System Operation

SYSTEM OPERATION
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.

On the front axle, the brake pressure is modulated separately for each wheel. On the rear axle, brake pressure is modulated by select low. Select low applies the same pressure to both rear brakes, with the pressure level being determined by the wheel on the lower friction surface. This maintains rear stability on split friction surfaces.

CBC
CBC influences the brake pressures, below the DSC and ABS thresholds, to counteract the yawing moment produced when braking in a corner. CBC produces a correction torque by limiting the brake pressure on one side of the vehicle.

DSC
DSC uses the brakes and powertrain torque control to help maintain the lateral stability of the vehicle. While the ignition is on the DSC function is permanently enabled unless selected off by the DSC switch. Even if DSC is deselected, driving maneuvers with extreme yaw or lateral acceleration may trigger DSC activity to assist vehicle stability.

DSC enhances driving safety in abrupt maneuvers and in understeer or oversteer situations which may occur in a bend. The ABS module monitors the yaw rate and lateral acceleration of the vehicle, and the steering input, then selectively applies individual brakes and signals for powertrain torque adjustments to reduce understeer or oversteer.

In general: in an understeering situation, the inner wheels are braked to counteract the yaw movement towards the outer edge of the bend, in an oversteering situation, the outer wheels are braked to prevent the rear end of the vehicle from pushing towards the outer edge of the bend.

The ABS module monitors the tracking stability of the vehicle using inputs from the wheel speed sensors, the steering angle sensor and the yaw rate and lateral acceleration sensor. The tracking stability is compared with stored target data and, whenever the tracking stability deviates from the target data, the ABS module intervenes by applying the appropriate brakes. On vehicles with an automatic transmission, when the DSC function is active, the ABS module also signals the Transmission Control Module (TCM) to prevent gear shifts. If necessary, the ABS module also signals:
^ The ECK to reduce engine torque.
^ The transfer box control module, to adjust the locking torque of the center differential.
^ The rear differential control module, to adjust the locking torque of the rear differential.

The DSC function overrides the differential locking torque requests from the terrain response system.

EBD
EBD limits the brake pressure applied to the rear wheels. When the brakes are applied, the weight of the vehicle transfers forwards, which reduces the ability of the rear wheels to transfer braking effort to the road surface. This can cause the rear wheels to slip and make the vehicle unstable.

EBD uses the anti-lock braking hardware to automatically optimize the pressure of the rear brakes, below the point where anti-lock braking would be invoked. Only the rear axle is under EBD control.

ETC
ETC attempts to optimize forward traction by reducing engine torque or braking a spinning wheel until it regains grip.

ETC is activated if an individual wheel speed is above that of the vehicle reference speed (positive slip) and the brake pedal is not pressed. The spinning wheel is braked, allowing the excess torque to be transmitted to the non spinning wheels through the drive line. If necessary, the ABS module also sends a high speed CAN bus message to the ECM to request a reduction in engine torque. Torque reduction requests are for either a slow or fast response: a slow response requests a reduction of throttle angle (4.0L and 4.4L only); a fast response requests an ignition cut-off (4.0L and 4.4L) or a fuel cut-off (2.7L Diesel).

When the DSC function is selected off with the DSC switch, the engine torque reduction feature is disabled.

On vehicles with an automatic transmission, when the ETC function is active the ABS module also signals the TCM to prevent gear shifts.

EBA
EBA assists the driver, in emergency braking situations, by automatically maximizing the braking effort. There are two situations when the ABS module will invoke EBA: when the brake pedal is pressed very suddenly and 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 the 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 the inputs from the stoplamp switch and from the pressure sensor in 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.

EBA 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.

EDC
EDC prevents wheel slip caused by any of the following:
^ A sudden decrease in engine torque when the accelerator is suddenly released.
^ The sudden engagement of the clutch after a downshift on manual transmission vehicles.
^ A downshift using the Command Shift TM on automatic transmission vehicles.

When the ABS module detects the onset of wheel slip without the brakes being applied it signals the ECM on the high speed CAN bus, to request a momentary increase in engine torque.

HDC
HDC uses brake intervention to control vehicle speed and acceleration during low speed descents in off-road and low grip on-road conditions. Generally, equal pressure is applied to all four brakes, but pressure to individual brakes can be modified by the ABS and DSC functions to retain stability. Selection of the HDC function is controlled by the HDC switch on the center console. HDC operates in both high and low ranges, at vehicle speeds up to 50 km/h (31.3 mph).

