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Principles of Operation




Anti-Lock Control - Four-Channel

Principles of Operation

Anti-lock Brake System

The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. The HCU contains 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, based on wheel speed sensor data, the ABS module senses that a wheel is about to lock up, 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 12 km/h (8 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 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 (also known as the sliding-car icon). If the concern is severe enough, the ABS may be disabled. Normal power-assisted braking remains available.

When the ignition switch is placed in the RUN position, the IPC carries out a preliminary self-check on the ABS warning indicator by illuminating the indicator for 3 seconds. If the Gross Vehicle Weight (GVW) is over 4,535 kg (10,000 lb), the warning indicator may remain on until the vehicle is driven at a speed above 24 km/h (15 mph) for 10 seconds. Bringing the vehicle up to this speed enables an ABS module self-check that turns the indicator off if no fault conditions are present.

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 control light. 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 that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC turns off he stability control light.

The traction control system can be disabled by the traction control switch (part of the center stack switch assembly) and the ABS module sends a message to the IPC to illuminate the stability/traction control indicator (sliding-car icon) OFF light. The traction control system resets and returns to normal operation when the ignition is cycled or when the stability/traction control switch is pressed and released a second time during the same ignition cycle.

Roll Stability Control (RSC(R))

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 anti-lock braking. The RSC(R) system also uses 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 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 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 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 RSC(R) system. Depending on the DTCs present, the yellow ABS warning indicator and/or the stability/traction control light (sliding-car icon) illuminates.

Trailer Sway Control System

Trailer sway control is a unique function of the RSC(R) system that uses information from the steering wheel rotation sensor and the yaw information from the RCM 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, it takes action similar to the action taken for Electronic Stability Control (ESC) and RSC(R). At the same time, the ABS module sends a message over the HS-CAN that a trailer sway event is taking place. When the IPC receives this message, it flashes the sliding-car icon and displays TRAILER SWAY REDUCE SPEED in the message center. If necessary, the PCM reduces engine torque as it does for ESC and RSC(R). Once the trailer sway has been corrected, the ABS module discontinues the event message over the HS-CAN. 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 system. Depending on the DTCs present, the yellow ABS warning indicator and/or the stability/traction light (sliding-car icon) illuminates.

If the vehicle is equipped with a Trailer Brake Control (TBC) system, it also receives the trailer sway message from the ABS module and takes appropriate action to assist in reducing the trailer sway. Refer to Auxiliary Brake System Testing and Inspection for information on the TBC system.

Trailer sway control only activates with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) 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

Unlike conventional ABS, the individual systems of AdvanceTrac(R) with RSC(R) can be deactivated by the driver through the stability/traction control switch (part of the Passenger Air Bag Deactivation (PAD)). The switch is hardwired to the IPC and when pressed, sends a ground signal to the IPC. The IPC then sends a message over the HS-CAN indicating that the driver has requested that the system be deactivated and illuminates the sliding-car icon. The circumstances present when the switch is pressed and the duration of the button press determine which systems are disabled.

If the switch is pressed and held for more than 30 seconds, a DTC is set in the ABS module.

Once disabled, the systems remain in that state until the driver presses the switch again, the ignition is cycled or the vehicle speed reaches or exceeds 56 km/h (35 mph). Once the vehicle speed reaches or exceeds 56 km/h (35 mph), RSC(R) and ESC re-activate but traction control remains deactivated.

The systems cannot be disabled when a MyKey(R)-restricted key is in use with the AdvanceTrac(R) always-on feature configured to "on". If an attempt is made to disable the ESC system while a MyKey(R)-restricted key is in use, ADVTRAC ON MY KEY SETTING displays in the IPC message center. For information about MyKey(R), Anti-Theft - Passive Anti-Theft System (PATS) Anti-Theft - Passive Anti-Theft System (PATS).

Hill Descent Control

Hill descent control uses the ABS pump motor and HCU to provide vehicle speed control while descending a downhill grade, information from the Restraints Control Module (RCM) is used to determine whether or not the vehicle is on a downhill grade and wheel speed sensor information to determine vehicle speed. When the hill descent control switch is pressed, a ground signal is sent to the IPC indicating that the driver has activated the system. The IPC sends a message to the ABS module along the HS-CAN requesting system activation. The ABS module uses the input from the RCM and wheel speed sensors to determine the mode of operation (active, stand-by or deactivated). During a hill descent while the hill descent control system is activated, the driver sets vehicle speed through the use of the accelerator and brake pedals. Once the desired speed has been set, the system maintains that speed until: the vehicle is no longer on a downhill grade, the driver presses the switch again, vehicle speed exceeds 32 km/h (20 mph) or the system is disabled for cooling.

During periods of sustained use, the hill descent control system may automatically deactivate in order to cool the brake system. When this occurs, the system notifies the driver with a message in the message center and by sounding a warning chime. At this time the driver needs to provide brake and/or throttle control to maintain the desired descent speed. The system remains disabled until it has sufficiently cooled. The length of time needed for cooling varies depending on operating conditions.

Hill Start Assist

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. The inputs to the system are:

- Road inclination.
- The brakes are applied 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 2 seconds giving the driver time to accelerate the vehicle. As the vehicle accelerates, hill start assist gradually reduces the brake pressure to make sure 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.
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