Electronic Brake Control Module: Description and Operation

CONTROLLER REAR WHEEL ANTILOCK BRAKES

Fig. 3 RWAL Cab:

The Controller Antilock Brakes (CAB) is a microprocessor which handles testing, monitoring and controlling the ABS brake system operation. The CAB functions are:
- Perform self-test diagnostics.
- Monitor the RWAL brake system for proper operation.
- Control the RWAL valve solenoids.

Fig. 4 CAB/HCU:

The CAB is mounted on the top of the hydraulic control unit. The CAB operates the ABS system and is separate from other vehicle electrical circuits. CAB voltage source is through the ignition switch in the RUN position.

NOTE: If the CAB needs to be replaced, the rear axle type and tire revolutions per mile must be programed into the new CAB. For axle type refer to Differential and Driveline.

System Self-test
When the ignition switch is turned-ON the microprocessor RAM and ROM are tested. If an error occurs during the test, a DTC will be set into the RAM memory. However it is possible the DTC will not be stored in memory if the error has occurred in the RAM module were the DTC's are stored. Also it is possible a DTC may not be stored if the error has occurred in the ROM which signals the RAM to store the DTC.

The CAB contains a self check program that illuminates the ABS warning light when a system fault is detected. Faults are stored in a diagnostic program memory and are accessible with the DRB scan tool.

ABS faults remain in memory until cleared, or until after the vehicle is started approximately 50 times. Stored faults are not erased if the battery is disconnected.

CAB Inputs
The CAB continuously monitors the speed of the differential ring gear by monitoring signals generated by the rear wheel speed sensor. The CAB determines a wheel locking tendency when it recognizes the ring gear is decelerating too rapidly. The CAB monitors the following inputs to determine when a wheel locking tendency may exists:
- Rear Wheel Speed Sensor
- Brake Lamp Switch
- Brake Warning Lamp Switch
- Reset Switch
- 4WD Switch (If equipped)

CAB Outputs
The CAB controls the following outputs for antilock braking and brake warning information:
- RWAL Valve
- ABS Warning Lamp
- Brake Warning Lamp