See Figures 1 and 2
The RWAL system was introduced to the S/T series trucks as standard equipment in 1989. It may be found on all 1989 and later trucks covered by this manual (except the MFI-Turbo equipped with the 4 wheel ABS system). 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 rear wheel lock-up during hard braking. On most 4wd vehicles, the system is deactivated when operating in four wheel drive. In this case the braking system acts as a normal hydraulic system. 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 Electronic Brake Control Module (EBCM), originally known as the RWAL Electronic Control Unit (ECU).
The control valve holds pressure when the control module energizes the isolation solenoid. This isolates the rear hydraulic circuit and prevents fluid from entering or leaving, therefore holding constant at a given pressure. Pressure is decreased when the module keeps the isolation solenoid energized and then energizes a dump solenoid which allows fluid from the rear hydraulic circuit to enter an accumulator, thereby reducing pressure and preventing wheel lockup. Pressure may be increased (though never over the driver's input) when both the isolation and dump solenoids are de-energized allowing the rear hydraulic circuit to function normally from full master cylinder pressure.
The RWAL ECU/EBCM is a separate and dedicated microcomputer mounted next to the master cylinder; it is not to be confused with the engine management computers also found in these vehicles. The ECU/EBCM receives signals from the speed sensor. The speed sensor sends its signals to the Vehicle Speed Sensor (VSS) buffer (previously known as the Digital Ratio Adapter Controller or DRAC) usually found within the instrument cluster. The buffer translates the sensor signal into a form usable by the computer module. The brake control module reads this signal and commands the control valve to function.
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 remaining illuminated after this initial test period.
If a fault is detected within the system, the control module will assign a diagnostic fault code and store the code in memory. The code may be read to aid in diagnosis, much in the same way codes are used in the engine emission control systems used by these vehicles.
No component of the RWAL system can be disassembled or repaired. Should the control module, control valve containing the isolation/dump valves or the speed sensor fail, each failed component must be replaced as an assembly. If the axle ratio or tire size is changed on the vehicle, the VSS buffer must be replaced with one of the appropriate calibration.
All electrical connections must be kept clean and tight. Make certain that all connectors are properly seated and all of the sealing rings on weather-proof connectors are in place. The low current and/or voltage found in some circuits require that every connection be the best possible. Special tools are required for servicing the GM Weather-Pack and Metric-Pack connectors. Use terminal remover tool J-28742 or equivalent for Weather-Pack and J-35689-A or equivalent for Metric-Pack connectors.
If removal of a terminal is attempted with a regular pick, there is a good chance the terminal will be bent or deformed. Once damaged, these connectors cannot be straightened.
Use care when probing the connections or replacing terminals; it is possible to short between adjacent terminals, causing component damage. Always use jumper wires between circuit connectors for testing circuits; never probe through weather-proof seals on connectors.
Oxidation or terminal misalignment may be hidden by the connector shell. When diagnosing open or intermittent circuits, wiggling the wire harness at the connector or component may reveal or correct the condition. When the location of the fault is identified, the connector should be separated and the problem connected. Never disconnect a harness connector with the ignition ON .
When working with the RWAL ECU/EBCM connectors, do not touch the connections or pins with the fingers. Do not allow the connectors or pins to contact brake fluid; internal damage to the RWAL ECU/EBCM may occur.