apply the brake pedal with just enough force to stop your truck without locking the front wheels. This way you can maintain steering control while the rear-wheel antilock system prevents the vehicle from skidding sideways.
DO allow enough distance to stop. Follow three seconds or more behind vehicles when driving in good conditions. Allow more time if conditions are hazardous.
DO practice driving with ABS. Become accustomed to pulsations that occur in the brake pedal when the antilock brakes are activated. Empty parking lots or other open areas are excellent places to practice emergency stops.
DO consult your vehicle owners manual for additional driving instructions regarding your antilock brake system.
DO NOT drive an ABS-equipped vehicle more aggressively than a vehicle without antilock brakes. Driving around curves faster, changing lanes abruptly or performing other aggressive steering maneuvers is risky in any vehicle.
DO NOT pump the brakes. With four-wheel ABS systems, pumping the brakes turns the antilock brake system on and off, which decreases braking efficiency and increases your stopping distance. ABS pumps the brakes for you automatically at a much faster rate than you could do it manually, and allows better steering control. What's more, it also pumps the brakes on individual wheels as needed -- which is something you cannot do.
DO NOT forget to steer. Four-wheel ABS can help you steer around hazardous situations, but your vehicle cannot steer itself.
DO NOT be alarmed by clicking noises and/or pedal pulsations when braking hard. These conditions are normal when ABS is active and lets you know the antilock brake system is working. There should be no ABS feedback, though, when braking normally on dry pavement. ABS should only come into play during panic stop situations or when the road is wet, icy, slick or covered with loose gravel.
A growing number of late model cars are equipped with antilock brake systems that can precharge the brakes in anticipation of braking (to reduce braking reaction time), or actually apply the brakes automatically as a function of the collision prevention system (automatic braking).
CAUTION: If you are doing any type of brake work on these vehicles, the braking system must be deactivated before you work on the brakes, otherwise the system may energize the brakes unexpectedly causing the caliper pistons to push outward with considerable force. This could pinch your fingers if they are between the pads and rotors. If the pads have been removed, the pistons can blow out of their calipers. This can happen even when the ignition and engine are off!
The antilock brake system can be deactivated by locating and removing the main power fuse for the ABS system, or by using a scan tool to temporarily deactivate the system. Disconnecting the battery also works, but this may cause loss of certain memory settings in various vehicle modules. See Battery Disconnect Cautions & Problems for more information on this subject.