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Home >> Land Rover >> 2012 >> Range Rover (LM) V8-5.0L >> Repair and Diagnosis >> Brakes and Traction Control >> Antilock Brakes / Traction Control Systems >> Description and Operation >> Part 2

Part 2





Anti-Lock Control - Traction Control

CONTROL DIAGRAM

NOTE:
A = Hardwired; D = High speed CAN (controller area network) bus; J = Diagnostic ISO 9141 K line; V = Private CAN (controller area network) bus.









OPERATION

Anti-lock Brake System
ABS (anti-lock brake system) 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 (anti-lock brake system) 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 (anti-lock brake system) 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 (anti-lock brake system) 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 (anti-lock brake system) 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 (anti-lock brake system) 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 (anti-lock brake system) module intervenes by applying the appropriate brakes. When the DSC function is active, the ABS (anti-lock brake system) module also signals the TCM (transmission control module) to prevent gear shifts, and the instrument cluster to flash the DSC warning indicator.
If necessary, the ABS (anti-lock brake system) module also signals:
- The ECM (engine control module), 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 Description and Operation

Electronic Brake Force Distribution
EBD (electronic brake force distribution) 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 (electronic brake force distribution) uses the ABS (anti-lock brake system) hardware to automatically optimize the pressure of the rear brakes, below the point where ABS (anti-lock brake system) intervention is normally invoked. Only the rear wheels are under EBD (electronic brake force distribution) 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/in2) in anticipation of the brakes being applied. This gives a quicker brake pedal response and consequently slightly shorter stopping distances.
When the ABS (anti-lock brake system) module detects rapid throttle lift off (from the signals received from the ECM (engine control module) over the high speed CAN (controller area network) bus), it controls the HCU (hydraulic control unit) 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 (anti-lock brake system) module also transmits a high speed CAN (controller area network) bus message to the ECM (engine control module) 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 (anti-lock brake system) module also signals the TCM (transmission control module) to prevent gear shifts.

Emergency Brake Assist
EBA (emergency brake assist) assists the driver during emergency braking situations by automatically maximizing the braking effort. There are two situations when the ABS (anti-lock brake system) module will invoke EBA (emergency brake assist):
- When the brake pedal is pressed very suddenly.
- When the brake pedal is pressed hard enough to bring the front brakes into ABS (anti-lock brake system) operation.
When the brake pedal is pressed very suddenly, the ABS (anti-lock brake system) module increases the hydraulic pressure to all of the brakes until the threshold for ABS (anti-lock brake system) operation is reached. This applies the maximum braking effort for the available traction. The ABS (anti-lock brake system) module monitors for the sudden application of the brakes using the stoplamp switch status broadcast on the high speed CAN (controller area network) by the ECM (engine control module), and from the pressure sensor within the HCU (hydraulic control unit). With the brake pedal pressed, if the rate of increase of hydraulic pressure exceeds the predetermined limit, the ABS (anti-lock brake system) module invokes emergency braking.
When the brake pedal is pressed hard enough to bring the front brakes into ABS (anti-lock brake system) operation, the ABS (anti-lock brake system) module increases the hydraulic pressure to the rear brakes up to the ABS (anti-lock brake system) threshold.
EBA (emergency brake assist) operation continues until the driver releases the brake pedal enough for the hydraulic pressure in the HCU (hydraulic control unit) to fall below a pre-determined threshold value stored in the ABS (anti-lock brake system) 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 (anti-lock brake system) module detects the onset of wheel slip without the brakes being applied, it transmits a message to the ECM (engine control module) via the high speed CAN (controller area network) 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 (anti-lock brake system) module detects the onset of understeer, it signals the ECM (engine control module) via the high speed CAN (controller area network) bus to request a decrease in engine torque. If required the ABS (anti-lock brake system) module will control the HCU (hydraulic control unit) 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 dependant 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 re-establishing 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 (anti-lock brake system) 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 (transmission control module) 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 (anti-lock brake system) module calculates a target deceleration value by comparing the set speed to the actual vehicle speed. The ABS (anti-lock brake system) module then operates the HCU (hydraulic control unit) in the active braking mode as required to achieve and maintain the target speed.
During active braking for HDC, the ABS (anti-lock brake system) module sends a high speed CAN (controller area network) message to the CJB (central junction box) to operate the stoplamps. For additional information, refer to Exterior Lighting 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 (anti-lock brake system) module adopts a default target speed.









As well as varying the target speed with the accelerator pedal, the target speed may also be varied by pressing the speed control '+' and '-' buttons (where fitted). For additional information, refer to Speed Control
For additional information, refer to Speed Control
During changes of target speed, the ABS (anti-lock brake system) module limits deceleration and acceleration to -0.5 m/s2 (-1.65 ft/s2) and +0.5 m/s2 (+1.65 ft/s2) respectively.
To provide a safe transition from active braking to brakes off, the ABS (anti-lock brake system) module invokes a fade out strategy that gradually releases the braking effort during active braking. The fade out strategy occurs if any of the following conditions are detected during active braking:
- HDC selected off with the HDC switch.
- Failure of a component used by HDC, but not critical to the 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 (anti-lock brake system) 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/s2 (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/s2 (3.3 ft/s2).
When fade out is invoked because of component failure, a warning chime is sounded, the HDC information warning indicator is extinguished and a message advising there is a fault is displayed in the message center.
When fade out is invoked because of brake overheat, a message advising that HDC is temporarily unavailable is displayed. At the end of fade out, the HDC information warning indicator flashes. The message is displayed, while HDC remains selected, until the brakes have cooled.
To monitor for brake overheat, the ABS (anti-lock brake system) 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 (anti-lock brake system) module invokes the fade out strategy. After the fade out cycle, the HDC function is re-enabled when the ABS (anti-lock brake system) module estimates that all of the brake temperatures are at less than 64% of the temperature limit.
When HDC is selected off, the instrument cluster message center displays a system off message.

Hill Start Assist
Hill start assist is an automatic feature that operates in a similar manner to gradient release control, but does not require HDC to be active. The feature is not driver selectable and there is no indication to the driver when in operation.
On steep slopes, hill start assist will hold a portion of the driver generated brake pressure for a short time to allow the driver to move their foot from the brake pedal to the throttle pedal without the vehicle rolling back.
The system will release the brake pressure in a controlled manner, either after the timer has expired or if the driver has generated sufficient drive-torque to move the vehicle forward up the hill.
The timer function is controlled by the ECM (engine control module) and transmitted to the ABS (anti-lock brake system) module on the high speed CAN (controller area network) bus.

Trailer Stability Assist
When the trailer electrical socket is connected, trailer stability assist operates automatically to enhance the existing DSC and terrain response functions of the vehicle when towing. The system detects sway movements caused by trailer oscillations at speeds in excess of 60 km/h (37 mph) and acts to eliminate them. It does this through braking and engine management. Braking management counterbalances the sway movement through symmetric and asymmetric braking, thereby slowing the vehicle and eliminating the oscillations. Engine management adapts engine torque output to support the braking management in stabilizing the vehicle and trailer.
Typical conditions when sway can occur include:
- Changing highway lanes.
- Traversing a lengthy bend.
- Acceleration.
- Braking.
The capability of trailer stability assist to respond early to the beginning of trailer-sway makes the system almost unnoticeable under normal driving conditions and keeps the vehicle and trailer under safe control. Trailer stability assist requires no input from the driver and operates up to the maximum vehicle speed.
Trailer stability assist will not operate while DSC is switched off.
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