Principles Of Operation

Anti-Lock Control

Principles of Operation

Anti-Lock Control

The anti-lock control system is controlled by the ABS module which continuously monitors and compares the rotational speed of 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 causes the strength of the magnetic field to increase (as a tooth passes the sensor) or decrease (as a gap passes the sensor) and generates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.

When the ABS module detects an impending wheel lock-up, brake fluid pressure to the appropriate brake caliper is modulated (pulsated). This is accomplished by opening and closing the appropriate solenoid valves inside the Hydraulic Control Unit (HCU) while the hydraulic pump motor is also actuated. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position and the hydraulic pump motor is deactivated.

The ABS module is self-monitoring and carries out self-tests at pre-determined times. When the ignition switch is turned to the RUN position, the ABS module carries out a preliminary electrical check of the wheel speed sensors and their circuits by sending voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 20 km/h (12 mph), and the brake pedal is not being pressed, the pump motor is commanded ON for approximately 0.5 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 by comparing wheel speed input to other sensor inputs. If a malfunction is detected, the ABS module deactivates the ABS and also sends a message over the High Speed Controller Area Network (HS-CAN). When the Instrument Cluster (IC) receives this message, it illuminates the yellow ABS warning indicator and (if equipped) the stability/traction control indicator (sliding-car icon).

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.

If ABS is disabled due to DTCs being present in the ABS module, Electronic Brake Distribution (EBD) 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 red brake warning indicator, the ABS warning indicator and (if equipped) the sliding-car icon illuminates.

Optimized Hydraulic Braking

The ABS module incorporates a strategy called optimized hydraulic braking. This strategy uses the pump motor in the HCU (Hydraulic Control Unit) to provide brake pedal boost in the event that the vacuum brake booster fails. The brake booster is equipped with a vacuum sensor. When vacuum in the booster falls below a predetermined level, a DTC is set in the ABS module. If the brake pedal is applied with low vacuum in the booster, the ABS module activates the pump motor in the HCU (Hydraulic Control Unit) to assist with braking. During pump motor operation, the driver may feel a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise may be heard.

for sensor removal and installation information.

Traction Control System

The traction control system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the ABS. The ABS module continuously monitors and compares the rotational speed of each wheel and when it detects a wheel spinning excessively, modulates brake pressure to the appropriate brake caliper. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a traction control event is taking place. When the PCM receives this message, it assists with traction control by adjusting engine timing and decreasing fuel injector pulses. When the IC (Instrument Cluster) receives this message, it flashes the stability/traction control indicator (sliding-car icon). Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the stability/traction control indicator.

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

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the traction control system is disabled. When the traction control system is disabled due to DTCs being present, both the ABS warning indicator and the stability/traction control indicator are illuminated.

Electronic Stability Control (ESC) System

The Electronic Stability Control (ESC) system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the anti-lock control system. The ESC (Electronic Stability Control) system also uses input from the steering wheel rotation sensor, the stability control sensors (yaw rate, longitudinal and lateral acceleration located in the Restraints Control Module (RCM)) and information from other modules sent over the HS-CAN (High Speed Controller Area Network) to help maintain vehicle stability. The ABS module uses all of these inputs to continuously monitor the vehicle's 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, it modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a stability event is taking place. When the PCM receives this message, it assists with vehicle stability by adjusting engine timing and decreasing fuel injector pulses. When the IC (Instrument Cluster) receives this message, it flashes the stability/traction control indicator (sliding-car icon). Once the stability condition has been corrected, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the stability/traction control indicator.

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the ESC (Electronic Stability Control) system is disabled. When the ESC (Electronic Stability Control) system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon are illuminated.

Roll Stability Control (RSC(R)) System

The Roll Stability Control (RSC(R)) system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the anti-lock control system. The RSC(R) (Roll Stability Control) system also uses input from the steering wheel rotation sensor, the stability control sensors (yaw rate, roll rate, longitudinal and lateral acceleration) and information from other modules sent over the HS-CAN (High Speed Controller Area Network) to help maintain vehicle stability. The ABS module uses all of these inputs to continuously monitor vehicle motion relative to the driver's intended course. If the ABS module determines from all these inputs that conditions exist 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 (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a stability event is taking place. When the PCM receives this message, it assists with vehicle stability by adjusting engine timing and decreasing fuel injector pulses. When the IC (Instrument Cluster) receives this message, it flashes the sliding-car icon. Once the stability condition has been corrected, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the sliding-car icon.

