Stability Assist

With the ignition switch in the START or RUN position, the anti-lock brake system module with stability assist functions similarly to a conventional anti-lock brake system 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 wheel speed sensor ring as they pass through the sensor's magnetic field. When the ABS detects an impending wheel lock, wheel spin, or vehicle motion that is inconsistent with the driver commands, brake pressure is modulated to the appropriate brake caliper(s). The ABS 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 ABS returns the solenoid valves to their normal position, and normal base brake operation is restored.

The ABS module communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 100 km/h (62 mph), when the drive wheels lose traction and begin to spin, the ABS 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 ABS 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.

The stability assist system constantly monitors the 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, the ABS continues to function as usual.

The stability assist 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 status is indicated by a stability assist warning lamp in the stability assist switch. An illuminated stability assist switch indicates that the stability assist system is switched off. In vehicles with a message center, the message T/C Off, T/C ON is displayed. The anti-lock brake function continues to work as designed unless the yellow ABS warning indicator is also illuminated. Normal braking 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 flashes
• 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 whooshing 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 is generated at the master cylinder, but no pressure is generated in the brake calipers.