Principles of Operation

Anti-Lock Control

Principles of Operations


Anti-Lock Braking

The ABS module prevents wheel lockup by automatically modulating the brake pressure to the brake calipers during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions. The ABS controls all 4 brake calipers separately. The brake pedal force required to engage the ABS function may vary with the road surface condition. A dry surface requires greater force than a slippery surface.

During the ABS operation, the driver may feel a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system. When the brakes are applied, brake fluid is forced from the brake master cylinder outlet ports to the Hydraulic Control Unit (HCU) inlet ports. The fluid pressure is transmitted through 4 normally open solenoid valves inside the HCU, through the outlet ports of the HCU and finally to the brake calipers. One circuit of the brake master cylinder feeds the front brakes while the other circuit feeds the rear brakes. If the ABS module senses that a wheel is about to lock, based on wheel speed sensor data, the solenoid valve will pulse closed, preventing more fluid from entering that circuit. The ABS module then reads the sensor signal from the affected wheel again. If the wheel is still decelerating, the normally closed solenoid valve for that circuit is opened. A controlled amount of hydraulic pressure between the normally open valve and the brake caliper is relieved into the HCU accumulator. Once the speed of the affected wheel matches the speeds of the other wheels, the ABS module returns the solenoid valves to their normal condition, allowing fluid flow to the affected brake caliper.

The ABS module is self-monitoring and will carry out self tests after the ignition key is turned to the RUN position, after an ABS stop and after an AdvanceTrac(R) system activation. 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 voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 21 km/h (13 mph) and the brake pedal is not being pressed, the HCU 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 anti-lock control system 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 message center indicator ("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 3 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.


AdvanceTrac(R) with Roll Stability Control (RSC(R))

The AdvanceTrac(R) with Roll Stability Control (RSC(R)) system provides the following stability enhancement features for certain driving situations:

- Traction Control System - helps to avoid excessive drive-wheel spin and loss of traction.
- Electronic Stability Control (ESC) - helps to avoid skids and lateral slides.
- RSC(R) - helps to avoid vehicle roll-over.

Traction Control System

The traction control 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 ABS module continuously monitors and compares the rotational speed of each wheel and when it detects a wheel spinning excessively, modulates brake pressure to the appropriate brake caliper. 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 bus that a traction control event is taking place. When the IC receives this message, it will flash the sliding-car icon. If the event is severe enough, the ABS module will also send a message to the PCM over the HS-CAN bus to assist with traction control. When the PCM receives this message, it will adjust engine timing and decrease 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 bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the sliding-car icon.

Once vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through PCM intervention.

Certain DTCs in the ABS module will disable the traction control system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon will be illuminated.

Electronic Stability Control (ESC) and Roll Stability Control (RSC(R)) Systems

The ESC and RSC(R) systems are controlled by the ABS module and use the same wheel speed sensors and tone rings that are used for anti-lock braking. The ESC and RSC(R) systems also use 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 bus 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 bus that a stability event is taking place. When the IC receives this message, it will flash the sliding-car icon. If the event is severe enough, the ABS module will also send a message to the PCM over the HS-CAN bus to assist with vehicle stability. When the PCM receives this message, it will adjust engine timing and decrease fuel injector pulses. Once the stability condition has been corrected, 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 bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the sliding-car icon.

Certain DTCs being present in the ABS module may disable the ESC and RSC(R) systems. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon will be illuminated.

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 trailer sway event is taking place, the module will modulate 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 bus that a trailer sway event is taking place. When the IC receives this message, it will flash the sliding-car icon and display TRAILER SWAY, REDUCE SPEED in the message center. If the event is severe enough, the ABS module will also send a message to the PCM over the HS-CAN bus to assist with vehicle stability. When the PCM receives this message, it will adjust engine timing and decrease fuel injector pulses. Once the trailer sway has been corrected, 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 bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the sliding-car icon.

If the vehicle is also equipped with a Trailer Brake Control (TBC) system, it will also receive the trailer sway message from the ABS module and take appropriate action to assist in reducing the trailer sway.

Trailer sway control will only activate with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) system will also disable 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 will be 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. The switch is hardwired to the IC and when pressed, sends a ground signal to the IC. The IC then sends a message over the HS-CAN bus 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 will determine what systems are disabled.

- Button pressed momentarily - only traction control is disabled and the sliding-car icon will illuminate solidly.
- Button pressed and held for more than 5 seconds with vehicle speed under 56 km/h (35 mph) - RSC(R), ESC and traction control are all disabled and the sliding-car icon will flash momentarily and then illuminate solidly.
- Button pressed after deactivation - all 3 systems will be enabled and the sliding-car icon will not be illuminated.
- Vehicle speed exceeds 56 km/h (35 mph) after the button is pressed and held for more than 5 seconds - RSC(R) and ESC will be enabled but traction control will be disabled. The sliding-car icon will remain illuminated solidly.

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

Once disabled, the systems will remain in that state until the driver presses the switch again, the ignition key 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 will re-activate but traction control will remain deactivated.

Stability/Traction Control Message Center Indicator ("Sliding-Car Icon")

Status of the traction control, ESC, RSC(R) and trailer sway control systems is indicated by a sliding-car icon located in the IC message center. The indicator will illuminate solidly when the driver disables one or more of the systems through the use of the stability/traction control switch or if there are DTCs present in the ABS module directly related to the specific system. The indicator will flash when one or more of the systems is currently active.


Stability Control Sensors

The stability control sensors for the AdvanceTrac(R) with RSC(R) system consist of the yaw rate sensor, roll rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over a dedicated CAN bus. If any of the sensors are defective, a new RCM must be installed.

- The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
- The roll rate sensor measures the rate of rotation along the front to rear horizontal axis.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.
- The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle's front and rear wheels.

Lateral acceleration has 2 forms. The first is the centrifugal acceleration that is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.


Steering Wheel Rotation Sensor

The steering wheel rotation sensor uses an optical sensor and a slotted wheel to measure the rate of rotation (angle) of the steering wheel. The steering wheel rotation sensor uses 2 signal lines to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The steering wheel rotation sensor does not indicate the absolute position of the steering wheel relative to straight-ahead. The ABS module learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The module confirms this position and modifies it as necessary during every new driving cycle.

ABS Module Configuration

Module configuration is a scan tool process which configures specific vehicle settings in a new module. If installing a new ABS module and/or a new HCU, the ABS module must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning