Rear Wheel Antilock (RWAL) Valve

RWAL VALVE

DESCRIPTION
The assembly is located on the drivers side inner fender under the hood. The assembly 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 assembly. 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.