Principles of Operation: Notes

The Anti-Lock Brake System (ABS) operates as follows:

• When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the Hydraulic Control Unit (HCU) inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU and then through the outlet ports of the HCU to each wheel.
• If the ABS module senses a wheel is about to lock, based on wheel speed sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.
• The ABS module then looks at the wheel speed sensor signal from the affected wheel(s) again.
• If that wheel(s) is still decelerating, it opens the closed solenoid valve for that circuit.
• Once the affected wheel comes back up to speed, the ABS module returns the valves to their normal condition, allowing fluid to flow to the affected brake.
• The ABS module monitors the electromechanical components of the system.
• A malfunction in the ABS will cause the ABS module to shut off or inhibit the system. However, normal power-assisted braking remains.
• Malfunctions are indicated by a Yellow ABS warning indicator in the instrument cluster (10849).
• The ABS is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will carry out 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.
• During vehicle operation, including normal and anti-lock braking, the ABS module monitors all electrical anti-lock functions and some hydraulic operations.
• Each time the vehicle is driven to approximately 32 km/h (20 MPH), the ABS module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the ABS module.
• Pedal pulsation coupled with noise while braking on loose gravel, bumps, wet or snowy roads is normal and indicates correct functioning of the vehicle's ABS system.