Antilock Brake System (Abs) (Service Information): Operation: Notes

Battery voltage is supplied by two fused B+ circuits from the Power Distribution Center (PDC) to the Antilock Brake System (ABS) module. A fused ignition RUN/START circuit from the RUN/START Relay in the PDC provides the ignition ON signal to the ABS module. The ABS module performs a system initialization procedure at start up along with an ABS motor check. Initialization consists of a static and dynamic self-check of system electrical components.

The static and dynamic checks occur at ignition ON. During the dynamic check, the ABS module briefly cycles solenoids to verify operation. An audible noise may be heard during this self-check. This noise should be considered normal.

If an ABS component exhibits a fault during initialization, the ABS module illuminates the amber warning light and registers a fault code in the microprocessor memory.

The ABS module monitors wheel speed sensor inputs continuously while the vehicle is in motion. However, the ABS module will not activate any ABS components as long as sensor inputs indicate normal braking.

During normal braking, the master cylinder, power booster and wheel brake units all function as they would in a vehicle without ABS and the Hydraulic Control Unit (HCU) components are not activated.

The purpose of the antilock system is to prevent wheel lockup. Preventing lockup helps maintain vehicle braking action and steering control.

The ABS module activates the system whenever sensor signals indicate periods of wheel slip.

The antilock system prevents lockup during a wheel slip condition by modulating fluid apply pressure to the wheel brake units.

Brake fluid apply pressure is modulated according to wheel speed, degree of slip and rate of deceleration. Sensors at each wheel convert wheel speed into electrical signals. These signals are transmitted to the ABS module for processing and determination of wheel slip and deceleration rate.

The ABS system has four fluid pressure control channels. Each wheel's brakes are controlled separately. A speed sensor input signal indicating a wheel slip condition activates the ABS module antilock program.

There are two solenoid valves (Isolation and Dump valve) which are used in each antilock control channel. The valves are all located within the HCU valve body and work in pairs to either increase, hold, or decrease apply pressure as needed in the individual control channels.

During an ABS stop the ISO valve is energized, which acts to prevent any more pressure build-up to the calipers. Then the Dump valve reduces pressure until the wheel unlocks. This will continue until the wheels stop slipping completely.