ABS Circuit Description

Battery voltage to the Electronic Rear Wheel Brake Control (RWAL) Module is supplied through the TAIL LPS Fuse by the ORN (40) wire. When the Ignition Switch is in RUN, battery voltage is also supplied by the PNK/WHT (350) wire through the BRAKE Fuse.

Each time the ignition is switched to RUN, the module performs a self-check and then lights the BRAKE Indicator for three seconds by grounding through the BLK (150) wire. Lighting the BRAKE Indicator serves as a bulb check and also indicates a successful completion of the self-check. When the Electronic Rear Wheel Brake Control Module detects a problem within the system, the Brake Indicator Lamp will remain ON.

Inputs to the Electronic Rear Wheel Brake Control Module consists of the Speed Signal from the WHT (696) wire and the Brake Switch from the PPL (420) wire. The speed signal pulse rage (128,000 RPM) is high so that any speed variations can be detected quickly. Battery voltage is supplied from the GAGS Fuse and PNK/BLK (39) wire with Light Duty MD8 Transmission or ECM I Fuse and PNK/BLK (439) wire with Heavy Duty MT1 Transmission to the Brake Switch and to the Brake Switch Input through the PPL (420) wire. It is removed by the Brake Switch which opens when the brake pedal is depressed.

When the Brake Switch Input is at zero, indicating the brake pedal is depressed, the Electronic Rear Wheel brake control Module monitors the Speed Input form the WHT (696) wire to check for any sudden change in speed. A sudden deceleration, for example, would indicate that one or both rear wheels are beginning to lock up. Should this occur, the module will operate the Antilock Brake Valve, to regulate the rear hydraulic brake line pressure. the Antilock Brake Valve consists of a accumulator and an Isolation Valve which maintains the rear brake pressure at its current level. The electronic Rear Wheel Brake Control Module can operate the Antilock brake Valve to either maintain the existing hydraulic pressure, release brake fluid through the Dump Valve to reduce pressure or measure the pressure by pulsing the isolation valve.

The Reset Switch in the Antilock brake Valve closes as soon as pressure is removed from the brake pedal. This signals the Electronic Rear Wheel Brake Control Module to stop the ABS sequence. The Brake Switch will provide the same indication, but it does not operate until the brake pedal is fully released.