Rear Wheel Antilock Brake System
Rear Wheel Antilock Brake System
SYSTEM OPERATION
The purpose of the rear wheel antilock (RWAL) brake is to maintain vehicle stability under severe conditions on most road surfaces. The RWAL system monitors the speed of the rear wheels during a braking maneuver. The electric brake control module (EBCM) processes these values to produce command and controls to prevent the rear wheels from locking.
BASE BRAKING MODE
During normal braking, the EBCM receives a signal from the brake switch and begins to monitor the vehicle speed line. The isolation valve is open and the dump valve is seated. This allows fluid under pressure to pass through the APV and travel to the rear brake channel. The reset switch does not move because hydraulic pressure is equal on both sides.
ANTILOCK BRAKING MODE
During a brake application the EBCM compares vehicle speed to the program built into it. When it senses a rear wheel lock-up condition, it operates the antilock pressure valve to keep the rear wheels from locking up. To do this the EBCM uses a three step cycle:
^ Pressure Maintain
^ Pressure Decrease
^ Pressure Increase
Pressure Maintain
During pressure maintain the EBCM energizes the isolation solenoid to stop the flow of fluid from the master cylinder to the rear brakes. The reset switch moves when the difference between. the master cylinder line pressure and the rear brake channel pressure becomes great enough. If this happens, it grounds the EBCM logic circuit.
Pressure Decrease
During pressure decrease the EBCM keeps the Isolation solenoid energized and energizes the dump solenoid. The dump valve moves off its seat and fluid under pressure moves into the accumulator. This action reduces rear pipe pressure preventing rear lock-up. The reset switch grounds to tell the EBCM that pressure decrease has taken place.
Pressure Increase
During pressure Increase the EBCM deenergizes the dump and isolation solenoids. The dump valve reseats and hold the stored fluid in the accumulator. The isolation valve opens and allows the fluid from the master cylinder to flow past it and increase pressure to the rear brakes. The reset switch moves back to its original position by spring force. This action signals the EBCM that pressure decrease has ended and driver applied pressure resumes.
Accumulator Bleed-Down
When the brake pedal is released, the accumulator spring and piston push the stored fluid back to the master cylinder past a lip seal on the dump valve.
System Self-Test
When the ignition switch is turned "ON," the EBCM performs a system self-test. It checks its internal and external default and performs a function test by cycling the Isolation and dump valves. The EBCM then begins its normal operation if no malfunctions are detected. Brake pedal pulsation and occasional rear tire "chirp" are normal during RWAL operation. The road surface and severity of the braking maneuver determine how much these will occur. Since these systems only control the rear wheels, it is still possible to lock the front wheels during certain severe braking conditions.
TIRES AND RWAL
Spare Tire
Using the spare tire supplied with the vehicle will not affect the performance of the RWAL system.
Replacement Tire
Tire size can affect the performance of the RWAL system. Replacement tires must be the same size, load range1 and construction on all four wheels.
Speedometer Calibration
The vehicle speed sensor (VSS) buffer, must be replaced when the rear axle ratio or tire size is changed.
WARNING LAMP OPERATION
BRAKE Lamp
The RWAL system uses the existing BRAKE warning lamp in the instrument panel to indicate system operation and malfunctions. The BRAKE lamp is also used to show a hydraulic system pressure differential condition and parking brake operation.
Normal Operation
A bulb check occurs each time the ignition switch Is turned "ON." The BRAKE lamp should light, remain on for about two seconds, then turn off. When system malfunctions occur, the BRAKE lamp will turn on and stay on until the malfunction is corrected. To determine the specific cause of a malfunction.
The BRAKE lamp will also light when the parking brake is applied or the combination valve switch is activated. Do not confuse these functions with an RWAL system malfunction.