Principles of Operation

Principles of Operation

Standard ABS
The standard ABS module manages anti-lock braking to maintain vehicle control during deceleration.

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. During this time a buzzing or humming noise may be heard and a vibration maybe felt in the brake pedal. This is a normal condition. 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 monitors and compares the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the sensor ring as it passes through the magnetic field of the sensor. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and commands the hydraulic control unit (HCU) to open and close the appropriate solenoid valves to modulate the brake pressure to the individual brake caliper(s). Once the affected wheel(s) return to the desired speed, the ABS module commands the HCU to return the solenoid valves to their normal position and normal base brake operation is restored.

In extreme cold conditions (temperature less than -25° C [-13° F]), the ABS module will briefly operate the hydraulic pump motor when the accelerator is pressed. This is done to warm the brake fluid.

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 prevent them from locking up during vehicle braking. If anti-lock braking is disabled due to DTCs being present in the ABS module, EBD will continue to function unless the DTCs are for wheel speed sensors or the HCU. Wheel speed sensor DTCs and HCU DTCs will disable the EBD function of the HCU. When EBD is disabled, the ABS warning indicator and (if equipped) the electronic stability control (ESC) warning indicator will illuminate.

ABS with Traction Control, Electronic Stability Control (ESC) and Roll Stability Control (RSC)
The ABS module with traction control and roll stability control (RSC) performs the same functions as the standard ABS module. It also performs additional functions specific to traction control and RSC.

Traction Control
When the drive wheels lose traction and begin to spin, with vehicle speed under 100 km/h (62 mph), the ABS module commands the hydraulic pump motor on and commands the HCU to open and close the appropriate solenoid valves to modulate the brake pressure to the brake caliper(s) while simultaneously sending a request to the PCM over the high speed controller area network (HS-CAN) bus to reduce engine torque to maintain vehicle 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.

Electronic Stability Control (ESC)
The electronic stability control (ESC) system constantly monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering input and the yaw rate sensor with that of the actual vehicle motion. The ESC system monitors information from the throttle position (TP) sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) and a yaw rate sensor (integral to the stability control sensor cluster) that measures changes in vehicle direction. If the ESC system determines from all these inputs that the vehicle is unable to travel in the intended direction, the system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle motion and reduces engine torque to allow the vehicle to follow the intended course.

When the ESC system 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 ESC indicator flashes
^ If the driver's foot is on the brake pedal, a vibration will be felt in the pedal much like ABS

Roll Stability Control (RSC)
The roll stability control (RSC) system constantly monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the 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 standard ABS and traction control continue to function as usual. The RSC system monitors information from the wheel speed sensors, the TP sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) and a roll-rate sensor (integral to the stability control sensor cluster) that measures changes in vehicle motion. If the RSC system determines from all these inputs that conditions exist for a potential roll-over event, the system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle roll motion and reduces engine torque to make the vehicle more stable. By adjusting brake torque, the system can reduce the cornering forces and, therefore, the total roll moment acting on the vehicle.

When the RSC system 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 RSC 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 enough 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.

Stability/Traction Control Switch
The stability/traction control switch is hard wired to the instrument cluster module. The instrument cluster module communicates the stability/traction control switch status to the ABS module via the HS-CAN bus.

The stability/traction control switch allows the driver to control use of the stability control/traction assist system. This is independent of the ABS function, which cannot be switched OFF by the driver. The stability control/traction assist system status is indicated by a light in the stability/traction control switch. When the stability/traction control switch is set to OFF, the system is disabled below 60 km/h (37 mph). The system will automatically restore to full functionality every time the vehicle speed exceeds 70 km/h (43 mph) or when the ignition switch is turned to the OFF position and back to the ON position. However, the stability/traction control switch OFF indicator remains illuminated until the switch is pressed. The ABS 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.

Stability Control Sensor Cluster
The stability control sensor cluster consists of the yaw rate sensor, roll-rate sensor, 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 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 the 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.

ABS Module Configuration
A new ABS module must be configured after installation. Refer to Programmable Module Installation Information Bus (Module Configuration).

If installing a new ABS module with RSC, the module must also be calibrated.

ABS Module Calibration

NOTE: The installation of a new stability control sensor cluster does not require ABS module calibration.

The ABS module must be calibrated if any of the following is occurring or have occurred:
^ A new ABS module has been installed.
^ A new HCU has been installed.
^ A new steering wheel rotation sensor has been installed.
^ The ABS warning indicator in the instrument cluster flashes once every 2 seconds.

Follow the scan tool directions for ABS module calibration. The ABS module should be configured before it is calibrated. Calibration is required for the ABS module to learn the "at rest" or "zero" position of the stability control components. If a DTC is set for any stability control component, the DTC must be cleared before carrying out the calibration procedure. The ESC warning indicator will flash after clearing the DTCs that are associated with the stability control system. If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart. ABS Module DTC Chart

Stability Control Sensor Cluster Calibration
When a new stability control sensor cluster is installed, the cluster must be calibrated. Follow the scan tool instructions for the calibration procedure. When carrying out the cluster calibration, the vehicle must be on a level surface, must not be moving and the ignition switch must be in the RUN position.