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 performs additional functions specific to traction control and Electronic Stability Control (ESC).
ABS Operation
When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check. 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 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 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) returns 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.
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 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. EBD will be disabled if DTCs are present for 2 or more wheel speed sensors. When EBD is disabled, the yellow ABS warning indicator and the sliding-car icon will be illuminated.
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 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 100 km/h (62 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 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 which is integral to the Restraints Control Module (RCM) and 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.
Traction Control Switch
The traction control switch is hardwired to the Instrument Panel Cluster (IPC). The IPC communicates switch status to the ABS module via the HS-CAN bus. The traction control switch allows the driver to control use of the traction control and ESC systems. Traction control system status is indicated by the sliding-car icon in the IPC. The system will automatically restore to full functionality when the ignition switch is cycled from OFF to RUN.
Stability Control Sensors
The stability control sensors for the ESC system include the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over a dedicated Controller Area Network (CAN) bus. If any of the sensors are defective, a new RCM must be installed.
- The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.
- 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 front and rear wheels.
Lateral acceleration has 2 forms. The first is the centrifugal acceleration that occurs 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.
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 2 signal lines 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 does not indicate the absolute position of the steering wheel relative to straight-ahead. The stability assist system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The stability assist system confirms this position and modifies it as necessary during every new driving cycle.
ABS Module Calibration
When a new ABS module, HCU, RCM or steering wheel rotation sensor 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 sensors and components.
If a DTC is logged for any component of the ESC system, the DTC must be repaired and then cleared before carrying out the calibration procedure. The need to calibrate the ABS module is also indicated by the sliding-car icon in the IPC flashing once every 2 seconds. The indicator flashes after clearing the DTCs that are associated with the ESC systems.
To calibrate the ABS module, carry out the IVD Initialization sequence using the scan tool. When carrying out the calibration procedure, the following criteria must be met.
- The vehicle must be on a level surface.
- The vehicle must not be moving.
- The transmission must be in NEUTRAL.
- The brake pedal must be applied.
If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart. Diagnostic Trouble Code Tests and Associated Procedures
ABS Module Configuration
When a new ABS module is installed on vehicles equipped with a 4.6L engine, it must be configured. Refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning
Vehicles equipped with a 4.0L or a 5.4L engine do not require PMI.