Principles of Operation

Special Tool(S):

Principles of Operation

Anti-Lock Brake System (ABS)
The anti-lock brake system (ABS) module manages anti-lock braking, traction control and engine control systems to maintain vehicle control during deceleration and acceleration.

When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check and, at approximately 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately 1/2 second. Any malfunction of the ABS disables the stability assist and the ABS warning indicator illuminates. However, the power-assist braking system functions normally.

The ABS module must be configured when a new ABS module is installed.

Electronic Brake Distribution (EBD)
The electronic brake distribution (EBD) controls the rear brake pressure and acts as an electronic proportioning valve. It is controlled by the ABS module. When EBD is disabled, the ABS warning indicator illuminates.

Stability Assist
With the ignition switch in the START or RUN positions, the ABS module functions similarly to a conventional anti-lock brake system 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 it passes through the sensor's magnetic field. When the ABS module 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 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 ABS module 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. When the drive wheels lose traction and begin to spin, with vehicle speed under 100 km/h (62 mph), the ABS 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 ABS module 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 and stability assist continue to function as usual.

The stability 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 status is indicated by a stability assist warning lamp in the stability traction control switch. An illuminated stability traction control switch indicates that the stability assist system is switched off. The anti-lock brake function continues to work as designed unless the yellow anti-lock brake 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 is 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 moves 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.

Roll Stability Control (RSC)
The roll stability control (RSC) system monitors the roll-rate sensor and existing stability assist sensors and calculates if the vehicle may be approaching a situation where rollover is probable. If such a situation exists, the RSC system applies preemptive action. When activated, the RSC system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle roll motion. By adjusting brake torque, the system can reduce the cornering forces and, therefore, the total roll moment acting on the vehicle. Systems with RSC have an additional roll-rate sensor located within the sensor cluster and additional programming within the ABS module to help control the vehicle during sudden maneuvers.

Information from the roll-rate sensor is fed into the ABS module. The computer uses information from the ABS wheel speed sensors, the throttle position sensor, the steering wheel angle, the steering wheel rate of change and a yaw rate sensor that measures the change in vehicle direction. If the computer determines from all these inputs that conditions exist for a potential roll over, the system applies one or more brakes and reduces engine torque to make the vehicle more stable.

Sensor Cluster
The sensor cluster (also called the accelerometer) consists of the yaw rate sensor, roll rate sensor (if equipped with roll stability control), lateral accelerometer and longitudinal accelerometer. The lateral accelerometer measures the acceleration which corresponds to the force involved when the vehicle slides sideways. This acceleration has 2 forms. The first is the centrifugal acceleration which is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity even though the vehicle is not moving.

The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle's front and rear wheels.

The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.

The roll rate sensor measures the rate of rotation along the front to rear horizontal axis.

Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The steering wheel rotation sensor uses 2 signal lines to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The steering wheel rotation sensor does not indicate the position of the steering wheel relative to straight-ahead. The stability assist system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The stability assist system confirms this position and modifies it as necessary during every new driving cycle.

Active Brake Booster
The active brake booster functions like a conventional brake booster with the added feature that it can be actuated electrically by the ABS module. The electrical brake booster actuation is necessary in severe stability assist events and ensures that the HCU can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather when the HCU cannot draw the brake fluid from the reservoir quickly at cold temperatures.

A solenoid within the active brake booster has a release switch to indicate when the driver is stepping on the brake pedal. The solenoid provides electrical actuation of the brake booster. Without the force of the input rod, the air valve is directly opened due to the movement of the energized solenoid. With increasing current applied to the solenoid, the air valve opens and output force is created. With decreasing current applied to the solenoid, the air valve is closed and the vacuum valve opens, reducing output force. The release switch indicates when the brake pedal has been applied. It is integrated into the booster key that normally sits against the rear shell of the boot in its rest position and it adjoins the valve body when in its balance position. The switch itself provides indication from 2 positions. In its rest position it supplies 2 signals, normally open and normally closed. Upon application of sufficient force to initiate movement of the key away from the valve body, the brake booster switch changes state, signifying a driver application.

When the ABS module is activating the brake booster, the brake pedal is pulled forward as the valve body moves. The release switch is held in position by the input rod spring and remains against the valve body. Although the stoplamp switch changes state when the brake pedal drops, the release switch remains inactive until it encounters an external force from the driver or a stability assist event controlled by the ABS module.

The brake pedal force switch sends a brake application input to the ABS module. This signal is called driver brake application. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not electrically actuated and no failure has been detected, the driver brake application signal indicates a brake application when either the stoplamp switch or the release switch (built into the brake booster) indicates that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the driver brake application signal indicates a brake apply only when the stoplamp switch indicates that the driver is applying the brake pedal.

When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy attempt to keep the driver brake application signal accurate. Control of the stoplamp operation is also required because the stoplamp switch can change states during a stability assist event when the driver is not applying the brake pedal. The switch can also change states during the system check of the brake booster. Only the stability assist system can interpret these changed states and determine which are actually due to driver brake application. Similarly, the stability assist system can take advantage of its other sensors to detect stoplamp switch failures.