Anti-Lock Brake System: Operation

When ignition is turned on, Yellow ANTI-LOCK and Red BRAKE warning lights will illuminate 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 illuminate (in most cases) and 4WAL functions will be disabled while ANTI-LOCK light is on. A related Diagnostic Trouble Code (DTC) will be stored. The Red BRAKE light illuminates if parking brake is applied or a mechanical brake problem is detected.

When brake pedal is pressed, brake switch voltage to 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, PCM/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.