Principles of Operation
Anti-Lock Control
Principles of Operation
Anti-Lock Braking
Anti-lock braking is controlled by the ABS module which continuously monitors and compares the rotational speed of each wheel. The ABS module uses a passive wheel speed sensor and a wheel speed sensor tone ring at each wheel end to monitor the rotational speed of each wheel. Each wheel speed sensor is independently connected to the ABS module by 2 circuits. One circuit is for the signal return and the other circuit is for ground. As the wheel spins, the wheel speed sensor tone ring passes through the magnetic field generated by the 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 voltage 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 will be 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 will carry out self-tests at pre-determined times. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check of the wheel speed sensors and their circuits by sending a 5 volt reference voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 5 km/h (3 mph) and the brake pedal is not being pressed, the pump motor will be 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 input. Depending on the nature of the malfunction detected, the ABS module will deactivate the ABS and send a message over the High Speed Controller Area Network (HS-CAN) bus to the Instrument Cluster (IC). When the IC receives this message, it will illuminate the yellow ABS warning indicator and/or (if equipped) the stability/traction control indicator (also known as a 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 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 will be disabled if there are 2 or more wheel speed sensor DTCs present in the ABS module. When EBD is disabled, the red brake warning indicator, the yellow ABS warning indicator and (if equipped) the sliding-car icon will be illuminated.
Engine Only Traction Control (EOTC) System
The Engine Only Traction Control (EOTC) system is controlled by the PCM and uses the same wheel speed sensors and tone rings that are used for ABS. The ABS module continuously monitors and compares the rotational speed of each rear wheel and sends that information to the PCM over the HS-CAN bus. When the PCM detects a wheel spinning excessively it will assist with traction control by adjusting engine timing and decreasing fuel injector pulses. The PCM will also send a message to the IC over the HS-CAN bus indicating that a traction control event is taking place. When the IC receives this message, it will flash the sliding-car icon. Once the affected wheel returns to normal speed, the PCM returns engine timing and fuel injectors to normal operation and sends another message over the HS-CAN bus indicating that the traction event has ended. When the IC receives this message, it extinguishes the sliding-car icon.
If wheel speed sensor DTCs are present in the ABS module or communication DTCs are present in the IC, PCM or ABS module, the EOTC system will also be disabled. When the system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon indicator will be illuminated.
Stability/Traction Control Switch
Unlike ABS, the EOTC system can be deactivated by the driver through the use of the stability/traction control switch. The switch is hardwired to the PCM and is a momentary contact-type switch that contains an LED to indicate EOTC system status. When the LED is illuminated, the system is OFF, when the LED is not illuminated the system is ON. When the switch is pressed, a ground signal is sent to the PCM. The PCM then sends a message over the HS-CAN bus to the IC indicating that the driver has requested that the system be deactivated. When the IC receives this message it sends a message to the Lighting Control Module (LCM) along the Standard Corporate Protocol (SCP) network. Once the LCM has received the message, it illuminates or extinguishes the LED in the switch. When the system is deactivated by the switch, the PCM will ignore any traction control events until the switch is pressed again or the ignition key is cycled.
Stability/Traction Control Indicator (Sliding-Car Icon)
Status of the traction control system is indicated by a sliding-car icon located in the IC. The indicator will flash during a traction control event. If the traction control system is disabled due to DTCs being present in the ABS module, the sliding-car icon will illuminate solidly in conjunction with the yellow ABS warning indicator.
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 will be disabled if there are 2 or more wheel speed sensor DTCs present in the ABS module. When EBD is disabled, the red brake warning indicator and the yellow ABS warning indicator will be illuminated.