Controller rear wheel antilock brakes. Rwal valve

Controller rear wheel antilock brakes

DESCRIPTION

The Controller Antilock Brakes (CAB) is a microprocessor which handles testing, monitoring and controlling the ABS brake system operation (Fig. 1). The CAB functions are:

• Perform self-test diagnostics.
• Monitor the RWAL brake system for proper operation.
• Control the RWAL valve solenoids.

NOTE: If the CAB needs to be replaced, the rear axle type and tire revolutions per mile must be programed into the new CAB. For axle type refer to Group 3 Differential and Driveline. For tire revolutions per mile refer to Group 22 Tire and Wheels. To program the CAB refer to the Chassis Diagnostic Manual.

Fig. 1 RWAL CAB

1 - RWALCAB

OPERATION

SYSTEM SELF-TEST

When the ignition switch is turned-on the microprocessor RAM and ROM are tested. If an error occurs during the test, a DTC will be set into the RAM memory. However it is possible the DTC will not be stored in memory if the error has occurred in the RAM module were the DTC's are stored. Also it is possible a DTC may not be stored if the error has occurred in the ROM which signals the RAM to store the DTC.

CAB INPUTS

The CAB continuously monitors the speed of the differential ring gear by monitoring signals generated by the rear wheel speed sensor. The CAB determines a wheel locking tendency when it recognizes the ring gear is decelerating too rapidly. The CAB monitors the following inputs to determine when a wheel locking tendency may exists:

• Rear Wheel Speed Sensor
• Brake Lamp Switch
• Brake Warning Lamp Switch
• Reset Switch
• 4WD Switch (If equipped)

CAB OUTPUTS

The CAB controls the following outputs for antilock braking and brake warning information:

• RWAL Valve
• ABS Warning Lamp
• Brake Warning Lamp

Rwal valve

DESCRIPTION

The valve is located on the drivers side inner fender under the hood. The valve modulates hydraulic pressure to the rear brakes during an RWAL stop.

OPERATION

If the CAB senses that rear wheel speed deceleration is excessive, it will energize an isolation solenoid by providing battery voltage to the 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 (the CAB energizes the dump solenoid by providing battery voltage to the 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 rear wheel deceleration matches the desired slip rate programmed into the CAB. The system will switch to normal braking once wheel locking tendencies are no longer present.

A predetermined maximum number of consecutive dump cycles can be performed during any RWAL stop. If excessive dump cycles occur, a DTC will be set and stored in the CAB memory. If during a RWAL stop, the driver releases the brake pedal, the reset switch contacts will open. This signal to the CAB is an indication that pressure has equalized across the RWAL valve. The CAB will then reset the dump cycle counter in anticipation of the next RWAL stop. Additionally, any fluid stored in the accumulator will force its way past the dump valve, back into the hydraulic circuit and return to the master cylinder.

A fuse internal to the CAB, provides a fail-safe device which prevents unwanted control over the isolation and dump solenoids. The fuse is in series with the isolation and dump solenoids output circuits. If the internal fuse is open, the CAB cannot provide voltage to energize either solenoid and RWAL stops are prevented. If the fuse is open, the braking system will operate normally but without antilock control over rear brake pressure.