Principles of Operation
Anti-Lock Control
Principles of Operations
Anti-Lock Control
The ABS module receives wheel speed readings from each wheel speed sensor and processes this information to determine if an ABS event is necessary. The wheel speed sensor electrically senses each tooth of the wheel speed sensor indicators as it passes through the wheel speed sensor magnetic field.
The active wheel speed sensor generates a signal that is sent to the ABS module. The wheel speed sensor circuitry connects to the ABS module through 2 wires and a connector at each wheel speed sensor. When the ignition is turned to the RUN position, the ABS module carries out a self-test by sending a reference voltage to all of the wheel speed sensors through their circuitry to determine if they are functional.
The ABS module continuously monitors and compares the rotational speed of each wheel and when it detects an impending wheel lock, modulates brake pressure to the appropriate brake caliper. This is accomplished by triggering the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position, and normal (base) braking resumes.
The HCU has additional internal solenoid valves incorporated to enable control modulation of the wheel brake pressures.
The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will do a preliminary electrical check of the wheel speed sensors by sending voltage through the sensor and checking for the voltage to return. At speeds above 20 km/h (12 mph), without the brake pedal being pressed, the pump motor will be commanded ON for approximately one-half second to check pump motor operation. Also, during all phases of operation the ABS module, with the vehicle in motion, checks for correct operation of the wheel speed sensors. Any malfunction of the ABS will cause the ABS to deactivate. Normal power-assisted braking, however, remains.
Electronic Brake Distribution (EBD)
The ABS incorporates a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the Hydraulic Control Unit (HCU) as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain 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.
Some drivers may feel a slight bump sensation in the brake pedal when EBD is active.
If anti-lock braking is disabled due to DTCs being present in the ABS module, EBD will continue to function unless the ABS module detects 2 wheel speed sensor faults on a single cycle or an internal HCU fault exists. When EBD is disabled, the ABS warning indicator, the red brake warning indicator and (if equipped) the stability assist warning indicator will illuminate.
Traction Control System - Front Wheel Drive (FWD)
In order to maintain maximum traction, the traction control system on Front Wheel Drive (FWD) vehicles is designed to limit wheel spin by modulating engine torque when driving on slippery or loose surfaces. The ABS module sends wheel speed information to the PCM over the High Speed Controller Area Network (HS-CAN) bus, the PCM uses this information to determine if traction control is necessary. When the drive wheels lose traction and begin to spin at different speeds, the PCM reduces engine torque by making minor incremental timing changes and fewer fuel injector pulses until the speed of the driven wheels return to the desired speed. The PCM also sends a request to the Instrument Cluster (IC) over the HS-CAN to flash the stability/traction control indicator ("sliding car" icon).
The traction control system can be disabled by pressing the stability/traction control switch and is indicated by the stability/traction control indicator in the IC (or message center if equipped). The traction control system will reset and return to normal operation when the ignition switch is cycled or when the stability/traction control switch is pressed and released a second time during the same ignition cycle.
Traction Control System - All-Wheel Drive (AWD)
In order to maintain maximum traction, the traction control system All-Wheel Drive (AWD) vehicles is designed to limit wheel spin by modulating engine torque and momentarily applying the brakes to the wheel that is slipping when driving on slippery or loose surfaces. The ABS module communicates with the PCM over the High Speed Controller Area Network (HS-CAN) bus to assist with traction control. With vehicle speed under 100 km/h (62 mph), when the drive wheels lose traction and begin to spin at different speeds, the ABS module sends a request to the PCM over the HS-CAN bus to reduce engine torque while simultaneously activating the Hydraulic Control Unit (HCU) to apply and release the appropriate brake caliper(s) to maintain vehicle traction. The PCM reduces engine torque by making minor incremental timing changes and fewer fuel injector pulses until the ABS module ends the traction control request. The request ends when the speed of the driven wheels return to the desired speed. The ABS module also sends a request to the Instrument Cluster (IC) over the HS-CAN to flash the stability/traction control indicator ("sliding car" icon). Once the vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through the PCM torque control.
The traction control system can be disabled by pressing the stability/traction control switch and is indicated by the stability/traction control indicator in the IC (or message center if equipped). The traction control system will reset and return to normal operation when the ignition switch is cycled or when the stability/traction control switch is pressed and released a second time during the same ignition cycle.
Electronic Stability Control (ESC) System
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 stability/traction control indicator ("sliding car" icon) flashes
- If the driver foot is on the brake pedal, a vibration will be felt in the pedal much like ABS
Stability/Traction Control Switch
The stability/traction control switch allows the driver to control use of the traction control system. This is independent of the standard ABS and Electronic Stability Control (ESC) functions, which cannot be switched off by the driver. The traction control system status is indicated by a stability/traction control indicator in the Instrument Cluster (IC) (or message center if equipped). When the traction control system is switched off, this indicator will illuminate. The standard ABS and ESC functions continue to operate 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 sensor cluster (also called the accelerometer) consists of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle slides sideways. This acceleration has 2 forms. The first is the centrifugal acceleration that 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 vehicle's front and rear wheels.
The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
The stability control sensor cluster sends the information to the ABS module over a dedicated Controller Area Network (CAN) bus.
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 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 rotation sensor does not indicate the position of the steering wheel relative to straight-ahead. The Electronic Stability Control (ESC) system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The ESC system confirms this position and modifies it as necessary during every new driving cycle.
ABS Module Configuration
Vehicles Equipped With Electronic Stability Control (ESC)
If it is necessary to replace the ABS module on a vehicle equipped with Electronic Stability Control (ESC), it will be necessary to configure the replacement module with information specific to the vehicle. To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus.
Vehicles Equipped With Standard ABS or ABS with Traction Control
Vehicles equipped with standard ABS or ABS with traction control (without ESC) do not require ABS module configuration.
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 steering wheel rotation sensor is 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 ESC components. If a DTC is present for any ESC specific component(s), the DTC must be repaired and cleared before carrying out the calibration procedure. The sliding-car icon will flash after clearing the DTCs that are associated with the ESC system.
To carry out the calibration procedure, connect the scan tool, proceed to the IVD Initialization sequence and follow the scan tool instructions. If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart. [1][2]Diagnostic Trouble Code Descriptions