Rear Wheel Anti-Lock Brake System

General Motors light duty trucks use a Rear Wheel Anti-Lock (RWAL) system to prevent wheel lock-up during heavy braking. This is done by regulating fluid pressure to rear wheel cylinders. Pressure regulating is adjusted by a control valve, located under the master cylinder. Control valve is controlled by an Electronic Control Unit (ECU), located next to master cylinder.

The ECU receives signals from a speed sensor, located in the transmission on 2WD models or in transfer case on 4WD models, and the stoplight switch. Speed sensor signal is sent through a digital ratio adapter which is part of the instrument cluster. If axle ratio or tire size is changed, recalibration of the digital ratio adapter must be performed. The system is disabled when vehicle is in 4WD.

On models equipped with automatic transmission, an in-line resistor, between module and stoplight switch, prevents battery voltage from energizing the transmission converter clutch solenoid. The RWAL system is connected to the existing brake warning light on the instrument panel. When ignition is turned on, warning light should come on for approximately 2 seconds to indicate system is operating.

The anti-lock brake system is covered under a separate article. See ANTI-LOCK BRAKE SYSTEM article in the BRAKES Section.