Principles of Operation

Anti-Lock Control - Non-Hybrid

Principles of Operation

Anti-Lock Control

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 speed sensor tone rings pass 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.

The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. 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. If the ABS module senses that a wheel is about to lock up, based on wheel speed sensor data, the module closes the normally open inlet valve for that wheel. This prevents more fluid from getting to the brake caliper. The ABS module then 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. This allows 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 that 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 switch 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 that the ABS module detects a concern, the module sets a DTC and send a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) bus to illuminate the yellow ABS warning indicator, the stability/traction control indicator (also known as the sliding-car icon) and/or the red brake warning indicator. 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 set 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 sliding-car icon illuminates.

AdvanceTrac(R) with Roll Stability Control (RSC(R))

The AdvanceTrac(R) with RSC(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.
- RSC(R) - helps to avoid vehicle roll-over.

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 bus that a traction control event is taking place. When the Instrument Panel Cluster (IPC) receives this message, it flashes the sliding-car icon (twice per second). If the event is severe enough, the ABS module also sends a message to the PCM over the HS-CAN bus to assist with traction control. When the PCM receives this message, it adjusts engine timing and decrease 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 bus indicating that 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 disables the traction control system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon illuminate.

Electronic Stability Control (ESC) and Roll Stability Control (RSC(R)) Systems

The ESC and RSC(R) systems are controlled by the ABS module and use the same wheel speed sensors and tone rings that are used for anti-lock braking. The ESC and RSC(R) systems also use input from the steering wheel rotation sensor, the stability control sensors (yaw rate, roll rate, longitudinal acceleration and lateral acceleration) which are internal to the Restraints Control Module (RCM) and information from other modules sent over the HS-CAN bus to help maintain vehicle stability.

The ABS module uses the various sensors to continuously monitor vehicle roll motion and direction of travel relative to the driver's intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction or that a condition exists for a potential roll-over event, it modulates brake pressure to the appropriate brake caliper(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 bus that a stability event is taking place. When the IPC receives this message, it flashes the sliding-car icon. During a stability event, conditions may require the involvement of the PCM to limit engine torque like it does for traction control. Once the stability condition has been corrected, 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 bus indicating that the traction 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 and RSC(R) systems. Depending on the DTCs present, the yellow ABS warning indicator and/or both the sliding-car icon and the sliding car OFF icon illuminate.

Stability Control Sensors

The stability control sensors for the AdvanceTrac(R) with RSC(R) system consist of 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. Refer to Supplemental Restraint System Service and Repair for additional information on the RCM.

- The yaw rate sensor measures the relative vehicle motion about the vertical axis through the vehicle center of gravity.
- 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 lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.

Lateral 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.

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 the HS-CAN bus 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 is integrated into the Electronic Power Assist Steering (EPAS) column assembly and is not serviced separately. For information and diagnostics on the EPAS system, refer to Steering System Testing and Inspection.

Stability/Traction Control Switch

The stability/traction control switch provides the driver with the ability to disable only the traction control feature. Anti-lock control, ESC and RSC(R) systems remain functional. The switch is hardwired to the ABS module. When the stability/traction control switch is pressed, a ground signal is sent to the ABS module. The ABS module then sends a message over the HS-CAN bus to the IC indicating that the driver has requested that the system be deactivated and the IC illuminates the sliding-car icon. The ABS module and the PCM then ignores any traction events until the driver presses the switch again, the ignition key is cycled or the vehicle speed reaches or exceeds 70 km/h (43 mph). Once the vehicle speed reaches or exceeds 70 km/h (43 mph), the traction control system re-activates but the indicator remains illuminated.

The AdvanceTrac(R) system cannot be disabled when a MyKey(TM)-restricted key is in use that has the AdvanceTrac(R) always-on feature configured to "on". If an attempt is made to disable the AdvanceTrac(R) system while a MyKey(TM)-restricted key is in use, ADVTRAC ON MYKEY SETTING displays in the IC message center. For information about MyKey(TM), refer to Anti-Theft - Passive Anti-Theft System (PATS) Testing and Inspection.

Stability/Traction Control Indicator (Sliding-Car Icon)

Status of the traction control system is indicated by a stability/traction control indicator (sliding-car icon) and a stability/traction control OFF indicator (sliding-car OFF icon) located in the IC (or message center if equipped). When the driver disables the traction control system through the use of the traction control switch, the sliding-car OFF icon illuminates solidly. When one or more of the systems is currently active, the sliding-car icon flashes, and when the system is disabled due to DTCs being present in the ABS module, the sliding-car icon illuminates. In some cases, the sliding-car icon illuminates in conjunction with the yellow ABS warning indicator. When a DTC is set that affects ESC/RSC(R), both the stability/traction control indicator (sliding-car icon) and a stability/traction control sliding car icon OFF indicator illuminate.

If the systems have been disabled through the stability/traction control switch and the vehicle speed reaches or exceeds 70 km/h (43 mph), the system activates, but the stability/traction control OFF indicator (sliding-car OFF icon) remains illuminated until the driver presses the switch again or the ignition switch is cycled.

Stability/Traction Control System Sensor Calibration

When an ABS system fault has been corrected or an ABS component has been replaced, it is necessary to calibrate the ABS module. The calibration procedure is required for the stability control sensors to learn the "zero-position" of the vehicle which means the vehicle must be on a level surface and must not be moving.

If a DTC is logged for any component of the stability/traction control system, the DTC must be cleared or repaired before carrying out the calibration procedure except those DTCs indicating a need for calibration. The need to calibrate is also indicated by the sliding car icon, flashing once every 2 seconds.

To carry out the calibration procedure, connect the scan tool, proceed to the IVD Initialization sequence and follow the scan tool instructions.

ABS Module Configuration

When a new ABS module is installed it must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).