Anti-Lock Brake System - With VCM: Description: Operation

When ignition is turned on, Yellow ANTI-LOCK and Red BRAKE warning lights will come on for 2 seconds as a bulb check. VCM performs a self-check of the 4WAL system once each ignition cycle when vehicle speed reaches 8 MPH. If faults are detected by VCM, ANTI-LOCK light will turn on (in most cases) and 4WAL functions will be disabled while ANTI-LOCK light is on. A related trouble code will be stored. The Red BRAKE light comes on if parking brake is applied or a mechanical brake problem is detected.

When the brake pedal is depressed, voltage to the VCM drops from 12 volts to one volt. At this point, VCM 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, VCM 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 VCM whether 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 VCM. This signal causes VCM to modify the anti-lock braking program for 4WD operation. Speed sensor, mounted in end of transmission, supplies VCM with rear wheel speed signal.