Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control - Stability Assist
COMPONENT LOCATION
INTRODUCTION
The Dynamic Stability Control system utilizes information on wheel speed, steering angle and vehicle body attitude to enhance the stability of the vehicle. This is achieved by controlling engine torque and application of the brakes at individual wheels. Dynamic Stability Control incorporates the following systems:
- ABS
- Traction Control
- Panic Brake Assist
- Active Yaw Control
- Braking in Curve Stability
The Dynamic Stability Control system software also controls the braking functions of the Adaptive Speed Control system. For additional information, refer to Speed Control Description and Operation
The Dynamic Stability Control software is contained within the ABS module and can apply or release braking force to each individual wheel, and increase or reduce engine torque to correct oversteer, understeer, wheel spin and wheel lock.
WARNING: Although the vehicle is fitted with Dynamic Stability Control, it remains the drivers responsibility to drive safely according to the prevailing conditions.
DYNAMIC STABILITY CONTROL SWITCH
The non-latching dynamic stability control switch is mounted in the floor console adjacent the L gate and forms part of the gear selector module assembly.
Dynamic Stability Control becomes active whenever the engine is running. A momentary press of the switch allows the driver to toggle between the standard Dynamic Stability Control settings and the optimized 'Trac DSC' settings. For more information on 'Trac DSC' refer to 'Principles of Operation'.
On early (06.5 model year) naturally aspirated vehicles, Dynamic Stability Control can be switched off by pressing and holding the switch for more than 3 seconds. On later (07.25 model year) naturally aspirated and supercharged vehicles the switch needs to be pressed and held for more than 10 seconds to switch Dynamic Stability Control off.
In each case the message 'DSC OFF' will be displayed in the instrument cluster message center to confirm Dynamic Stability Control has been switched off. The amber Dynamic Stability Control warning indicator in the instrument cluster will remain illuminated however. The system can be switched back on again by simply pressing and releasing the switch. The message 'DSC ON' will then appear in the instrument cluster message center to confirm the system is on.
NOTE:
Switch requests may be delayed if the switch is pressed while a Dynamic Stability Control operation is taking place. The switch request will be displayed in the instrument cluster but the ABS module will not initiate any stability changes until it is safe to do so.
If a fault is detected with the Dynamic Stability Control switch, the ABS module defaults to the 'DSC ON' setting and any switch requests are ignored.
WARNING: It is recommended that when using snow chains, Trac DSC is switched on.
WHEEL SPEED SENSORS
An active wheel speed sensor is installed in each wheel hub to provide the ABS module with a rotational speed signal from each road wheel. The head of each wheel speed sensor is positioned close to a 46 tooth sensor ring. The rear sensor rings are located on the outer diameter of the halfshaft constant velocity joints. The front sensor rings are located within the wheel hub and bearing assemblies. A fly lead connects each sensor to the vehicle wiring.
The wheel speed sensors each have a signal and a return connection with the ABS module. When the ignition is ON the ABS module supplies a signal feed to the wheel speed sensors and monitors the return signals. Any rotation of the road wheels induces current fluctuations in the return signals, which are converted into individual wheel speeds and overall vehicle speed by the ABS module.
The ABS module broadcasts the individual wheel speeds and the vehicle speed on the high speed Controller Area Network (CAN) bus for use by other systems.
If a wheel speed sensor fault is detected by the ABS module, 'ABS FAULT' will be displayed in the instrument cluster message center and an amber warning indicator lamp will illuminate. For additional information, refer to Information and Message Center Description and Operation
STEERING ANGLE SENSOR
The steering angle sensor measures the steering wheel angle and the rate of change of the steering wheel angle. These measurements are received by the ABS module and broadcast on the high speed CAN bus for use by other systems.
The steering angle sensor is mounted on the steering column upper shroud mounting bracket, immediately behind the multifunction switches, and is secured by 2 screws. A fly lead connects the sensor to the passenger compartment wiring harness via a 4 pin multiplug.
