Principles Of Operation
Anti-Lock Control
Principles of Operation
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 carries 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 inputs. If a malfunction is detected, the ABS module deactivates the ABS and also sends a message over the High Speed Controller Area Network (HS-CAN). When the Instrument Cluster (IC) receives this message, it illuminates the yellow ABS warning indicator and (if equipped) 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 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, Electronic Brake Distribution (EBD) continues to function unless the DTCs are for wheel speed sensors or the HCU. When EBD is disabled, the red brake warning indicator, the ABS warning indicator and (if equipped) the sliding-car icon illuminates.
Optimized Hydraulic Braking
The ABS module incorporates a strategy called optimized hydraulic braking. This strategy uses the pump motor in the HCU to provide brake pedal boost in the event that the vacuum brake booster fails. The brake booster is equipped with a vacuum sensor. When vacuum in the booster falls below a predetermined level, a DTC is set in the ABS module. If the brake pedal is applied with low vacuum in the booster, the ABS module activates the pump motor in the HCU to assist with braking. During pump motor 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 may be heard.
Refer to Power Brake Actuation Service and Repair for sensor removal and installation information.
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 the ABS. 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 that a traction control event is taking place. When the PCM receives this message, it assists with traction control by adjusting engine timing and decreasing fuel injector pulses. When the IC receives this message, it flashes the stability/traction control indicator (sliding-car icon). 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 indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the stability/traction control indicator.
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 disabled. When the traction control system is disabled due to DTCs being present, both the ABS warning indicator and the stability/traction control indicator are illuminated.
Electronic Stability Control (ESC) System
The Electronic Stability Control (ESC) 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 ESC system also uses input from the steering wheel rotation sensor, the stability control sensors (yaw rate, longitudinal and lateral acceleration located in the Restraints Control Module (RCM)) and information from other modules sent over the HS-CAN 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, 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 that a stability event is taking place. When the PCM receives this message, it assists with vehicle stability by adjusting engine timing and decreasing fuel injector pulses. When the IC receives this message, it flashes the stability/traction control indicator (sliding-car icon). 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 indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the stability/traction control indicator.
If the anti-lock control system is disabled due to DTCs being present in the ABS module, the ESC system is disabled. When the ESC system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon are illuminated.
Roll Stability Control (RSC(R)) System
The 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 RSC(R) system also uses input from the steering wheel rotation sensor, the stability control sensors (yaw rate, roll rate, longitudinal and lateral acceleration) and information from other modules sent over the HS-CAN to help maintain vehicle stability. The ABS module uses all of these inputs to continuously monitor vehicle motion relative to the driver's intended course. If the ABS module determines from all these inputs 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 while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN that a stability event is taking place. When the PCM receives this message, it assists with vehicle stability by adjusting engine timing and decreasing fuel injector pulses. When the IC receives this message, it flashes the sliding-car icon. 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 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 anti-lock control system is disabled due to DTCs being present in the ABS module, the RSC(R) systems are also disabled. When the RSC(R) system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon are 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 a trailer sway event is taking place, the module 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 that a trailer sway event is taking place. When the IC receives this message, it flashes the sliding-car icon and displays TRAILER SWAY, REDUCE SPEED in the message center. If the event is severe enough, the ABS module also sends a message to the PCM over the HS-CAN to assist with vehicle stability. When the PCM receives this message, it adjusts engine timing and decreases 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 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.
Trailer sway control only activates with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) system also disables 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 is re-enabled at each ignition key cycle.
Stability/Traction Control Switch
The stability/traction control switch provides the driver with the ability to disable only the traction control feature. Standard ABS, ESC and RSC(R) remain functional. The switch is hardwired to the ABS module. When the stability/traction control switch is pressed, a ground signal is sent to the ABS module. The ABS module then sends a message over the HS-CAN to the IC indicating that the driver has requested that the system be deactivated and the IC illuminates the stability/traction control indicator (sliding-car icon). The ABS module and the PCM ignores any traction events until the driver presses the switch again, the ignition key is cycled or the vehicle speed reaches or exceeds 70 km/h (43 mph). Once the vehicle speed reaches or exceeds 70 km/h (43 mph), the traction control system reactivates but the indicator remains illuminated.
The ESC system cannot be disabled when a MyKey(R)-restricted key is in use that has the AdvanceTrac(R) always-on feature configured "on". If an attempt is made to disable the ESC system while a MyKey(R)-restricted key is in use, ADVTRAC ON MY KEY SETTING displays in the IC message center. For information about MyKey(R), refer to Anti-Theft - Passive Anti-Theft System (PATS) Anti-Theft - Passive Anti-Theft System (PATS).
Stability/Traction Control Indicator "Sliding-Car Icon"
Status of the traction control, ESC and RSC(R) systems is indicated by a stability/traction control indicator (sliding-car icon) located in the IC (or message center if equipped). When the driver disables the systems through the use of the stability/traction control switch, this indicator illuminates continuously, when one or more of the systems is currently active, the indicator flashes and when the system is disabled due to DTCs being present in the ABS module, the indicator illuminates in conjunction with the yellow ABS warning indicator.
Stability Control Sensors
The stability control sensors for the ESC system include the yaw rate sensor, lateral and longitudinal accelerometers. The yaw rate sensor, the lateral and longitudinal accelerometers are able to detect and measure changes in vehicle direction that indicate the need for the ABS to make corrections that help prevent vehicle roll over or spin outs. The sensors are housed in the Restraints Control Module (RCM) which sends sensor information to the ABS module over the Controller Area Network (CAN). Install a new RCM if any of the sensors are damaged.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the slotted ring 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 does not indicate the position of the steering wheel relative to straight-ahead. The AdvanceTrac(R) system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The AdvanceTrac(R) system confirms this position and modifies it as necessary during every new driving cycle.
Stability/Traction Control System Sensor Calibration
When a new RCM is installed, the sensor(s) must be calibrated. The calibration procedure is required for the sensor(s) to learn the "zero-position" of the vehicle which means the vehicle must be on a level surface and must not be moving.
If a DTC is logged for any component of the AdvanceTrac(R) system, the DTC must be cleared or repaired before carrying out the calibration procedure. The need to calibrate is also indicated by the stability/traction control indicator flashing once every 2 seconds.
To carry out the calibration procedure, connect the scan tool, proceed to the IVD Initialization sequence and follow the scan tool instructions.
ABS Module Configuration
The ABS module must be configured when a new module is installed. Refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).