Principles of Operation

Anti-Lock Control

Principles of Operation


Anti-Lock Braking

The standard ABS module manages anti-lock braking to maintain vehicle control during deceleration.

The ABS module monitors and compares the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the sensor ring as it passes through the magnetic field of the sensor. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and commands the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves to modulate the brake pressure to the individual brake caliper(s). Once the affected wheel(s) return to the desired speed, the ABS module commands the HCU to return the solenoid valves to their normal position and normal base brake operation is restored.

The ABS module is self-monitoring. When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check and, at approximately 12 km/h (8 mph), the hydraulic pump motor is turned on for approximately one-half second. During this time a buzzing or humming noise may be heard and a vibration maybe felt in the brake pedal. This is a normal condition. If a malfunction is detected in the system, the module disables the ABS system and illuminates the yellow ABS warning indicator. However, the base power-assist braking system functions normally.


Traction Control

Traction control utilizes ABS components to detect and control excessive rear wheel spin on low-traction surfaces.

When the drive wheels lose traction on a low-traction surface, with vehicle speed under 100 km/h (62 mph), the ABS module commands the hydraulic pump motor on and commands the HCU to open and close the appropriate solenoid valves to modulate the brake pressure to the rear brake caliper(s) while simultaneously sending a request to the PCM over the High Speed Controller Area Network (HS-CAN) bus to reduce engine torque to maintain vehicle traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the ABS module ends the request. The request ends when the driven wheel speed returns to the desired speed. After the vehicle speed exceeds 100 km/h (62 mph), the traction control is accomplished only through the PCM torque control. During a traction control event, the traction control indicator in the instrument cluster will flash.

The intention of traction control is to detect and control excessive rear wheel spin on low-traction surfaces. On high-traction surfaces, a controlled amount of rear wheel spin will be allowed in the interest of quick acceleration. In order to avoid unnecessary traction control activation, the traction control strategy uses several inputs to determine when traction control activation is beneficial to the driver. The PCM relays throttle position information to the ABS module over the HS-CAN bus. The throttle position is utilized to determine if the driver intends to accelerate quickly. If this is the case, traction control will allow a controlled amount of rear wheel spin. If rear wheel spin is permitted, input from the front wheel speed sensors will be used to calculate vehicle acceleration. If the vehicle is accelerating while wheel spin is occurring, the ABS module will check for a difference between the left front and right front wheel speeds to determine if the vehicle is accelerating in a straight line. If the vehicle is not accelerating (rear wheels spinning on a slippery surface) or if the vehicle is turning, traction control will activate to help make better use of the available traction.


Traction Control Switch

The traction control switch allows the driver to control use of the traction control system. This is independent of the standard ABS function, which cannot be switched off by the driver. The traction control system status is indicated by a light in the traction control switch and an indicator in the instrument cluster. When the traction control system is switched off, the indicator in the switch will illuminate. When the traction control system is turned off, the standard ABS functions continue to operate as designed unless the yellow ABS warning indicator is also illuminated. Normal braking function always occurs, unless the red brake warning indicator is illuminated.