The sensor is housed in a 'U' shaped plastic casing and contains 2 offset Light Emitting Diode's (LED's) facing 2 detectors.
An encoder ring is mounted on the inner steering column shaft and intersects the LED's and detectors. The encoder ring contains 60 slots which break and restore the light beams between the LED's and the detectors as the steering wheel is rotated. The ABS module is able to determine the direction of rotation of the steering wheel by monitoring when the light beams change state. The LED's and detectors are mounted in such a way that only one beam will change state, either to broken or restored, at any one time.
The center (straight ahead) position of the steering wheel has to be learned by the ABS module every time the ignition is switched ON. The steering angle sensor is unable to determine the center position so inputs from the yaw rate and lateral acceleration sensor and wheel speed signals are also used by the ABS module to help it perform this process. If extreme weather conditions are present, for example ice causing extreme wheel spin or understeer/oversteer, the ABS module may not be able to determine the center position of the steering wheel. In this situation 'DSC NOT AVAILABLE' will be displayed in the instrument cluster message center and the amber warning indicator will illuminate. For additional information, refer to Information and Message Center Description and Operation
'DSC NOT AVAILABLE' will also be displayed if the ABS module detects a steering angle sensor fault. The amber warning indicator will also illuminate until the fault is rectified.
YAW RATE AND LATERAL ACCELERATION SENSOR
The yaw rate and lateral acceleration sensor is mounted on the transmission tunnel, forward of the gear selector module. The sensor is secured by 2 screws and connects to the vehicle wiring via a 4 pin multiplug.
When the ignition is ON, the sensor receives a power feed from the ABS module. The ABS module also provides a ground path for the sensor. The sensor measures the yaw rate and lateral acceleration of the vehicle, providing values to the ABS module via a dedicated, private high speed CAN bus connection. The ABS module broadcasts these values on the high speed CAN bus for use by other systems.
If a sensor fault is detected by the ABS module, 'DSC NOT AVAILABLE' will be displayed in the instrument cluster message center and the amber warning indicator will illuminate. For additional information, refer to Information and Message Center Description and Operation
STOPLAMP SWITCH
The stoplamp switch is mounted on the brake pedal bracket and is connected to the vehicle harness via a 4 pin multiplug.
When the brake pedal is pressed, the switch contacts close. This allows a hard wired signal feed to be sent to the Engine Control Module (ECM). A stoplamp switch status message is then sent from the ECM to the ABS module on the high speed CAN bus. The ABS module is then able to control braking force accordingly in conjunction with the Hydraulic Control Unit (HCU).
NOTE:
The stoplamp switch also forms part of the speed control system. For additional information, refer to Speed Control Description and Operation
ABS MODULE
The ABS module is secured to a mounting bracket located in the LH front wheel arch. The module is mounted on the rear face of the Hydraulic Control Unit (HCU), which it uses to control all braking and stability functions by modulating hydraulic pressure to the individual wheel brakes.
Two different ABS modules are available; one for vehicles with standard Speed Control, one for vehicles fitted with Adaptive Speed Control. Both incorporate analogue valves which offer refined pressure control and reduced noise levels over previously used systems.
If an ABS modulator fault is detected, 'ABS FAULT' will be displayed in the instrument cluster message center and the amber warning indicator will illuminate. For additional information, refer to Information and Message Center Description and Operation
CAUTION: The ABS module and the Hydraulic Control Unit (HCU) are a single unit and must not be separated.
HYDRAULIC CONTROL UNIT (HCU)
The HCU is a 4 channel unit, secured to a mounting bracket located in the LH front wheel arch. The HCU modulates the supply of hydraulic pressure to the brakes under the control of the ABS module. For additional information, refer to Hydraulic Brake Actuation Description and Operation
CONTROL DIAGRAM
NOTE:
A = Hardwired; D = High Speed CAN bus; N = Medium Speed CAN bus; V = Private CAN bus
PRINCIPLES OF OPERATION
Trac DSC
'Trac DSC' optimizes the Dynamic Stability Control settings to maximize traction in difficult conditions such as deep snow, gravel, sand or when snow chains are fitted. This makes greater traction possible, without losing the benefits of Dynamic Stability Control, however the stability of the vehicle may be less than normal.
