Antilock Brakes / Traction Control Systems: Description and Operation

Anti-Lock Control
The 4-Wheel Anti-Lock Brake System (4WABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions.

The 4WABS controls both front and rear brakes separately. The brake pedal force required to engage the 4WABS function may vary with the road surface condition. A dry surface requires greater force than a slippery surface.

During the 4WABS operation, the driver may feel a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.

When the brakes are applied, brake fluid is forced from the brake master cylinder outlet ports to the HCU inlet ports. The fluid pressure is transmitted through three normally open solenoid valves inside the HCU, through the outlet ports of the HCU to the brakes. One circuit of the brake master cylinder feeds the front brakes while the other circuit feeds the rear brakes. If the anti-lock brake control module senses that a wheel is about to lock, based on wheel speed sensor data, the solenoid valve will pulse closed, preventing more fluid from entering that circuit. The anti-lock brake control module then reads the sensor signal from the affected wheel again. If the wheel is still decelerating, the normally closed solenoid valve for that circuit is opened. A controlled amount of hydraulic pressure between the normally open valve and the brake is relieved into the HCU accumulator. Once the affected wheel returns to vehicle speed, the anti-lock brake control module returns the solenoid valves to their normal condition, allowing fluid flow to the affected brake.

The anti-lock brake control module monitors the electromechanical components of the system. Malfunction of the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains. Malfunctions are indicated by the yellow ABS warning indicator in the instrument cluster.

The 4WABS system is self-monitoring. When the ignition switch is placed in the RUN position, the anti-lock brake control module performs a preliminary self-check on the anti-lock electrical system indicated by a three-second illumination of the yellow ABS warning indicator in the instrument cluster.

If the vehicle is over 10,000 GVW and has a build date after 2/22/99, the warning indicator may remain on until the vehicle is driven at a speed above 24 km/h (15 mph) for 10 seconds. Bringing the vehicle up to this speed enables a system self-check that turns the indicator off if no fault conditions are present.

A self-test of the solenoid valve coils and the pump motor is performed when the vehicle is started. During starting, the brake pedal must be released to run the self-test. If the brake pedal is depressed or a failed brake switch occurs, the self-test is run at 6 km/h (4 mph) regardless of brake pedal position.

The anti-lock brake control module and the hydraulic control unit can be replaced separately.