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the RSC(R) (Roll Stability Control) systems are also disabled. When the RSC(R) (Roll Stability Control) system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon are illuminated.

Trailer Sway Control System

Trailer sway control is a unique function of the RSC(R) (Roll Stability Control) system that uses information from the steering wheel rotation sensor and the yaw information from the RCM (Restraints Control Module) to determine if a trailer sway event is taking place. Trailer sway is the undesirable yaw force a trailer can apply to the towing vehicle. If the ABS module determines that a trailer sway event is taking place, the module modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a trailer sway event is taking place. When the IC (Instrument Cluster) receives this message, it flashes the sliding-car icon and displays TRAILER SWAY, REDUCE SPEED in the message center. If the event is severe enough, the ABS module also sends a message to the PCM over the HS-CAN (High Speed Controller Area Network) to assist with vehicle stability. When the PCM receives this message, it adjusts engine timing and decreases fuel injector pulses. Once the trailer sway has been corrected, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the sliding-car icon.

Trailer sway control only activates with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) (Roll Stability Control) system also disables trailer sway control. Trailer sway control can also be disabled by entering the setup menu in the message center. Refer to the Owner's Literature for the disabling/enabling procedure. Regardless of the chosen state (enabled or disabled), trailer sway control is re-enabled at each ignition key cycle.

Stability/Traction Control Switch

The stability/traction control switch provides the driver with the ability to disable only the traction control feature. The conventional ABS, ESC (Electronic Stability Control) and RSC(R) (Roll Stability Control) 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 (High Speed Controller Area Network) to the IC (Instrument Cluster) indicating that the driver has requested that the system be deactivated and the IC (Instrument Cluster) illuminates the stability/traction control disabled indicator (sliding-car OFF icon). The ABS module and the PCM ignore 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 reactivates but the sliding-car OFF icon remains illuminated.

The traction control system cannot be disabled when a MyKey(R) restricted key is in use that has the AdvanceTrac(R) always-on feature configured "on". If an attempt is made to disable the traction control system while a MyKey(R) restricted key is in use, ADVTRAC ON MY KEY SETTING displays in the IC (Instrument Cluster) message center. For information about MyKey(R), refer to Antitheft and Alarm Systems.

Stability/Traction Control Indicators

When the driver disables the traction control system through the use of the traction control switch, the sliding-car OFF icon will illuminate solidly. When one or more of the AdvanceTrac(R) systems (traction control, ESC (Electronic Stability Control) or RSC(R) (Roll Stability Control)) is currently active, the sliding-car icon will flash (twice per second) and when one or more of the systems is disabled due to DTCs being present in the ABS module, the sliding-car icon will illuminate solidly. In some cases, the sliding-car icon will illuminate in conjunction with the yellow ABS warning indicator.

If the traction control system has been disabled through the traction control switch, the system will reactivate when the driver presses the switch again or the ignition switch is cycled.

Stability Control Sensors

The stability control sensors for the ESC (Electronic Stability Control) system include the yaw rate sensor, lateral and longitudinal accelerometers. The yaw rate sensor, the lateral and longitudinal accelerometers 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 Restraints Control Module (RCM) which sends sensor information to the ABS module over a private HS-CAN (High Speed Controller Area Network).

Steering Wheel Rotation Sensor

The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the slotted 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 AdvanceTrac(R) system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The AdvanceTrac(R) system confirms this position and modifies it as necessary during every new driving cycle.

Stability/Traction Control System Sensor Calibration

When a new RCM (Restraints Control Module) is installed, the sensor(s) must be calibrated. The calibration procedure is required for the sensor(s) 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 AdvanceTrac(R) system, the DTC must be cleared or repaired before carrying out the calibration procedure. The need to calibrate is also indicated by the stability/traction control indicator 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

The ABS module must be configured when a new module is installed. Refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning