Schematic Of Hydraulic Control Unit: Operation

ANTI-LOCK BRAKE SYSTEM

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 in order 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.

ACTIVE ROLL MITIGATION

The ARM function uses the brake system and engine torque control to attempt to restore vehicle stability if the vehicle is forced into such a harsh maneuver that it risks tipping over.

The ABS module monitors driver inputs and vehicle behavior using various powertrain signals and inputs from the wheel speed sensors, steering angle sensor and the yaw-rate and lateral acceleration sensor. These are compared with modeled behavior and, if vehicle behavior reaches a given risk level, the ABS module initiates a reduction in engine power, or brakes one or more wheels sufficiently to correct the vehicle stability and assist the driver remain in control.

While the ignition is on, ARM is permanently enabled, even when DSC is selected off.

CORNER BRAKE CONTROL

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.

DYNAMIC STABILITY CONTROL

DSC uses the brakes and engine 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 that 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 engine 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. When the DSC function is active, the ABS module also signals the TCM to prevent gear shifts, and the instrument cluster to flash the DSC warning indicator.

If necessary, the ABS module also signals:

The ECM, to reduce engine torque.
The transfer box control module, to adjust the locking torque of the center differential.
The active on-demand coupling module, to adjust the locking torque of the rear differential.

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

If DSC is selected off, a DSC system off message is displayed in the message center.

For additional information, refer to: Information and Message Center .

ELECTRONIC BRAKE FORCE DISTRIBUTION

EBD limits the brake pressure applied to the rear wheels. When the brakes are applied, the weight transfer of the vehicle reduces the adhesion of the rear wheels on the road surface. This may cause the rear wheels to slip and make the vehicle unstable.

EBD uses the ABS hardware to automatically optimize the pressure of the rear brakes, below the point where ABS intervention is normally invoked. Only the rear wheels are under EBD control.

ELECTRONIC BRAKE PREFILL

Electronic brake prefill senses any rapid throttle lift off, activating a small brake hydraulic pressure build-up of approximately 3 to 5 bar (43.5 to 72.5 lbf/in?² ) in anticipation of the brakes being applied. This gives a quicker brake pedal response and consequently slightly shorter stopping distances.

When the ABS module detects rapid throttle lift off (from the signals received from the ECM over the high speed CAN bus), it controls the HCU to apply a low brake pressure to assist in a quicker brake application.

ELECTRONIC TRACTION CONTROL

ETC attempts to optimize forward traction by reducing engine torque or braking a spinning wheel until traction is restored.

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 transmits a high speed CAN bus message to the ECM requesting 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; a fast response requests an ignition cut-off.

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

When the ETC function is active the ABS module also signals the TCM to prevent gear shifts.

EMERGENCY BRAKE ASSIST

EBA assists the driver during 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.
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 the threshold for ABS operation is reached. This applies the maximum braking effort for the available traction. The ABS module monitors for the sudden application of the brakes using the stoplamp switch status broadcast on the high speed CAN by the ECM, and from the 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.

EBA operation continues until the driver releases the brake pedal enough for the hydraulic pressure in the HCU to fall below a predetermined threshold value stored in the ABS module.

ENGINE DRAG-TORQUE CONTROL

EDC prevents wheel slip caused by any of the following:

A sudden decrease in engine torque when the accelerator is suddenly released.
A downshift using the CommandShift™.

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

ENHANCED UNDERSTEER CONTROL

Understeer logic control monitors the vehicle for understeer by comparing signals from the yaw rate and lateral acceleration sensor with signals from the steering angle sensor and wheel speed sensors.

When the ABS module detects the onset of understeer, it signals the ECM via the high speed CAN bus to request a decrease in engine torque. If required the ABS module will control the HCU to apply brake pressure to the inside rear wheel to correct the understeer. If the vehicle continues to understeer, enhanced understeer control is activated and uses multiple brakes (maximum of three brakes) to rapidly reduce the vehicle speed.

GRADIENT ACCELERATION CONTROL

Gradient acceleration control is an automatic feature and is always available when HDC is not selected.

When HDC is not selected, gradient acceleration control will intervene to limit downhill acceleration on a steep descent.

The feature uses generated brake pressure to control acceleration in situations where the driver could lose control of the vehicle on a steep incline.

Gradient acceleration control keeps the vehicle to a speed and throttle pedal dependent acceleration limit when the vehicle is moving in the intended direction of travel, for example:

Descending an incline forwards, with D (drive) selected.
Descending an incline backwards, with R (reverse) selected.

When the vehicle is moving against the intended direction of travel, for example: descending a slope, but facing uphill with D selected, gradient acceleration control will prevent the vehicle accelerating above 5 km/h (3 mph) for up to 20-30 seconds to aid the driver in reestablishing control of the vehicle.

GRADIENT RELEASE CONTROL

Gradient release control is an automatic feature which is always available when HDC is selected.

If the vehicle is brought to a standstill on a slope using the foot brake, gradient release control will become active (except in the terrain response, sand program). Subsequently, when the foot brake is released gradient release control will automatically delay and graduate the brake release. This allows time for the foot to be moved from the brake pedal to the accelerator pedal so that the vehicle can move smoothly away.

When descending a hill, a similar brake hold and gradual release is employed to provide a smooth transition into HDC. Gradient release control operates in forward and reverse gears and requires no driver intervention.

HILL DESCENT CONTROL

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 and the terrain response rotary control located on the floor console. HDC operates in both high and low ranges, at vehicle speeds up to 50 km/h (31.3 mph).

WARNING: Incorrect use of the HDC function may compromise the stability of the vehicle, resulting in a dangerous and uncontrolled hill descent. Driving with the transmission in neutral while HDC is active will prevent engine braking from assisting the vehicle. The brakes will overheat and induce the HDC fade out strategy. In this condition there will be no control over the vehicle during a descent.

NOTE: With the HDC function selected, HDC is operative even when the transmission is in the neutral. It is not recommended to drive the vehicle further than is absolutely necessary with HDC selected and the transmission in neutral.

HDC may be used in D, R and CommandShift 1 in high range, and in D, R and all CommandShift gears in low range. When in D, the TCM will automatically select the most appropriate gear.

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 warning indicator is permanently illuminated if a valid gear is selected.
At speeds from between 50 to 80 km/h (31 to 50 mph) the HDC information warning indicator flashes and 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 warning indicator will not illuminate and HDC will not be selected.
If the speed increases to 80 km/h (50 mph), the HDC function is switched off, the information warning indicator is extinguished, a warning chime sounds and 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 deceleration value by comparing the set speed to 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 sends a high speed CAN message to the CJB to operate the stoplamps.

For additional information, refer to: Exterior Lighting .

Applying the foot brake during active braking may result in a pulse through the brake pedal.

The target speed varies between minimum and maximum values for each gear and transmission range, depending on driver input with the accelerator pedal. If the accelerator pedal is not operated, the ABS module adopts a default target speed.