Anti-Lock Brake System: Operation

When ignition is turned on, BPMV performs a self-check of the 4WAL electrical system. The Yellow ANTI-LOCK and Red BRAKE warning lights will come on for 2 seconds. ANTI-LOCK light should turn off if no electrical 4WAL faults are detected by BPMV. The Red BRAKE light may come on or stay on if parking brake is applied or a mechanical brake problem is detected.

Once vehicle speed has exceeded 8 MPH and driver has fully released the brake pedal, BPMV performs a self-check on electro-hydraulic portion of the system. This includes pump, control valves and reset switches. If BPMV detects a problem during this self-check or initial self-test, an indicator light will come on and 4WAL system will be disabled. A related trouble code will be stored.

When the brake pedal is depressed, voltage to the BPMV drops from 12 volts to one volt. At this point, BPMV monitors wheel speed through an AC signal generated by speed sensors located at each wheel. If the deceleration rate of wheel speed reaches a preprogrammed rate, BPMV will activate various control valves to prevent wheel lock-up by increasing or decreasing hydraulic pressure to each channel: left front, right front or rear wheels.

On 4WD models, a front-axle mounted switch is used to inform BPMV when vehicle is in 2WD or 4WD mode. Switch is open during 2WD operation, thus sending a one volt or less signal. During 4WD operation, switch closes, sending a 12-volt signal to BPMV. This signal causes BPMV to modify the anti-lock braking program for 4WD operation.

Speed sensor signal is sent through a Vehicle Speed Sensor (VSS) buffer located behind glove box. If axle ratio or tire size is changed, VSS buffer must be replaced to match axle ratio or tire size as necessary.