Operation

When brakes are applied, fluid is forced from the 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, then through outlet ports of the HCU to each wheel. The primary (rear) circuit of the master cylinder feeds the right front and left rear brakes. The secondary (front) circuit feeds the left front and right rear brakes.

When the Anti-Lock Brake System (ABS) module senses wheel lock conditions based on wheel speed sensor data, it pulses against the normally open solenoid valve, closing the circuit. This prevents any more fluid from entering the circuit. The module senses the wheel again. If the wheel is still decelerating, the module pulses the normally closed valve open, decreasing the pressure trapped in line.

The ABS module monitors the electro-mechanical components of the
system. An anti-lock brake system malfunction will cause the module to shut off or inhibit the anti-lock function, while retaining the normal power assisted braking. Malfunctions are indicated by one or two warning lamps inside the vehicle. Loss of hydraulic fluid in the HCU reservoir also will disable the anti-lock brake system.

The four wheel anti-lock brake system is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will perform a preliminary self-check on the anti-lock electrical system, indicated by a three or four second illumination of the amber "Check Anti-Lock Brakes" lamp on the instrument cluster. During vehicle operation, including normal and anti-lock braking, the module monitors all electrical anti-lock functions and some hydraulic system operation.

For most malfunctions of the anti-lock brake system, the amber "Check Anti-Lock Brakes" and/or the red "Brake" lamp will be illuminated. The sequence of illumination of these warning lamps, combined with the problem symptoms, can determine the appropriate diagnostic test to perform. Most malfunctions are recorded as a coded number in the controller memory, pinpointing the exact component requiring service.