Principles of Operation: Notes

The stability assist module executes control of the anti-lock brakes, full speed traction control and stability assist functions to enhance the drivers control of the vehicle. The stability assist manages the interactions between the anti -lock brakes, traction control, and engine control systems to optimize the vehicle traction during deceleration and acceleration. In addition, stability assist function modulates brake pressure during braking and nonbraking conditions as required to counteract excessive vehicle rotation while cornering.

The stability assist module is operational with the ignition switch in the RUN or START positions.

The stability assist module continuously monitors and compares the rotational speed of each wheel. The wheel speeds are measured by the anti -lock brake sensors which electrically sense each tooth of the antilock sensor indicator as they pass through the sensors magnetic field. When stability assist detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the drivers commands, it modulates brake pressure to the appropriate brakes. This is accomplished by triggering the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to the desired speed, the stability assist module returns the solenoid valves to their normal position, and normal base brake operation resumes.

The stability assist module is self-monitoring. When the ignition switch is turned to the RUN position, the stability assist module will do a preliminary electrical check and the pump motor is turned on for approximately one half-second. Any malfunction of the anti-lock brake control system will cause the ABS/TC/Stability Assist to shut off and anti-lock brake warning indicator to illuminate, however the power assist braking system will function normally.

The stability assist module communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 72 km/h (45 mph) the stability assist 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 stability assist module ends the request (when the driven wheel speed returns to normal). After the vehicle speed exceeds 72 km/h (45 mph), the traction control is accomplished only through the PCM torque control.

An all new stability assist system helps maintain vehicle stability at the limits of tire adhesion. System effectiveness varies with vehicle speed, road conditions, and steering inputs.

The stability assist system constantly monitors vehicle motion relative to the driver's intended course. Sensors compare various inputs with the actual motion of the vehicle.

The driver's steering input is measured from a steering wheel rotation sensor. The yaw rate sensor determines rotation about the vertical axis of the vehicles. The lateral accelerometer determines left or right vehicle slippage. The yaw rate sensor and lateral accelerometer are located in a unit under the driver-side seat. The ABS wheel speed sensors determine wheel speed. When there is a discrepancy between the driver's inputs and the vehicle motion, the stability assist changes the force at each tire to help control the vehicle. If the vehicle is beginning to rotate excessively left or right, spin out, or slide sideways, the system will attempt to correct the excessive motion. If the vehicle is not responding to steering inputs, the system will attempt to increase the turning response of the vehicle.

The stability assist system defaults to ON when the engine is started. The system does not activate when the vehicle is traveling in reverse. In reverse, ABS and traction control will continue to function as usual. The system uses ABS and traction control (including control of the engine) as well as its more advanced controls to enhance vehicle stability.

Two indicators located on the instrument cluster display the stability assist system status.

The "TRAC OFF" indicator illuminates when the stability assist is disabled.

Stability assist is disabled by the stability assist/traction control switch and when there is a stability assist system fault.

The "TRAC ACTIVE" indicator illuminates during a stability assist or when a traction control event occurs.

During a stability assist event you may experience any of the following behaviors, which are normal:

• 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 illuminate
• if your foot is on the brake pedal, you will feel a vibration in the pedal much like ABS. As with any vehicle equipped with four-wheel anti-lock brakes, do not pump the brakes, but instead, press firmly on the pedal.
• if the event is severe and your foot is not on the brake, the brake pedal will move to apply higher brake forces. A whoosh sound may also be heard from under the instrument panel in an event this severe. All of these behaviors are normal during a stability assist event.

The stability assist system continually monitors and checks all of the sensors and actuators used to improve the stability of the vehicle. Some drivers may notice a slight movement of the brake pedal when the system checks itself. If the brake system has not been bled correctly, the brake pedal movement may become more significant. 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 calipers. This test will occur after the vehicle has been running approximately five minutes. The test will only be run if the vehicle is stable, the driver is not braking and the accelerator pedal is depressed at least a small amount. 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.