Operation

When the 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 inside the HCU, then through the HCU outlet ports to each wheel. The primary (rear) circuit of the master cylinder feeds the right front and left rear brakes. The secondary (front) circuit of the master cylinder feeds the left front and right rear brakes. If the ABS module senses that a wheel is about to lock, based on wheel speed sensor data, it closes the solenoid valve for that circuit. This prevents any more fluid from entering the circuit. Once the affected wheel comes back up to speed, the ABS module returns the valves to their normal operating condition allowing fluid to flow to the affected brake.
The ABS module monitors the electro-mechanical components of the system. Malfunction of the anti-lock brake system will cause the ABS module to shutoff or inhibit system function, while retaining normal power assisted braking. Malfunctions are indicated by one or two warning lamps inside the vehicle.
Loss of hydraulic fluid in the HCU reservoir will disable the ABS system.
The four wheel ABS 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 ABS electrical system indicated by a three or four second illumination of the amber ``Check Anti-Lock Brakes'' lamp 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 system operation.
Each time the vehicle is driven, as soon as vehicle speed reaches about 19 mph, the module turns on the pump motor for about 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.
For most malfunctions of the ABS 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 ABS module memory, pinpointing the exact component requiring service.