Bleeding the System
ABS system bleeding requires conventional bleeding methods plus use of the DRB scan tool. The procedure involves performing a base brake bleeding, followed by use of the scan tool to cycle and bleed the HCU pump and solenoids. A second base brake bleeding procedure is then required to remove any air remaining in the system.
- Perform base brake bleeding. Refer to the appropriate section.
- Connect the scan tool to the data link connector beneath the dashboard.
- Select Antilock Brake followed by Miscellaneous , then ABS Brake . Follow the instructions displayed until the unit displays "Test Complete", then disconnect the scan tool and proceed.
- Perform a base brake bleeding a second time.
- Top off master cylinder fluid level and verify proper brake operation before moving vehicle.
Diagnosis & Testing
Perform ABS verification test as follows:
- Turn the ignition off.
- Connect all previously disconnected components and connectors.
- Verify all accessories are turned off and the battery is fully charged.
- Verify that the ignition is on, with the scan tool, erase all Diagnostic Trouble Codes from All modules. Start the engine and allow it to run for 2 minutes and fully operate the system that was indicating the failure.
- Turn the ignition off and wait 5 seconds. Turn the ignition on and using the scan tool, read DTCs from all modules.
If any Diagnostic Trouble Codes are present, return to symptom list and trouble shoot new or recurring symptom.
NOTEFor Sensor Signal and Pump Motor faults, the ABM must sense all 4 wheels at 12 km/h (7.5 mph) before it will extinguish the ABS indicator.
- If there are no DTCs present after turning ignition on, road test the vehicle for at least 5 minutes. Perform several anti-lock braking stops.
- Again, with the scan tool read DTCs . If any DTCs are present, refer to the Table of Contents in the applicable Section for the diagnostic test procedure and troubleshoot the new or recurring symptom.
- If there are no Diagnostic Trouble Codes (DTCs) present, and the customer's concern can no longer be duplicated, the repair is complete.
When the brakes are applied, hydraulic fluid is routed from the master cylinder's secondary circuit to the RWAL valve. From there hydraulic fluid is routed to the rear brakes. The Antilock Brake Controller (ABM) contains an Electronic Variable Brake Proportioning (EVBP) control algorithm, which proportions the applied braking force to the rear wheels during braking. The EVBP function of the Rear Wheel Antilock (RWAL) system takes the place of a conventional hydraulic proportioning valve. The ABM monitors the rear wheel speed through the rear wheel speed sensor and calculates an estimated vehicle deceleration. When an established deceleration threshold is exceeded, an isolation valve is closed to hold the applied brake pressure to the rear brakes constant. Upon further increases in the estimated vehicle deceleration, the isolation valve is selectively opened to increase rear brake pressure in proportion to the front brake pressure. If impending rear wheel lock-up is sensed, the ABM signals the RWAL valve to modulate hydraulic brake pressure to the rear wheels to prevent lock-up.
Since the RWAL valve also performs the EVBP or proportioning function, vehicle deceleration under normal braking may be sufficient to trigger the EVBP function of the RWAL system without full RWAL activity as would normally occur during an impending rear wheel lock-up. As previously mentioned, the isolation valve is selectively closed and opened to increase rear brake pressure in proportion to the front brake pressure under EVBP control. Slight brake pedal pulsations may be noticed as the isolation valve is opened
Rear Wheel Antilock (RWAL) brake system is standard equipment on 1500 series vehicles. The RWAL brake system is designed to prevent rear wheel lock-up on virtually all types of road surfaces. RWAL braking is desirable because a vehicle which is stopped without locking the rear wheels will retain directional stability. This allows the driver to retain greater control of the vehicle during braking.
The valve is located on the drivers side inner fender under the hood. The valve modulates hydraulic pressure to the rear brakes.
The RWAL components include:
Removal & Installation
- Before servicing the vehicle, refer to the Precautions Section.
- Disconnect the battery cables from the battery.
- Remove the battery.
- Disconnect the electrical harness connector.
- Remove the brake lines from the RWAL valve.
Remove RWAL valve mounting nuts and remove the RWAL valve.
- Install RWAL valve and tighten the nuts to 11 ft. lbs. (15 Nm).
- Install the brake lines to the RWAL valve and tighten to 170 in. lbs. (19 Nm).
- Install the electrical harness connector to the rwal valve and secure the connector.
- Install the battery.
- Install the battery cables to the battery.
- Bleed ABS brake system.
Rear Wheel Sensor
- 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 harness to 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.
NOTEUse the original or replacement sensor bolt only. The bolt is special and must not be substituted.
- Lower the vehicle.
Refer to Control Valve.