Principles of Operations

Anti-Lock Control

Principles of Operations

Anti-Lock Control

The anti-lock control system is controlled by the ABS module which continuously monitors and compares the rotational speed of 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 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 square-wave 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 is 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 carries out self-tests at pre-determined times. When the ignition switch is turned to the RUN position, the ABS module 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 20 km/h (12 mph), and the brake pedal is not being pressed, the pump motor is 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. If a malfunction is detected, the ABS module deactivates the anti-lock control system and also sends a message over the High Speed Controller Area Network (HS-CAN) bus. When the Instrument Cluster (IC) receives this message, it illuminates the yellow ABS warning indicator and the stability/traction control 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 (Hydraulic Control Unit) 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.

If ABS is disabled due to DTCs being present in the ABS module, EBD (Electronic Brake Distribution) continues to function unless the DTCs are for wheel speed sensors or the HCU (Hydraulic Control Unit). When EBD (Electronic Brake Distribution) is disabled, the red brake warning indicator, the ABS warning indicator and the sliding-car icon is illuminated.

Traction Control System

The traction control system uses the ABS module and the PCM to limit wheel spin. When the ABS module detects a wheel spinning excessively, brake pressure to the appropriate brake caliper is modulated. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) bus that a traction control event is taking place. When the PCM receives this message, it adjusts engine timing and decrease fuel injector pulses. When the IC (Instrument Cluster) receives this message, it flashes the sliding-car icon (twice per second). Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the sliding-car icon.

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

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the traction control system is also disabled. When the traction control system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon is illuminated.

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

The AdvanceTrac(R) with Roll Stability Control (RSC(R)) system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the anti-lock control system. The AdvanceTrac(R) with RSC(R) (Roll Stability Control) system also uses input from the steering wheel rotation sensor on Hydraulic Power Assist Steering (HPAS) -equipped vehicles, the Power Steering Control Module (PSCM) on Electronic Power Assist Steering (EPAS) -equipped vehicles, inertial signals (such as yaw and roll rate and lateral and longitudinal acceleration) from the Restraints Control Module (RCM), and information from other modules sent over the HS-CAN (High Speed Controller Area Network) bus to help maintain vehicle stability. The ABS module uses all of these inputs to continuously monitor the vehicle's 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 conditions exist 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 (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) bus that a stability event is taking place. When the PCM receives this message, it adjusts engine timing and decrease fuel injector pulses. When the IC (Instrument Cluster) receives this message, it flashes the sliding-car icon (twice per second). Once the stability condition has been corrected, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the sliding-car icon.

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the AdvanceTrac(R) with RSC(R) (Roll Stability Control) system is also disabled. When the AdvanceTrac(R) with RSC(R) (Roll Stability Control) system is disabled due to DTCs being present, the sliding-car icon is illuminated. In some cases, the sliding-car icon illuminates in conjunction with the yellow ABS warning indicator.

Trailer Sway Control

Trailer sway control is a unique function of the RSC(R) (Roll Stability Control) system 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 trailer sway event is taking place, the ABS module modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) bus that a trailer sway event is taking place. When the IC (Instrument Cluster) receives this message, it flashes the sliding-car icon and display TRAILER SWAY, REDUCE SPEED in the message center. If the event is severe enough, the ABS module sends a message to the PCM over the HS-CAN (High Speed Controller Area Network) bus to assist with vehicle stability. When the PCM receives this message, it adjusts engine timing and decrease fuel injector pulses. Once the trailer sway has been corrected, the ABS module returns the solenoid valves in the HCU (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC (Instrument Cluster) extinguishes the sliding-car icon.

The trailer sway control system activates when the vehicle speed exceeds 64 km/h (40 mph). The system can be disabled during any key cycle. Refer to the Owner's Literature for information on disabling the system. The system is re-enabled when the ignition cycled from RUN to OFF and back to RUN.

Traction Control Switch

Unlike the standard anti-lock control system, the traction control system can be deactivated by the driver through the traction control switch. The switch is hardwired to the IC (Instrument Cluster) and when pressed sends a ground signal to the IC (Instrument Cluster). The IC (Instrument Cluster) then sends a message over the HS-CAN (High Speed Controller Area Network) bus indicating that the driver has requested that the system be deactivated and illuminates the sliding-car "OFF" icon. After receiving this message, the ABS module and the PCM ignore any traction events until the driver presses the switch again or the ignition is cycled.

