Traction Control and Stability Assist

With the ignition in the START or RUN positions, the stability assist module functions similarly to a conventional anti-lock brake module by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor which electrically senses each tooth of the anti-lock sensor ring 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 lose 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 ends when the driven wheel speed returns to the desired speed. 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/traction control 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/traction control switch. An illuminated stability assist/traction control switch indicates that the stability assist system is switched off. In vehicles with a message center, the message "ADVANCE TRAC 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 illuminate. The anti-lock brake function continues to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. The normal brake function always occurs, unless the red brake warning indicator is illuminated.

When stability assist activates, any of 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
• if the driver's foot is on the brake pedal, a vibration will be felt in the pedal much like ABS
• if the event is severe and the driver's foot is not on the brake, the brake pedal will move to apply 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 is not bled correctly, the brake pedal movement can become more significant.