Antilock Brakes / Traction Control Systems: Description and Operation

Fig. 1 Bendix Anti-Lock 10 Braking System Components:

NOTE: For late model vehicles see '91 Anti-lock brake system.

The purpose of the Anti-Lock Brake System is to prevent wheel lock-up under heavy braking conditions on virtually any type of road surface. Anti-Lock braking is desirable because a vehicle which is stopped without locking the wheels will normally stop in a shorter distance than a car with locked wheels, while retaining directional stability and some steering capability. This allows the driver to retain greater control of the vehicle during heavy braking.

This is accomplished by using a sensor at each wheel to send wheel speed information to the Anti-lock Brake Control Module (ABCM). If the ABCM detects any wheel slip while braking, it sends signals to the hydraulic assembly to control the brake pressure in that braking circuit.

The system components include a hydraulic modulator, wheel speed sensors (WSS), reluctor rings, the ABCM, driver warning lamps, and associated fuses, connectors and wiring.

The hydraulic assembly functions as the booster and master cylinder. In addition, the hydraulic assembly modulates brake pressure for each wheel circuit, during anti-lock braking.

OPERATION AND PERFORMANCE
The Anti-Lock Brake System (ABS) offers the driver increased safety and control during braking through a sophisticated system of electrical and hydraulic components. There are several performance characteristics that are different from conventional vacuum assisted brake systems which are normal operating characteristics of ABS.

Pedal Feel
The system utilizes hydraulic power assist for both normal power assisted braking and to provide a source of high pressure hydraulic fluid during anti-lock braking. An advantage of this system is the ability to reduce total pedal travel during normal braking resulting in a firm, short travel pedal feel during normal braking. The pedal will seem springy when the vehicle is not in motion.

Due to the hydraulic power assist, it is often not possible to diagnose brake system problems by judging brake pedal feel. With hydraulic assist, the master cylinder is isolated from the brake pedal by the booster servo circuit. Thus conditions such as air in the brake system or a generally low pedal may not be perceivable when the system is under hydraulic assist. Only with the hydraulic accumulator discharged can the brake pedal feel be evaluated. Without power assist, the system reverts to a manual mode, pedal pressure acts directly upon the master cylinder without the benefit of power assist.

Anti-Lock Operation
During Anti-Lock Braking, brake pressure at the wheels is modulated by cycling electric valves. The cycling of these valves can be heard as a series of popping or ticking noises. In addition, the cycling may be felt as a pulsation in the brake pedal, although no pedal movement will be noticed. If anti-lock operation occurs during hard braking, some pulsation may be felt in the vehicle body due to fore and aft movement of the suspension as brake pressures are modulated.

ABS operation is automatically turned off at speeds below 3 to 5 mph. Therefore wheel lock-up may be perceived at the very end of an anti-lock stop and should be considered normal.

Tire Noise & Marks
Maximum braking performance is achieved at a wheel slip of 30%. The ABS system is designed to achieve this slip value. Thus this slip may result in some tire "chirping," depending on the road surface. This sound should not be interpreted as total wheel lock-up.

Complete wheel lock-up normally leaves black tire marks on dry pavement. However, anti-lock braking will not leave dark black tire marks since the wheel never reaches a locked condition. Tire marks may however be noticeable as light patched marks.

Vehicle Performance
Anti-Lock Brakes provide the driver with shorter stopping distance on most road surfaces and some steering control during hard braking. However, there are conditions where the system does not provide any benefit, for example, hydroplaning and high speed cornering may cause vehicle skidding, independent of vehicle braking.

One of the significant benefits of the ABS system is that of steering control during hard braking or during braking on slippery surfaces. It is therefore possible to steer the vehicle while braking on virtually any road surface.

System Self-Diagnostics
There are two self checks the system performs every time the vehicle is started. First, when the key is turned on, the system performs an electrical check called Power-On-Reset (PORS). During this check, the BRAKE warning lamp and the ANTI-LOCK warning lamp are illuminated, and then turned off at the end of the test, after about 1-2 seconds. As soon as the vehicle reaches a speed of about 3-4 mph, the system performs a functional check called BITE (Built In Test Equipment). During BITE, all hydraulic valves are activated briefly to test function. BITE can be detected as a series of rapid clicks when initially driving the car after starting. If the brake pedal is applied during BITE, some feedback will be noticed from the brake pedal. Both of these conditions are a normal part of the system self test. Some fault conditions will set an ABS Fault Code in the ABCM, which can be retrieved to aid in fault diagnosis.