Four Wheel Antilock Brake System

Four Wheel Antilock Brake System






SYSTEM OPERATION

The brake pressure modulator valve (BPMV) receives input signals from the vehicle speed sensor (VSS) buffer for the rear wheels, wheel speed sensors (WSS) located at each front wheel, brake switch, and transfer case switch on 4W0 models.

BASE BRAKING MODE

Applying the brake pedal causes the brake switch to open the circuit to the BPMV. When the BPMV loses the signal from the brake switch, it begins to monitor the signals from the VSS buffer and WSSs for a departure from vehicle speed. It remains in this stand-by mode until it sees one speed signal indicating a lock-up condition or the brake switch indicates braking has stopped. The BPMV does not affect the front or rear brake hydraulic channels at this time.

ANTILOCK BRAKING MODE

When the BPMV sees a change in the speed signals that indicate one or more wheels is locking up, it takes control of the hydraulic channel to the affected wheel. The system has the ability to quickly control hydraulic pressure to the left front and right front wheels independently and the rear wheels together. It cycles the solenoids for the affected wheel to maintain, decrease, or increase hydraulic pressure until the speed signal stabilizes with the other speed inputs.

Brake pedal pulsation and occasional tire "chirping" are normal during an antilock stop. The road surface and severity of the braking maneuver determine how much these will occur.

System Self-Test

The first time vehicle speed reaches approximately 8 mph after an ignition cycle, the BPMV performs a system self-test. It checks its internal and external circuit and performs a function test by cycling the pump motor, isolation solenoids, and PWM solenoids; During this self-test some noise may be heard from the area of the BPMV. This is normal during the selftest cycle. The BPMV then begins its normal operation if no malfunctions are detected.

TIRES AND 4WAL

Spare Tire

Using the spare tire supplied with the vehicle will not affect the performance of the 4WAL system.

Replacement Tires

Replacement tires should be the same size, load range, and construction on all four wheels. Failure to comply with this can affect the performance of the 4WAL system.

Speedometer Calibration

The vehicle speed sensor (VSS) buffer,. must be replaced when the rear axle ratio or tire size is changed.

WARNING/INDICATOR LAMP OPERATION

The 4WAL system uses an ANTILOCK indicator lamp in the instrument cluster to show. system operation and malfunctions. Should a malfunction occur in the ANTILOCK lamp circuit, the BRAKE warning lamp serves as a back up.

Normal Lamp Operation

A bulb check occurs each time the ignition switch is turned "ON." The ANTILOCK and BRAKE lamps should light, remain on for about two seconds, then turn off. The ANTILOCK lamp also indicates 4WAL system malfunctions~. When the BPMV detects a malfunction in the system, it turns.the ANTILOCK lamp on. The lamp may remain on or turn back off depending on the malfunction. To determine the specific cause of the malfunction.