Rear Wheel Antilock Brake System With VCM
Rear Wheel Antilock Brake System With VCM
SYSTEM OPERATION
The purpose of the VCM/RWAL brake system is to maintain vehicle stability under severe braking conditions on most road surfaces. The VCM monitor the speed of the rear wheels during a braking maneuver. It processes these values and processes command controls to prevent the rear wheels from locking,
BASE BRAKING MODE
During normal braking1 applying the brake pedal causes the voltage from the brake switch to the VCM to drop to 0 volts. The VCM then begins to monitor the vehicle speed line. The isolation valve 15 open and the dump valve is seated. Fluid under pressure passes through the APV and travels to the rear brake channel. The reset switch does not move because hydraulic pressure Is equal on both sides of it.
ANTILOCK BRAKING MODE
During a brake application, the VCM compares vehicle sped to the program built into It. When it senses a rear wheel lock-up condition, it coperates the antilock pressure valve to keep the rear wheels from locking up. To do this the VCM uses a three step cycle:
^ Pressure Maintain
^ Pressure Decrease
^ Pressure Increase
Pressure Maintain
During pressure maintain the VCM 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 VCM logic circuit.
Pressure Decrease
During pressure decrease the VCM 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 VCM that pressure decrease has taken place.
Pressure Increase
During pressure Increase the VCM deenergizes the dump and Isolation solenoids. The dump valve reseats and holds the stored fluid in the accumulator. The Isolation valve opens and allows 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 VCM 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 VCM performs a system self-test. It checks its internal and external circuit and performs a function test by cycling the isolation and dump valves. The VCM then begins its normal operation if no malfunctions are detected.
Brake pedal pulsation and occasional rear tire "chirping" are normal during VCM/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.
TIRE AND VCM/RWAL
Spare Tire
Using the spare tire supplied with the vehicle will not affect the performance of the VCM/RWAL system.
Replacement Tire
Tire size can affect the performance of the VCM/RWAL system. Replacement tires must be the same size, load range, and construction on all four wheels.
WARNING LAMP OPERATION
BRAKE Lamp
The VCM/RWAL system uses the existing BRAKE warning lamp in the instrument cluster to Indicate system operation and malfunctions. The BRAKE lamp is also used to show a hydraulic system pressure differential condition and parking bake 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, refer to "Functional Test"
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 a VCM/RWAL system malfunction.