Rear Wheel Anti-Lock Brake System

Rear Wheel Antil-lock (RWAL) System 1993 C/K System:

The Rear Wheel Anti-Lock (RWAL) and Zero Pressure Rear Wheel Anti-Lock (ZPRWAL) systems, Fig. 3, reduce the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. The systems monitor the speed of the rear wheels during braking. The Electronic Brake Control Module (EBCM) processes these values to produce command controls to prevent the rear wheels from locking.

This system uses four basic components to control hydraulic pressure to the rear brakes. These components are a Electronic Brake Control Module (EBCM), Anti-Lock Pressure Valve (APV), Vehicle Speed Sensor (VSS) and Vehicle Speed Sensor Buffer (VSS). The EBCM mounted on a bracket next to the master cylinder, contains a microprocessor and software for system operation. The APV mounted to the combination valve under the master cylinder, has an isolation valve to maintain or increase hydraulic pressure and a dump valve to reduce hydraulic pressure. The VSS located on the left rear of the transmission on two wheel drive trucks and on the transfer case of four wheel drive vehicles, produces an AC voltage signal that varies in frequency according to the output shaft speed. The VSS buffer located behind the glove box, changes the AC sine-wave signal from the VSS to a DC square-wave signal.
If the axle ratio or tire size is changed, it will be necessary to replace the VSS buffer.

During normal braking, the EBCM receives a signal from the stop lamp switch and begins to monitor the vehicle speed line. The isolation valve is open and the dump valve is seated. This allows fluid under pressure to pass through the Anti-Lock Pressure Valve (APV) and travel to the rear brake channel.

During Anti-Lock braking, the EBCM senses a rear wheel lock-up condition and operates the APV valve to keep the rear wheels from locking up. To keep the rear wheels from locking up the EBCM uses a three-step cycle, these three steps are as follows; pressure maintain, pressure decrease and pressure increase. During pressure maintain, the EBCM energizes the isolation solenoid to stop the flow of fluid from the master cylinder to the rear brakes. During pressure decrease, the EBCM keeps the isolation solenoid energized and energizes the dump solenoid. The dump solenoid then moves off its' seat and fluid under pressure moves into the accumulator, which reduces rear brake pressure. During pressure increase the EBCM de-energizes the dump and isolation solenoids. The dump valve reseats and holds the stored fluid in the accumulator. The isolation valve opens and allows fluid from the master cylinder to flow past it and increase pressure to the rear brakes.

When the brake pedal is released, the accumulator spring and piston push the stored fluid back to the master cylinder past a lip seal on the dump valve.

When the ignition switch is turned on, the EBCM performs a system self-test. The EBCM checks its' internal and external circuit and performs a function test by cycling the isolation and dump valves.

The EBCM is programmed to perform a self-diagnostic function. When the EBCM detects a system malfunction, it sets a diagnostic trouble code (DTC) which represents the malfunction, turns on the BRAKE lamp and disables all RWAL functions for as long as the brake lamp remains on.