Principles Of Operation

Anti-Lock Control

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 will close 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 will carry 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 will be 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 will set a DTC and send a message to the Instrument Cluster (IC) over the High Speed Controller Area Network (HS-CAN) bus 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, ABS may be disabled. Normal power-assisted braking, however, remains.

When the ignition switch is placed in the RUN position, the IC carries out a preliminary self-check on the ABS warning indicator by illuminating the indicator for 3 seconds. If the 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.


Engine Only Traction Control

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 system is to provide wheel speed information to the PCM along the High Speed Controller Area Network (HS-CAN) bus. The PCM uses the wheel speed information to determine when traction control is needed.

The traction control system can be disabled by the traction control switch (located on the instrument panel center finish panel). The traction control system will reset and return to normal operation when the ignition switch is cycled or when the traction control switch is pressed and released a second time during the same ignition cycle.

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