Principles Of Operation

Anti-Lock Control

Principles of Operation

Base ABS

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

When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check and, at approximately 12 km/h (7 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 may be felt in the brake pedal. This is a normal condition. Any malfunction of the ABS disables the ABS function and the ABS warning indicator illuminates. However, the power-assist braking system functions normally.

The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished using 4 active wheel speed sensors, one for each wheel. The wheel speed sensors are connected to the ABS module by 2 circuits. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module. As the wheels spin, the wheel bearing magnetic strip passes through the magnetic field generated by the active wheel speed sensor. This creates a square-wave signal that is sent to the ABS module to indicate individual wheel speed. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and 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 (Hydraulic Control Unit) to return the solenoid valves to their normal position, restoring normal base brake operation.

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 (Hydraulic Control Unit) 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. 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 (Electronic Brake Distribution) continues to function unless the DTCs are for wheel speed sensors or the HCU (Hydraulic Control Unit). When EBD (Electronic Brake Distribution) is disabled, the ABS warning indicator, the red brake warning indicator, stability/traction control indicator (sliding car icon) and the stability/traction control indicator (sliding car Off icon) illuminate.

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) (Roll Stability Control).

Traction Control

When the drive wheels lose traction and begin to spin, the ABS module commands the hydraulic pump motor on and the HCU (Hydraulic Control Unit) 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) 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.

The traction control system allows the driver to control use of the traction control system by enabling or disabling it. This is independent of the ABS, ESC (Electronic Stability Control) and RSC(R) (Roll Stability Control) functions, which cannot be disabled by the driver. To disable or enable the traction control system, change the TRACTION CNTRL setting in the message center to ON or OFF. When the driver uses the message center and turns the traction control system off, the Instrument Panel Cluster (IPC) communicates traction control system status to the ABS module via the HS-CAN (High Speed Controller Area Network). The ABS module turns off the traction control system and commands the IPC (Instrument Panel Cluster) to illuminate the stability/traction control indicator (sliding car icon) OFF, which indicates to the driver that the traction control system is disabled. The system automatically restores to full functionality when the ignition switch is cycled from OFF to RUN.

Electronic Stability Control (ESC)

The ESC (Electronic Stability Control) 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 (Electronic Stability Control) system monitors information from the SCCM (Steering Column Control Module) to determine steering wheel angle and rate of change and a yaw rate sensor (integral to the Restraints Control Module (RCM)) that measures changes in vehicle direction. If the ESC (Electronic Stability Control) system determines from all of 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 (Electronic Stability Control) 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 can be felt in the pedal much like ABS

Roll Stability Control (RSC(R))

The RSC(R) (Roll Stability Control) 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) (Roll Stability Control) system monitors information from the wheel speed sensors, the Throttle Position (TP) sensor, the SCCM (Steering Column Control Module) (steering wheel angle and rate of change) and a roll-rate sensor that measures changes in vehicle motion. If the RSC(R) (Roll Stability Control) system determines from all of 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) (Roll Stability Control) 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 can 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.

Hill Start Assist

Hill Start Assist is used to help the driver when accelerating from a stop on an incline or side of a hill. Hill Start Assist supports the driver during drive-off situations on upward gradients when the driver releases the vehicle brake by preventing the vehicle from rolling back in the opposite direction. The system engages automatically when the following conditions are met:

- Road inclination of approximately 4 degrees or more is detected.

- The driver applies the vehicle brake and the vehicle is at a complete stop.

- The vehicle transmission information indicates that the driver selects the correct gear, indicating the intention to drive up the hill.

- The parking brake is not engaged.

When the driver releases the brake pedal, Hill Start Assist acts to hold the car stationary for up to 2 seconds giving the driver time to accelerate the vehicle. As the vehicle accelerates Hill Start Assist gradually reduces the brake pressure to make sure that the vehicle is neither rolling back nor driving off until there is sufficient driving torque to accelerate the vehicle forward. Hill Start Assist functions if the vehicle is facing downhill if the reverse gear is selected.

Stability Control Sensors

The stability control sensors for the ESC (Electronic Stability Control) system include the yaw rate sensor, lateral and longitudinal accelerometers. The yaw rate sensor, the lateral and longitudinal accelerometers are able to detect and measure changes in vehicle direction that indicate the need for the ABS to make corrections that help prevent vehicle roll over or spin outs. The sensors are housed in the RCM (Restraints Control Module) which sends sensor information to the ABS module over the Controller Area Network (CAN). Install a new RCM (Restraints Control Module) if any of the sensors are defective.

Steering Sensor

The steering sensor is used by the SCCM (Steering Column Control Module) to determine speed and direction of the steering wheel. This information is transmitted to the ABS module along the HS-CAN (High Speed Controller Area Network). The sensor is attached to the steering column assembly and is not serviced separately.

ABS Module Configuration

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