Principles of Operation

Anti-Lock Control - Three-Channel

Principles of Operation

The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished through the use of 3 active wheel speed sensors, one sensor for each front wheel and the third sensor mounted on top of the differential housing. 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 (Hydraulic Control Unit) contains solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid flows through 3 normally open inlet valves inside the HCU (Hydraulic Control Unit), and through the outlet ports of the HCU (Hydraulic Control Unit) 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 (Hydraulic Control Unit) accumulator. The ABS pump motor runs during an ABS braking event so that sufficient brake fluid pressure is consistently available to the HCU (Hydraulic Control Unit) 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. 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 traction control indicator (also known as the sliding-car icon). If the concern is severe enough, the ABS may be disabled, however, normal power-assisted braking remains.

When the ignition switch is placed in the RUN position, the IPC (Instrument Panel Cluster) 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 self-check that turns the indicator off if no fault conditions are present.

Trailer Sway Control System

Trailer sway control is a unique function 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, it takes action similar to the action taken for Electronic Stability Control (ESC), 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 IPC (Instrument Panel Cluster) 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 (Electronic Stability Control) and RSC(R) (Roll Stability Control). Once the trailer sway has been corrected, the ABS module discontinues the event message over the HS-CAN (High Speed Controller Area Network). The PCM returns engine timing and fuel injectors to normal operation and the IPC (Instrument Panel Cluster) 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 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. for information on the TBC (Trailer Brake Control) system.

Trailer sway control only activates with vehicle speed greater than 64 km/h (40 mph). 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.

Engine Only Traction Control (EOTC)

In order to achieve maximum traction, the Engine Only Traction Control (EOTC) system is designed to limit wheel spin by modulating engine torque when driving on slippery or loose surfaces. Engine torque modulation is achieved by regulating fuel and spark to the cylinders. This means that the traction control system is controlled by the PCM and not the ABS module. The only function of the ABS module in an EOTC (Engine Only Traction Control) system is to provide wheel speed information to the PCM along the HS-CAN (High Speed Controller Area Network). The PCM uses the wheel speed information to determine when traction control is needed.

The traction control system can be disabled by the stability/traction control switch which is part of the center stack switch assembly (the assembly contains the Passenger Air Bag Deactivation (PAD) indicator and the stability/traction control switch). The traction control system resets and returns 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. When the traction control system is disabled the message center (if equipped) displays TRACTION CONTROL OFF.

The traction control system is contained in the PCM and is configured when the PCM is configured. Refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning

The brake pressure transducer (if equipped) is mounted on the bottom of the master cylinder near the rear and sends a variable voltage to the ABS module depending on the pressure applied to the brake pedal. The ABS module receives this information directly from the pressure transducer.