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 three normally open solenoid valves contained inside the HCU and then through the outlet ports of the HCU to each wheel.
• If the ABS control 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 control module then checks 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 control module returns the valves to their normal condition, allowing fluid to flow to the affected brake.
• The ABS control module monitors the electromechanical components of the system.
• A malfunction in the ABS will cause the ABS control module to shut off or inhibit the system. However, normal power - assisted braking remains.
• Malfunctions are indicated by the yellow ABS warning indicator, the red BRAKE warning indicator, or both warning indicators which are located in the instrument cluster.
• The ABS is self-monitoring. When the ignition switch is turned to the RUN position, the ABS control module will carry out a preliminary self-check on the ABS 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 control module monitors all electrical anti-lock functions and some hydraulic operations.
• The first time the ignition switch is turned to the ON position and the vehicle is driven to approximately 10 km/h (6.25 mph), the ABS control 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 control module.
• Pedal pulsation coupled with noise while braking on loose gravel, bumps, wet or snowy roads is normal and indicates correct functioning of the ABS control system.