Principles of Operation

Anti-Lock Control

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 one-half 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 control 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)

On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module then uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a predetermined threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.

A slight bump sensation may be felt in the brake pedal when Electronic Brake Distribution (EBD) is active.

If ABS 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. When EBD is disabled, the ABS warning indicator and (if equipped) the Electronic Stability Control (ESC) warning indicator will be illuminated.


ABS with Traction Control, Electronic Stability Control (ESC) and Roll Stability Control (RSC(R))

The ABS module with traction control and Roll Stability Control (RSC(R)) performs the same functions as the standard ABS module. It also performs additional functions specific to traction control and RSC(R).

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 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 Steering Angle Sensor Module (SASM) (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 stability/traction control indicator "sliding-car icon" 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(R))

The RSC(R) 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(R) system monitors information from the wheel speed sensors, the Throttle Position (TP) sensor, the SASM (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(R) 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(R) 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(R) indicator flashes
- If the driver's foot is on the brake pedal, a vibration will be felt in the pedal much like ABS

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 hardwired to the Instrument Cluster (IC). The IC communicates switch status to the ABS module via the HS-CAN bus. The stability/traction control switch allows the driver to control use of the traction control system. This is independent of the ABS, ESC and RSC(R) functions, which cannot be switched off by the driver. Traction control system status is indicated by the stability/traction control indicator "sliding-car icon" in the IC. The system will automatically restore to full functionality when the ignition switch is cycled from OFF to RUN.


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 Angle Sensor Module (SASM)

The SASM measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The SASM 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 (PMI) in Information Bus. If installing a new ABS module with RSC(R) the module must also be calibrated. Programming and Relearning


ABS Module Calibration

The ABS module should be configured before it is calibrated. The ABS module must be calibrated if any of the following is occurring or has occurred:

- A new ABS module has been installed
- A new HCU has been installed
- A new stability control sensor cluster is installed
- A new SASM has been installed
- The stability/traction control indicator "sliding-car icon" flashes once every 2 seconds
- DTC C1991 is present in the ABS module

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 stability/traction control 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

To calibrate the ABS module, carry out the IVD Initialization sequence. Follow the instructions provided by the scan tool.