Principles of Operation
Anti-Lock Control
Principles of Operation
Standard ABS
The ABS module receives front wheel speed information directly from the front wheel speed sensors and rear wheel speed information from the PCM over the High Speed Controller Area Network (HS-CAN) bus. The PCM receives the rear wheel speed sensor information from the transmission Output Shaft Speed (OSS). The ABS module then processes this information, along with other sensor inputs, to determine if an ABS event is necessary. The front wheel speed sensors sense each tooth on the sensor ring as it passes through the magnetic field of the wheel speed sensor. The OSS is a Hall-effect type sensor and senses the gear teeth on the park gear as they pass through the magnetic field of the sensor.
The ABS module continuously monitors and compares the rotational speed of each front wheel and the speed of the transmission output shaft. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and commands the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves to modulate the brake pressure to the individual brake caliper(s). Once the affected wheel(s) return to the desired speed, the ABS module commands the HCU to return the solenoid valves to their normal position and normal base brake operation is restored.
The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module does a preliminary electrical check. At approximately 20 km/h (12 mph), the pump motor is turned on for approximately one-half second. Any ABS malfunction causes the module to deactivate the system and the ABS warning indicator to illuminate; however, normal power-assisted braking remains.
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 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.
A slight bump sensation may be felt in the brake pedal when Electronic Brake Distribution (EBD) is active.
EBD is disabled if 2 or more wheel speed sensor DTCs are set. When EBD is disabled, the red brake warning indicator and the yellow ABS warning indicator is illuminated.
Engine Only Traction Control (EOTC)
Some vehicles may be equipped with an Engine Only Traction Control (EOTC) system. In order to achieve maximum traction, the 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 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 HS-CAN bus. The PCM uses the wheel speed information to determine when traction control is needed. The stability/traction control indicator "sliding-car icon" in the Instrument Panel Cluster (IPC) flashes during a traction control event and illuminates without flashing if the system is inoperative due to DTCs present in the ABS module. The stability/traction control indicator "sliding car OFF icon" is used to alert the driver that the traction control portion of the AdvanceTrac(R) system has been disabled by the driver. This icon illuminates continuously and does not flash.
The EOTC system can be switched off by pressing the traction control switch. Traction control system status is indicated by the sliding-car icon in the IPC being illuminated steadily. The system resets and returns 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 system is contained in the PCM and is configured when the PCM is configured. Refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).
When the EOTC system activates, either of the following can occur:
- A small deceleration or a reduction in the acceleration of the vehicle
- The sliding-car icon in the IPC flashes
ABS Module Configuration
A new ABS module must be configured after installation. Refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).