On manual transmission vehicles, HDC may be used in first and reverse gears in high range and all gears in low range. Once the vehicle is moving, the clutch pedal should be fully released. The vehicle should not be driven with HDC active and the transmission in neutral.

On automatic transmission vehicles, HDC may be used in D, R and Command Shift 1 in high range, and in D, R and all Command Shift gears in low range. When in D, the transmission control module will automatically select the most appropriate gear. The vehicle should not be driven with HDC active and the transmission in N.

HDC can be selected at speeds up to 80 km/h (50 mph), but will only be enabled at speeds below 50 km/h (31.3 mph). When HDC is selected:

At speeds up to 50 km/h (31.3 mph), the HDC information indicator is permanently illuminated if a valid gear is selected and, on manual transmission vehicles, the clutch pedal is not depressed.
^ At speeds from 50 to 80 km/h (31.3 to 50 mph) the HDC information indicator flashes and, on vehicles with the high line instrument cluster, a message advising that the speed is too high is displayed in the message center. If the HDC switch is pressed while vehicle speed is more than 80 km/h (50 mph), the HDC information indicator will not illuminate and HDC will not be selected.
^ If the speed reaches 80 km/h (50 mph) or more, a warning chime sounds, the HDC function is switched off, the information indicator goes off and, on vehicles with the high line instrument cluster, a message advising that HDC has been switched off is displayed in the message center.

When HDC is enabled, the ABS module calculates a target speed and compares this with the actual vehicle speed. The ABS module then operates the HCU, in the active braking mode, as required to achieve and maintain the target speed. During active braking for HDC, the ABS module also energizes the HDC relay to operate the stop lamps. Applying the foot brakes during active braking may result in a pulse through the brake pedal, which is normal.

The target speed varies, between minimum and maximum values for each gear and transmission range, depending on driver inputs through the foot pedals. If the foot pedals are not operated, the ABS module adopts a default target speed.





Low Range Target Speeds





High Range Target Speeds
The target speed is varied between the minimum and maximum values using the accelerator pedal.
The target speed can also be varied by pressing the speed control `+' and `-' buttons (where fitted).
During changes of target speed, the ABS module limits deceleration and acceleration to -0.5 m/s 2 (-1.65 ft/s2 ) and +0.5 m/s 2 (+1.65 ft/s2 ) respectively.
On manual transmission models target speed changes are suspended during gear changes, to prevent unwanted braking when the accelerator pedal is released to change gear. The ABS module determines a gear change is occurring from:
^ Gear position information on the high speed CAN bus.
^ The rate of release of the accelerator pedal.
^ The status of the clutch pedal.

To provide a safe transition from active braking to brakes off, the ABS module invokes a fade out strategy, which gradually discontinues the braking effort, if it detects any of the following during active braking:
^ HDC selected off with the HDC switch.
^ Failure of a component used by HIDC, but not critical to fade out function.
^ Accelerator pedal pressed when transmission is in neutral.
^ Brake overheat.

If fade out is invoked because of deselection or component failure, the HDC function is cancelled by the ABS module. If fade out is invoked because the accelerator pedal is pressed with the transmission in neutral, or because of brake overheat, the HDC function remains in standby and resumes operation when the accelerator pedal is released or the brakes have cooled.

The fade out strategy increases the target speed, at a constant acceleration rate of 0.5 m/s 2 (1.65 ft/s2 until the maximum target speed is reached or until no active braking is required for 0.5 s. If the accelerator pedal is positioned within the range that influences target speed, the acceleration rate is increased to 1.0 m/s 2 (3.3 ft/s2

When fade out is invoked because of component failure, a warning chime sounds and the HDC information indicator is extinguished. The HDC warning indicator is illuminated (low line instrument cluster) or a message advising there is a fault is displayed in the message center (high line instrument cluster).

When fade out is invoked because of brake overheat on vehicles with the high line instrument cluster, a message advising that HDC is temporarily unavailable is displayed. On vehicles with the low line instrument cluster, the HDC warning indicator flashes. At the end of fade out, the HDC information indicator flashes. The flashing indicators and/or message continue while HDC remains selected until the brakes have cooled.

To monitor for brake overheat, the ABS module monitors the amount of braking activity and, from this, estimates the temperature of each brake. If the estimated temperature of any brake exceeds a preset limit, the ABS module invokes the fade out strategy. After the fade out cycle, the HDC function is re-enabled when the ABS module estimates that all of the brake temperatures are at less than 64% of the temperature limit.

ANTI-LOCK CONTROL DIAGRAM








NOTE: A = Hardwired connections, D = High speed CAN bus