Principles of Operation

Principles of Operation

The anti-lock brake system (ABS) 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 ABS controls both front and rear brakes separately. The brake pedal force required to engage the ABS function may vary with the road surface condition. A dry surface requires greater force than a slippery surface.

During the ABS 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 hydraulic control unit (HCU) inlet ports. The fluid pressure is transmitted through 3 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 ABS module senses that a wheel is about to lock, based on wheel speed sensor data, the inlet solenoid valve pulses closed, preventing more fluid from entering that circuit. The ABS module then reads the sensor signal from the affected wheel again. If the wheel is still decelerating, the normally closed inlet solenoid valve for that circuit is opened. A controlled amount of hydraulic pressure between the normally open inlet valve and the brake is relieved into the HCU accumulator. Once the affected wheel returns to vehicle speed, the ABS module returns the solenoid valves to their normal condition, allowing fluid flow to the affected brake.

The ABS is self-monitoring. When the ignition switch is placed in the RUN position, the ABS module carries out a preliminary self-check on the anti-lock electrical system indicated by a 3-second illumination of the yellow ABS warning indicator in the instrument cluster. Malfunction of the system causes the ABS 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.

If the vehicle GVW is over 4,535 kg (10,000 lbs), 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 carried out when the vehicle is started. During starting, the brake pedal must be released to run the self-test. If the brake pedal is applied or a failed brake switch occurs, the self-test is run at 6 km/h (4 mph) regardless of brake pedal position.

The ABS module receives several messages from the PCM over the high-speed controller area network (HS-CAN) bus. The ABS module uses these messages for diagnostic purposes (comparing wheel speed to vehicle speed and brake pedal position) as well as vehicle functionality in relation to ABS functionality (what gear the transmission is in and is it currently shifting gear).

Electronic Brake Distribution (EBD)

On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module then uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a predetermined threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.

A slight bump sensation may be felt in the brake pedal when electronic brake distribution (EBD) is active.

If ABS is disabled due to DTCs being present in the ABS module, EBD will continue to function unless the DTCs are for wheel speed sensors or the HCU. When EBD is disabled, the red brake warning indicator and the ABS warning indicator will be illuminated.