Anti-Lock Brake System: Operation

When ignition is turned on, ABS and BRAKE warning lights will illuminate for 2 seconds, then turn off as bulb check. EBCM performs self-check of 4WAL system, once each ignition cycle, when vehicle speed reaches 8 MPH. If faults are detected, ABS indicator light will illuminate (in most cases) and 4WAL functions will be disabled. A related Diagnostic Trouble Code (DTC) will be stored. The BRAKE light illuminates if parking brake is applied or a mechanical brake problem is detected.

When brake pedal is depressed, brake switch voltage to PCM/VCM drops from 12 volts to one volt. At this point, PCM/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.