WARNING: 'Trac DSC' must only be used when necessary for the prevailing conditions. The vehicle should then only be driven appropriately for those conditions. When conditions improve sufficiently, full stability of the vehicle must be regained by switching 'Trac DSC' off and returning the vehicle to the Dynamic Stability Control mode.
Pressing and releasing the Dynamic Stability Control switch will switch the vehicle between normal Dynamic Stability Control settings and 'Trac DSC' settings. To confirm which setting has been selected, either 'DSC ON' or 'Trac DSC' will be temporarily displayed in the instrument cluster message center.
When 'Trac DSC' is selected, the amber Dynamic Stability Control warning indicator located in the instrument cluster will illuminate. The warning indicator will remain illuminated until 'Trac DSC' becomes active. At this point the warning indicator will flash.
NOTE:
If speed control is engaged it will automatically disengage if Dynamic Stability Control or 'Trac DSC' becomes active. For additional information, refer to Speed Control
ABS
ABS controls the speed of all road wheels to ensure optimum wheel slip when braking at the adhesion limit. This prevents the wheels from locking, which helps to retain effective steering control of the vehicle in a braking situation.
Traction Control
Traction control attempts to optimize forward traction by reducing engine torque or braking a spinning wheel until it regains grip.
Traction control is activated if an individual wheel speed is above that of the vehicle reference speed and the brake pedal is not pressed. The spinning wheel is braked, allowing the excess torque to be transmitted to the non-spinning wheel through the driveline. If necessary, the ABS module also sends a high speed CAN bus message to the Engine Control Module (ECM) to request a reduction in engine torque.
Panic Brake Assist
Panic Brake Assist (PBA) helps the driver bring the vehicle to a halt in an emergency braking situation by automatically maximizing the braking effort. There are 2 situations when the ABS module will invoke PBA:
- When the brake pedal is pressed very suddenly
- When the brake pedal is pressed hard enough to bring the front brakes into ABS operation.
When the brake pedal is pressed very suddenly, the ABS module increases hydraulic pressure to all of the brakes until they reach the threshold for ABS operation, thus applying the maximum braking effort for the available traction. The ABS module monitors for the sudden application of the brakes using inputs from the stoplamp switch and from a pressure sensor within the HCU. With the brake pedal pressed, if the rate of increase of hydraulic pressure exceeds the predetermined limit, the ABS module invokes emergency braking.
When the brake pedal is pressed hard enough to bring the front brakes into ABS operation, the ABS module increases the hydraulic pressure to the rear brakes up to the ABS threshold.
PBA operation continues until the driver releases the brake pedal enough for the hydraulic pressure in the HCU to drop below a threshold value stored in the ABS module.
Active Yaw Control
Active Yaw Control monitors the vehicle for oversteer or understeer by comparing signals from the yaw rate and lateral acceleration sensor with signals from the steering angle sensor. If the ABS module determines the vehicle is not responding correctly to steering inputs from the driver it will control the HCU to;
- apply brake pressure to the inner wheels to correct understeer
- apply brake pressure to the outer wheels to correct oversteer.
Braking in Curve Stability
Braking in Curve Stability influences the brake pressures below the Dynamic Stability Control and ABS thresholds to counteract the yawing movement produced when braking in a corner. Braking in Curve Stability produces a correction torque by limiting the brake pressure on one side of the vehicle.
System Malfunction
If an error is detected in the Dynamic Stability Control system, a message will be displayed in the instrument cluster message center and the amber warning indicator will illuminate. In this condition the vehicle is still safe to drive but the system may not react under slide or wheel spin conditions. For additional information, refer to Anti-Lock Control - Stability Assist