The Rear Wheel Antilock Brake System (RWAL) is designed to prevent rear wheel lockup under heavy braking conditions. The main system components are: the RWAL valve, controller (CAB), rear wheel speed sensor mounted on the axle, and exciter ring on the differential.
When the brakes are applied, hydraulic fluid is routed from the master cylinder's secondary circuit through the combination valve, to the RWAL valve. From there the fluid is routed to the rear brake wheel cylinders. The CAB monitors rear wheel speed through the sensor on the axle. If a wheel is about to lock up, the CAB signals the RWAL valve which then modulated hydraulic pressure to the rear wheels to prevent lock up.
During light brake application, rear wheel deceleration is not sufficient to activate the antilock system. During a normal stop the brake fluid flows without restriction to the rear wheel cylinders to stop the vehicle. The antilock solenoids are inactive. The isolation valve is open and the dump valve is closed allowing normal fluid flow to the rear wheel cylinders.
If the CAB senses impending rear wheel lockup, it will energize the isolation solenoid. This prevents a further increase of driver induced brake pressure to the rear wheels. If this initial action is not enough to prevent rear wheel lock-up, the CAB will momentarily energize a dump solenoid. This opens the dump valve to vent a small amount of isolated rear brake pressure to an accumulator. The action of fluid moving to the accumulator reduces the isolated brake pressure at the wheel cylinders. The dump (pressure venting) cycle is limited to very short time periods (milliseconds). The CAB will pulse the dump valve until the rear wheel deceleration reaches the desired slip rate programmed into the CAB. The system will switch to normal braking once wheel-locking tendencies are no longer present.
Note that the RWAL system controls both rear wheel brakes simultaneously.
Bleeding the System
This is accomplished using the same methods as for the base brake system. Refer to those procedures.
Diagnosis & Testing
Diagnosis of base brake conditions, which are mechanical in nature, should be performed first. This includes brake noise, lack of power assist, parking brake, or vehicle vibration during normal braking.
The RWAL brake system performs several self-tests every time the ignition switch is turned on and the vehicle is driven. The CAB monitors the system inputs and outputs circuits to verify the system is operating properly. If the CAB senses a malfunction in the system it will set a DTC into memory and trigger the warning lamp.Diagnostic Codes
The red brake warning lamp and amber ABS warning lamp are located in the instrument cluster. The red brake warning lamp is used to alert the driver of a hydraulic fault or that the parking brake is applied. The red brake-warning lamp also is used to alert the driver of a problem with the RWAL system.
The red brake-warning lamp illuminates when a message is sent over the bus to the cluster to illuminate the bulb. A ground is provided when:
The amber ABS warning lamp is used to alert the driver of an RWAL problem and identify DTCs stored in the CAB memory.
If a system fault is detected during the self-test, or at any other time, the control module will illuminate the Antilock indicator lamp and store the fault code in the microprocessor memory. If a fault code is generated, the module will remember the code after the ignition is switched OFF . The microprocessor memory will store and display only one fault code at a time. The stored code can be displayed by grounding the RWAL diagnostic connector and counting the number of flashes on the indicator lamp. To clear the fault code, disconnect the control module connector or disconnect the battery for at least 5 seconds. During system retest, wait 30 seconds to make sure the fault code does not reappear.
Removal & Installation
- Remove the differential case from the axle housing.
- Clamp the differential case in a soft-jaw vise.
- Remove and discard the ring gear bolts.
- Tap the ring gear off with a rawhide or plastic mallet.
- Remove the exciter ring with a soft-faced hammer.
- Be sure to align the exciter ring tab with the slot in the differential case.
- Invert the differential case and start two ring gear bolts. This will provide case to ring gear bolt hole alignment.
- Press the exciter ring onto the differential case using the ring gear as a pilot.
Install new ring gear bolts and tighten in a cross pattern as follows:
For 8 1 / 4 in. axles, 75 ft. lbs. (102 Nm)
Removal & Installation
- Remove the RWAL valve harness connector from the controller.
- Remove the brake lines from the valve.
- Remove the valve mounting bolt and remove the valve from the bracket.
- Installation is the reverse of removal. Tighten the mounting bolt to 15-20 ft. lbs. (20-27 Nm). Tighten the brake line fittings to 14 ft. lbs. (19 Nm).
Removal & Installation
- Raise the vehicle.
- Remove the brake line mounting nut, if fitted, and remove the brake line from the sensor stud.
- Remove the mounting stud from the sensor and shield.
- Remove the sensor and shield from the differential housing.
- Disconnect the wire and remove the sensor.
- Connect the harness to the sensor. Be sure the seal is securely in place between the sensor and wiring connector.
- Install O-ring on sensor if removed.
- Install sensor on differential; housing, sensor shield, mounting stud. Tighten to 18 ft. lbs. (24 Nm).
- Install the brake line on the sensor stud and install the nut.
- Lower the vehicle.