Antilock Brake System

Principles of Operation
The Electronic Stability Program (ESP) module executes control of the Anti-Lock Brakes (ABS), full speed Traction Control (T/C), and stability assist functions to enhance driver control of the vehicle. The stability assist module manages the interactions between the anti-lock, traction control, and engine control systems to optimize the vehicle traction during deceleration and acceleration. In addition, the stability assist function modulates brake pressure during braking and non-braking 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 rotational speed is measured by the anti-lock brake sensor which electrically senses each tooth of the anti- lock sensor indicator as they pass through the sensors magnetic field. When stability assist detects an impending wheel lock, wheel spin or a vehicle motion that is inconsistent with the driver commands, it modulates brake pressure to the appropriate brake caliper(s). 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 carry out a preliminary electrical check, and at approximately 20 km/h (12 mph) the pump motor is turned on for approximately one half-second. Any malfunction of the stability assist system will cause the stability assist system to shut off, the ABS warning indicator to illuminate and the power assist braking system to function normally.

The stability assist module communicates with the Powertrain Control Module (PCM) requesting assistance with traction control. At speeds under 40 km/h (25 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 ignition timing changes and fewer fuel injector pulses until the driven wheel speed returns to normal and the stability assist module ends the request. After the vehicle speed exceeds 40 km/h (25 mph), the traction control is accomplished only by the PCM controlling torque.

The 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. This is done by using added sensors to compare the steering inputs from the driver with the actual motion of the vehicle.

The driver steering input is measured by a steering wheel rotation sensor. The motion of the vehicle is determined from a yaw rate sensor which measures rotation about the vehicle's vertical axis, a accelerometer which measures the acceleration generated from the vehicle sliding sideways, and the wheel speeds used for ABS and T/C function. When there is a discrepancy between the driver inputs and the vehicle motion, the stability assist system 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 stability assist system will attempt to correct the 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 function when the vehicle is traveling in reverse. In reverse, ABS and T/C function normally. The stability assist system uses ABS and T/C as well as its more advanced controls to enhance vehicle stability. The Traction Control Switch (TCS) allows the driver to control ON/OFF operation of the traction control system (independent of the ABS function). The ABS system cannot be switched off by the driver. The stability assist system status is indicated by a T/C/stability assist warning light. The illuminated T/C stability assist indicates the stability assist system is switched off.

During an stability assist event you may experience any of the following normal behaviors:
^ a rumble or grinding sound much like ABS or traction control
^ a small deceleration of the vehicle
^ the ABS indicator will flash
^ 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 ABS, 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 hiss sound (as when releasing a vacuum) may also be heard from under the instrument panel in an event this severe. All of these behaviors are normal during an stability assist event.

The stability assist system continually monitors all sensors and actuators used to improve the stability assist 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 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 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 ABS indicator will be illuminated. The ABS function will continue to work as designed unless the yellow ABS warning indicator is also illuminated. The normal brake function should always occur, unless the red brake warning indicator is illuminated.