Principles Of Operation

Anti-Lock Control

Principles of Operation

ABS with Traction Control and Electronic Stability Control (ESC)

The ABS module with traction control manages anti-lock braking to maintain vehicle control and carries out additional functions specific to traction control and Electronic Stability Control (ESC).

ABS Operation

When the ignition is in the RUN position the ABS module does a preliminary electrical check. At approximately 16 km/h (10 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 stability assist and illuminates the ABS warning indicator. 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 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) returns 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.

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

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 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 (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 light, stability/traction control indicator (sliding car icon) and the stability/traction control indicator (sliding car icon) off illuminate.

Traction Control

When the drive wheels lose traction and begin to spin, with vehicle speed under 108 km/h (67 mph), 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), and may simultaneously send 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 with 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 108 km/h (67 mph), traction control is accomplished using only the PCM torque control function. The traction control system can be deactivated by pressing the traction control switch in the instrument panel.

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 the yaw rate and lateral accelerometer sensors to compare the steering input with that of the actual vehicle motion. The ESC (Electronic Stability Control) system monitors information from the Throttle Position (TP) sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) a yaw rate sensor and lateral accelerometer sensor which is integral to the Restraints Control Module (RCM) and measures changes in vehicle direction. If the ESC (Electronic Stability Control) 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.

Traction Control Switch

The traction control switch is hardwired to the Instrument Panel Cluster (IPC). The IPC (Instrument Panel Cluster) communicates switch status to the ABS module via the HS-CAN (High Speed Controller Area Network). The traction control switch allows the driver to control the use of the traction control and ESC (Electronic Stability Control) systems. Traction control system status is indicated by the stability/traction control indicator (sliding car icon) off in the IPC (Instrument Panel Cluster). The ESC (Electronic Stability Control) system automatically restores to full functionality when the ignition is cycled from OFF to RUN.

Stability Control Sensors

The stability control sensors for the ESC (Electronic Stability Control) system include the yaw rate sensor and the lateral accelerometer. The yaw rate sensor and the lateral accelerometer 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 damaged.

Steering Sensor

The steering sensor is used by the Power Steering Control Module (PSCM) 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 Electronic Power Assist Steering (EPAS) column assembly and is not serviced separately.

ABS Module Calibration

When a new ABS module, HCU (Hydraulic Control Unit) , or RCM (Restraints Control Module) is installed, the ABS module must be calibrated. The calibration procedure is required for the ABS module to learn the zero-position of the various ESC (Electronic Stability Control) sensors and components.

If a DTC(s) sets for any component of the ESC (Electronic Stability Control) system, correct the fault condition and clear the DTC before carrying out the calibration procedure. The need to calibrate the ABS module is also indicated by the stability/traction control indicator (sliding car icon) in the IPC (Instrument Panel Cluster) flashing once every 2 seconds. The indicator flashes after clearing the DTCs associated with the ESC (Electronic Stability Control) systems.

To calibrate the ABS module, carry out the IVD Initialization sequence using the scan tool.

If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart Antilock Brakes / Traction Control Systems.