Principles Of Operations

Anti-Lock Control

Principles of Operations

Anti-Lock Braking

The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished through the use of 4 active wheel speed sensors, 1 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. The active wheel speed sensor sends a square wave signal to the ABS module to indicate individual wheel speed.

The ABS module uses a Hydraulic Control Unit (HCU) to isolate and modulate hydraulic pressure to the caliper(s) and wheel cylinder(s). The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid flows through 4 normally open inlet valves inside the HCU, and through the outlet ports of the HCU to the brake calipers and wheel cylinders. If the ABS module senses that a wheel is nearing a lockup condition based on wheel speed sensor data and deceleration algorithms, the module closes the normally open inlet valve for that wheel. This isolates the corresponding caliper or wheel cylinder. The ABS module reads the wheel speed sensor signal from the affected wheel again. If the wheel is still decelerating (locking up), the module opens the normally closed outlet valve allowing a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. The ABS pump motor runs during an ABS braking event so sufficient brake fluid pressure is consistently available to the HCU valves. This procedure of closing and opening valves is repeated several times per second until the speed of the affected wheel matches the speeds of the other wheels. Once this has been accomplished, the ABS module returns the solenoid valves to their normal position.

The ABS module is self-monitoring. When the ignition is turned to the RUN position, the ABS module carries out a preliminary electrical check of the wheel speed sensor circuitry 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 is commanded on for approximately one-half second to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors. In the event the ABS module detects a concern, the module sets a DTC and sends a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) to illuminate the yellow ABS warning indicator and/or the stability/traction control indicator (sliding-car icon). If the concern is severe enough, ABS may be disabled. Normal power-assisted braking, however, remains.

Electronic Brake Distribution (EBD)

The ABS module incorporates a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the 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 solenoid 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.

If there is a DTC in the ABS module specifically for the HCU or there are 2 or more wheel speed sensor DTCs set, EBD is disabled. When EBD is disabled, the ABS warning indicator, the red brake warning indicator and the stability/traction control indicator (sliding-car icon) illuminate.

AdvanceTrac(R) Stability Enhancement System

The AdvanceTrac(R) system provides the following stability enhancement features for certain driving situations:

- Traction Control System - helps to avoid excessive drive-wheel spin and loss of traction.
- Electronic Stability Control (ESC) System - helps to avoid skids and lateral slides.

Traction Control System

The traction control system uses the ABS and the PCM to limit wheel spin. When the ABS module detects a wheel spinning excessively, brake pressure to the appropriate brake caliper is modulated. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN that a traction control event is taking place. When the IPC receives this message, it flashes the stability/traction control indicator (sliding-car icon). If the event is severe enough, the ABS module sends a message to the PCM over the HS-CAN to assist with traction control. When the PCM receives this message, it adjusts engine timing and decreases fuel injector pulses. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN indicating the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the sliding-car icon.

Once vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through PCM intervention.

Certain DTCs in the ABS module disable the traction control system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon is illuminated.

The driver is able to enable and disable the traction control portion of the Advance Trac(R). This is independent of the ABS and ESC functions, which cannot be disabled by the driver. To disable or enable the traction control system, the TRACTION CNTRL setting in the message center is changed from ON to OFF. When the driver disables the traction control system, the IPC communicates traction control system status to the ABS module over the HS-CAN bus. The ABS module will no longer intervene when a traction event is detected. The ABS module sends a message to the IPC over the to HS-CAN bus illuminate the stability/traction control OFF indicator (sliding-car OFF icon), which indicates to the driver that the traction control system is disabled. The system automatically restores to full functionality when the ignition is cycled to OFF and then back to RUN.

Electronic Stability Control (ESC) System

The ESC system is controlled by the ABS module and uses the same wheel speed sensors and wheel bearing magnetic strips used for anti-lock braking. The ESC system also uses input from the stability control sensors for the ESC system to help maintain vehicle stability. They include the yaw rate sensor, lateral accelerometer and longitudinal accelerometer and are located in the Restraints Control Module (RCM).

The ABS module uses the various sensors to continuously monitor vehicle direction of travel relative to the intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction, it modulates brake pressure to the appropriate brake caliper(s) or wheel cylinder(s) by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN that a stability event is taking place. When the IPC receives this message, it flashes the sliding-car icon. Under certain conditions during a stability event, the PCM may be requested to reduce engine torque as it does for traction control. Once the stability condition has been corrected, the ABS module sends another message over the HS-CAN indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the sliding-car icon.

Certain DTCs being present in the ABS module may disable the ESC system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon is illuminated.

Hill Start Assist (Automatic transmission only)

Hill start assist helps 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 must indicate that the driver intends to drive up the hill.
- The parking brake is not engaged.

When the driver releases the brake pedal in drive gear, hill start assist holds the car stationary for up to 3 seconds giving the driver time to accelerate the vehicle. As the vehicle accelerates hill start assist gradually reduces the brake pressure to ensure 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.

There is 1 message displayed by the multifunction display that informs the driver of the status of the hill start assist. HILL START ASSIST NOT AVAILABLE is displayed and the hill start assist telltale in the IPC is illuminated when it is not available.

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). If any of the sensors are defective, install a new RCM.

- 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 Sensor

The Power Steering Control Module (PSCM) uses the steering sensor to determine speed and rotation direction of the steering wheel. This information is transmitted to the ABS module along the HS-CAN. The sensor is attached to the Electronic Power Assist Steering (EPAS) column assembly and cannot be serviced separately.

ABS Module Configuration

ABS module configuration is a scan tool process which configures specific vehicle settings in a new ABS module. If installing a new ABS module, it must be configured. Refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).

ABS Module Calibration

The calibration procedure is also required for the ABS module to learn the "zero-position" of the steering and stability control sensors.

To calibrate the ABS module, connect the scan tool and carry out the IVD Initialization sequence, following the scan tool directions. The vehicle must be on a level surface and not moving during calibration. If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart. [1][2]Diagnostic Trouble Code Descriptions