Principles Of Operation
Anti-Lock Control - Non-Hybrid
Principles of Operations
Anti-Lock Braking
The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished through the use of 4 active wheel speed sensors, 1 for 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 creates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.
The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid flows through 4 normally open inlet valves inside the HCU, and through the outlet ports of the HCU to the brake calipers. If the ABS module senses that a wheel is about to lock up, based on wheel speed sensor data, the module will close the normally open inlet valve for that wheel. This prevents more fluid from getting to the brake caliper. The ABS module then reads the wheel speed sensor signal from the affected wheel again. If the wheel is still decelerating (locking up), the module opens the normally closed outlet valve. This allows a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. The ABS pump motor runs during an ABS braking event so that sufficient brake fluid pressure is consistently available to the HCU valves. This procedure of closing and opening valves is repeated several times per second until the speed of the affected wheel matches the speeds of the other wheels. Once this has been accomplished, the ABS module returns the solenoid valves to their normal position.
The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check of the wheel speed sensor circuitry by sending voltage through the sensor and checking for the voltage to return. At speeds above 15 km/h (9 mph), without the brake pedal being pressed, the pump motor will be commanded ON for approximately one-half 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. In the event that the ABS module detects a concern, the module will set a DTC and send a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) bus to illuminate the yellow ABS warning indicator, the stability/traction control indicator (also known as the sliding-car icon) and/or the red brake warning indicator. If the concern is severe enough, ABS may be disabled. Normal power-assisted braking, however, remains.
Electronic Brake Distribution (EBD)
The ABS module incorporates a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the HCU as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain threshold, the ABS module closes the appropriate solenoid 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 there is a DTC in the ABS module specifically for the HCU or there are 2 or more wheel speed sensor DTCs, EBD will be disabled. When EBD is disabled, the ABS warning indicator, the red brake warning indicator and the sliding-car icon will be illuminated.
Optimized Hydraulic Braking (3.0L All-Wheel Drive (AWD) and 3.5L only)
When the vacuum sensor in the booster indicates vacuum is below a predetermined level, a DTC will be set in the ABS module. A low vacuum DTC will be set after a pre-determined time, depending on driving conditions. It could take as long as 10 minutes to set the DTC with vehicle speed above 2 km/h and greater than 30% throttle. If the brake pedal is applied with low vacuum in the booster, the ABS module will activate the pump motor in the HCU to assist with braking.
Refer to Power Brake Assist for sensor removal and installation information.
AdvanceTrac(R) Stability Enhancement System
The AdvanceTrac(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) System - helps to avoid skids and lateral slides.
Traction Control System
The traction control system uses the ABS 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 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 IPC receives this message, it will flash the sliding-car icon (twice per second). 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 IPC 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) System
The ESC 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 ESC system also uses input from the steering wheel rotation sensor (3.5L) or Power Steering Control Module (PSCM) (2.5L, 3.0L and Hybrid), and the stability control sensors (yaw rate, longitudinal acceleration and lateral acceleration), which are internal to the Restraints Control Module (RCM). Additional information from other modules is sent over the HS-CAN bus to help maintain vehicle stability.
The ABS module uses the various sensors to continuously monitor vehicle 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 bus that a stability event is taking place. When the IPC receives this message, it will flash the sliding-car icon. Under certain conditions during a stability event, the PCM may be requested to reduce engine torque as it does for traction control. Once the stability condition has been corrected, the ABS module 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 IPC extinguishes the sliding-car icon.
Certain DTCs being present in the ABS module may disable the ESC system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon will be illuminated.
Traction Control Switch
Unlike conventional ABS, the traction control portion of the AdvanceTrac(R) can be deactivated by the driver through the use of the traction control switch. The switch is hardwired to the IPC. When pressed, the switch sends a ground signal to the IPC. The IPC 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 OFF icon. Once disabled, the system will remain in that state until the driver presses the switch again or the ignition key is cycled.
Stability Control Sensors
The stability control sensors for the AdvanceTrac(R) system consist of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over the dedicated Controller Area Network (CAN) bus. If any of the sensors are defective, a new RCM must be installed. Refer to Restraint Systems for additional information on the RCM.
- The yaw rate sensor measures the relative vehicle motion about the vertical axis through the vehicle center of gravity.
- 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.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.
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 Position Information
On vehicles equipped with Electronic Power Assist Steering (EPAS) (2.5L, 3.0L and Hybrid vehicles), the steering wheel position information is sent to the ABS module from the PSCM along the HS-CAN bus. The PSCM calculates steering position based on motor position within the EPAS steer gear. For information on EPAS, refer to Steering.
On vehicles equipped with Hydraulic Power Assist Steering (HPAS) (3.5L vehicles only), the steering wheel rotation information is sent from the steering wheel rotation sensor. The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation 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 ESC system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The ESC system 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
ABS Module Calibration
Non-hybrid vehicles do not require a module calibration. For module calibration information for hybrid vehicles, refer to ABS Module Calibration in Anti-Lock Control - Hybrid.