Principles of Operation

Principles of Operation

ABS
The ABS module manages anti-lock braking, traction assist and roll stability control (RSC) systems to maintain vehicle control during deceleration, acceleration and vehicle maneuvers.

When the ignition switch is in the RUN position, the ABS module carries out a self-test by sending a reference voltage to each wheel speed sensor to determine if the sensors and related wiring are functional. At the same time, the hydraulic pump motor is turned on for approximately 1/2 second and the valves in the hydraulic control unit (HCU) are actuated to determine operational status of the HCU. Any malfunction of the ABS components disables the system and the ABS warning indicator illuminates. However, the standard power-assist braking system functions normally.

The active wheel speed sensor generates a signal that is sent to the ABS module. The wheel speed sensor circuitry connects to the ABS module through 2 wires and a connector at each wheel speed sensor. When the ignition is turned to the RUN position, the ABS module carries out a self-test to all of the wheel speed sensors through their circuitry to determine if they are functional.

The ABS module monitors and compares the rotational speed of each wheel through the use of active wheel speed sensors and wheel speed sensor rings. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the wheel speed sensor ring as it passes through the magnetic field of the sensor. This information is sent back to the ABS module which is used to determine if a wheel lock is pending. When the ABS module detects an impending wheel lock, the module commands the HCU to activate the pump motor and to open and close the appropriate solenoid valves in order to modulate brake fluid pressure to the appropriate brake caliper(s). Once the affected wheel(s) return to the desired speed the ABS module commands the HCU to deactivate the pump motor and return the solenoid valves to their normal positions.

ABS Module Configuration
When a new ABS module is installed, it must be configured. Refer to Programmable Module Installation in Information Bus.

Electronic Brake Distribution (EBD)
The HCU is used as an electronic proportioning valve for electronic brake distribution (EBD). The HCU controls the brake fluid pressure to the rear brakes to help prevent them from applying before the front brakes. If the ABS is disabled due to DTCs being present in the module, the EBD will continue to function unless there is a complete failure of the HCU. When EBD is disabled, the ABS warning indicator also illuminates.

Stability Assist System
The stability assist system is comprised of the traction assist system and the roll stability control (RSC) system. The ABS module is the electronic control unit for the ABS, the traction assist system and the RSC system. The traction assist system helps the vehicle maintain traction during a wheel slip event and the RSC system helps maintain control of the vehicle during sudden maneuvers.

Traction Assist
The ABS module communicates with the PCM to assist with vehicle traction. When the drive wheels lose traction and begin to spin, with vehicle speed under 100 km/h (62 mph), the ABS module detects the difference in wheel speed and requests the PCM to reduce engine torque, while simultaneously commanding the HCU to activate the pump motor and to open and close the appropriate solenoid valves in order to modulate brake fluid pressure to the appropriate brake caliper(s) to slow the wheel(s) and maintain traction. The PCM reduces engine torque with 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. With the vehicle speeds over 100 km/h (62 mph), traction assist is accomplished only through the PCM.

Roll Stability Control (RSC)
The ABS module 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 ABS module activates the RSC system to apply 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 movement acting on the vehicle. Vehicles 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.

The ABS module constantly monitors the vehicle motion relative to the intended course. This is done by using information from the wheel speed sensors, the throttle position sensor, the steering wheel position sensor, the steering wheel rate of change and a yaw rate sensor that measures the change in vehicle direction. The ABS module compares the sensor information with that of the actual vehicle motion. If the module 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.

The RSC system does not activate when the vehicle is traveling in reverse; however, the ABS and traction assist continue to function as usual.

When the RSC system activates, any of the following can occur:
^ A rumble or grinding sound much like ABS
^ A small deceleration or a reduction in the acceleration of the vehicle
^ The stability assist indicator flashes
^ 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 some noise and slight movement of the brake pedal when the system carries out a self-test. 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.

Stability/Traction Control Switch
The stability/traction control switch allows the driver to control the 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.

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.

The sensor cluster receives power and ground from the ABS module and communicates with the ABS module along a secondary, dedicated communication area network (CAN2) bus. All 4 circuits are hardwired to the ABS module.

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 the high speed communication area network (HS-CAN) bus 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 is integrated into the electronic power assisted steering (EPAS) module and is not serviced separately.