Principles of Operation

The anti-lock-brake system (ABS) module receives wheel speed readings from each wheel speed sensor and processes this information to determine if an ABS event is necessary. The wheel speed sensor electrically senses each tooth of the wheel speed sensor indicators as it passes through the wheel speed sensor magnetic field.

The ABS module continuously monitors and compares the rotational speed of each wheel and, when it detects an impending wheel lock, modulates brake pressure to the appropriate brake caliper. This is accomplished by triggering the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position, and normal (base) braking resumes.

The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will do a preliminary electrical check, and at approximately 20 km/h (12 mph) the pump motor is turned on for approximately one half-second. Any malfunction of the ABS will cause the ABS or anti-lock brake system/traction control (ABS/TC) to shut off and the yellow anti-lock brake warning indicator to illuminate, however the power assist braking system functions normally.

The ABS module and traction control communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 40 km/h (25 mph) the ABS module requests the PCM to reduce engine torque, while simultaneously applying and releasing the appropriate brake to restore traction when one or both drive wheels lose traction and begin to spin. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the ABS module ends the request (when driven wheel speed returns to normal). After the vehicle speed exceeds 40 km/h (25 mph), the traction control is accomplished only through the PCM torque control.

If the traction control system is cycled excessively, the brake portion of the system will shut down to prevent the rear brakes from overheating. A limited traction control function using only engine torque reduction will still control the wheels from over-spinning. When the rear brakes have cooled down the system will again function normally. Anti-lock braking is not affected by this condition and will function normally during the cool down period.

The traction control system can be disabled by pressing the traction control switch. The traction control system will reset and return to normal traction assist when the ignition switch is cycled or when the traction control switch is pressed again.