See Figures 1 and 2
The Kelsey Hayes RWAL system is found on Chevrolet and GMC products including Suburban and C/K Series pick-ups. The system is particularly useful because of the wide variations of loading the vehicle may experience. Preventing rear wheel lock-up often makes the difference in controlling the vehicle during hard or sudden stops.
Found on both 2WD and 4WD vehicles, the RWAL system is designed to regulate rear hydraulic brake line pressure, preventing wheel lock-up at the rear. Pressure regulation is managed by the control valve, located under the master cylinder. The control valve is capable of holding, increasing or decreasing brake line pressure based on electrical commands from the RWAL Electronic Control Unit (ECU) or Electronic Brake Control Module (EBCM).
The RWAL ECU is a separate and dedicated microcomputer mounted next to the master cylinder; it is not to be confused with the engine management ECU. The RWAL ECU receives signals from the speed sensor. The speed sensor sends its signals to the Vehicle Speed Sensor buffer (previously known as the Digital Ratio Adapter Controller or DRAC) within the instrument cluster. The buffer translates the sensor signal into a form usable by the ECU. The RWAL ECU reads this signal and commands the control valve to function. If commanded to release pressure, the dump valve releases pressurized fluid into the accumulator where it is held under pressure. If a pressure increase is called for, the isolator valve within the control valve pulses, releasing pressurized fluid into the system.
The RWAL system is connected to the BRAKE warning lamp on the instrument cluster. A RWAL self-check and a bulb test are performed every time the ignition switch is turned to ON . The BRAKE warning lamp should illuminate for about 2 seconds and then go off. Problems within the RWAL system will be indicated by the BRAKE warning lamp staying illuminated.
If a fault is detected within the system, the RWAL ECU will assign a fault code and store the code in memory. The code may be read to aid in diagnosis.