The AdvanceTrac(R) with RSC(R) (Roll Stability Control) system cannot be disabled when a MyKey(TM)-restricted key is in use that has the AdvanceTrac(R) always on feature configured to "on". If an attempt is made to disable the AdvanceTrac(R) system while a MyKey(TM)-restricted key is in use, ADVTRAC ON MY KEY SETTING displays in the IC (Instrument Cluster) message center. For information about MyKey(TM), refer to Antitheft and Alarm Systems.

Stability/Traction Control Indicator (Sliding Car Icon)

Status of the traction control and AdvanceTrac(R) systems is indicated by a stability/traction control indicator (sliding-car icon) and a stability/traction control "OFF" indicator (sliding-car "OFF" icon) located in the IC (Instrument Cluster) (or message center, if equipped). When the driver disables the systems through the use of the stability/traction control switch, the stability/traction control sliding car "OFF" icon illuminates solidly. When one or more of the systems is currently active, the stability/traction control sliding car icon flashes, and when the system is disabled due to DTCs being present in the ABS module, the sliding-car icon illuminates in conjunction with the yellow ABS warning indicator. The yellow ABS indicator and the stability/traction control indicator (sliding car icon) remain illuminated on the next key cycle until the ABS module carries out the self-tests and confirms that the malfunction is resolved.

If the traction control system has been disabled through the traction control switch, the system re-activates when the driver presses the switch again or the ignition switch is cycled.

Stability Control Sensors

The RCM (Restraints Control Module) inertial signals consist of the yaw rate sensor, roll rate sensor, lateral accelerometer and longitudinal accelerometer. The RCM (Restraints Control Module) is usually mounted as close as possible to the center point of the vehicle to get the most accurate readings. Information from the RCM (Restraints Control Module) sensors is sent to the ABS module over a dedicated Controller Area Network (CAN) bus.

- 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 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.

Hydraulic Power Assist Steering (HPAS)

The steering wheel rotation sensor (on vehicles equipped with HPAS (Hydraulic Power Assist Steering)) measures the rate of rotation of the steering wheel by monitoring a clock spring slotted wheel as it passes through the sensor gap. The 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 rotation sensor installed on HPAS (Hydraulic Power Assist Steering) vehicles is a relative sensor and does not indicate the position of the steering wheel relative to straight ahead. At each consecutive ignition cycle, the steering wheel angle sensor "zeroes" itself

The SASM (Steering Angle Sensor Module) measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The SASM (Steering Angle Sensor Module) uses the HS-CAN (High Speed Controller Area Network) bus 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.

Hydraulic Power Assist Steering (HPAS) Steering Wheel Angle Sensor

The steering wheel rotation sensor (on vehicles equipped with HPAS (Hydraulic Power Assist Steering)) measures the rate of rotation of the steering wheel by monitoring a clock spring slotted wheel as it passes through the sensor gap. The 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 rotation sensor installed on HPAS (Hydraulic Power Assist Steering) vehicles is a relative sensor and does not indicate the position of the steering wheel relative to straight ahead. At each consecutive ignition cycle, the steering wheel angle sensor "zeroes" itself and transmit the 0 value to ABS PID SWA regardless of the steering wheel position. If the front wheels are not turned to the straight ahead position when the ignition is turned on, the zero position taken as the initial wheel position. If the front wheels are then turned to the straight ahead position, the value read in ABS PID SWA no longer transmits 0. This is normal operation and does not indicate a problem in either the steering wheel angle sensor or the ABS module. Vehicles equipped with EPAS (Electronic Power Assist Steering) do not have a steering wheel rotation sensor. Autopark equipped vehicles have an absolute steering wheel angle sensor module that transmits the steering wheel angle data over the HS-CAN (High Speed Controller Area Network) bus.

For more information on EPAS (Electronic Power Assist Steering), Refer to Steering.

For more information about the Active Park Assist steering wheel angle sensor, Refer to Active Parking Assist

ABS Module Configuration

The ABS module must be configured when any one of the following components are installed new:

- ABS module

- HCU (Hydraulic Control Unit)

To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus. If a new ABS module is installed, it must also be calibrated. Programming and Relearning

ABS Module Calibration

The ABS module must be configured before it is calibrated. The ABS module must be calibrated if any one of the following components are installed new:

- ABS module

- HCU (Hydraulic Control Unit)

- RCM (Restraints Control Module)

Calibration is required for the ABS module to learn the "at rest" or "zero position" of the stability control components. If a DTC is set for any stability control component, the DTC must be repaired before carrying out the calibration procedure. The sliding-car icon flashes after clearing any stability control DTCs, indicating that the system is not calibrated.

If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart. Antilock Brakes / Traction Control Systems

To calibrate the ABS module, carry out the IVD Initialization sequence. Follow the instructions provided by the scan tool.