Principles of Operation

NOTE: The diagnostic tool refers to the stability assist module as an "ABS module". The stability assist module is an ABS module at its core.

The stability assist module simultaneously manages the anti-lock braking, traction control, and engine control systems to maintain vehicle control during deceleration and acceleration.

When the ignition switch is put in the RUN position, the stability assist module will do a preliminary electrical check, and at approximately 20 km/h (12 mph) the hydraulic pump motor is turned on for approximately one half-second. Any malfunction of the anti-lock brake control system will disable the ABS/TC/Stability Assist and the anti-lock brake warning indicator will illuminate, however the power-assist braking system will function normally.

With the ignition in the START or RUN positions, the stability assist module functions similar to a conventional anti-lock brake module by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the anti-lock brake sensor which electrically senses each tooth of the anti-lock sensor indicator as they pass through the sensor's magnetic field. When the stability assist module detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the driver's commands, brake pressure is modulated to the appropriate brake caliper(s). The stability assist module triggers the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel(s) return to the desired speed, the stability assist module returns the solenoid valves to their normal position, and normal base brake operation is restored.

The stability assist module communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 40 km/h (25 mph), when drive wheels loose traction and begin to spin, the stability assist module requests the PCM to reduce engine torque, while simultaneously applying and releasing the appropriate brake caliper(s) to maintain traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the stability assist module ends the request, the request is ended when the 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.

The stability assist system constantly monitors vehicle motion relative to the driver's intended course. This is done by using sensors to compare the driver's steering input and brake application with that of the actual vehicle motion. The system does not activate when the vehicle is traveling in reverse, however, ABS and traction control will continue to function as usual.

The stability assist/Advance Trac® switch allows the driver to control use of the stability assist system. This is independent of the ABS function, which cannot be switched off by the driver. The stability assist system's status is indicated by a stability assist warning light in the stability assist/Advance Trac® switch. An illuminated stability assist/Advance Trac® switch indicates that the stability assist system is switched off. In vehicles with a message center, the message "ADVANCETRAC OFF" is displayed. If a failure is detected somewhere in the stability assist system which affects either the yaw control or traction control functions, the stability assist indicator will be illuminated. The anti-lock brake function will continue to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. The normal brake function will always occur, unless the red brake warning indicator is illuminated.

When stability assist activates, the following can occur:

a rumble or grinding sound much like ABS or traction control.
a small deceleration or a reduction in the acceleration of the vehicle.
the stability assist indicator will flash.
a vibration in the brake pedal much like ABS when the pedal is applied.
if the event is severe and the brake pedal is not applied, the pedal will move to generate higher brake forces. A whoosh sound can also be heard from under the instrument panel in an event this severe.

Some drivers may notice a slight movement of the brake pedal when the system checks itself. The brake pedal moves when an active test of the brake booster is run. During this test a small amount of pressure will be generated at the master cylinder, but no pressure will be generated in the brake calipers. This test will occur above 48 km/h (30 mph) after the vehicle has been running approximately 8 minutes. The test will only be run if the vehicle is stable, the driver is not braking and the accelerator pedal is slightly depressed. If the brake system has not been bled correctly, the brake pedal movement can become more significant.