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Home >> Land Rover >> 2011 >> LR2 (LF) L6-3.2L >> Repair and Diagnosis >> Brakes and Traction Control >> Antilock Brakes / Traction Control Systems >> Description and Operation >> Anti-Lock Control - Stability Assist >> Anti-Lock Control - Stability Assist with Hill Descent Control (HDC) >> Part 2

Part 2





Anti-Lock Control - Stability Assist

HCU Schematic Diagram









The HCU features 3 operating modes:
- Normal braking/ EBD
- ABS braking
- Active braking.

Normal Braking/EBD Mode
Initially, all of the solenoid-operated valves are de-energized. Operating the brake pedal produces a corresponding increase or decrease of pressure in the brakes, through the open pilot valves and inlet valves. If the ABS module determines that EBD is necessary, it energizes the inlet valves for both the rear brakes, to isolate the brakes from any further increase in hydraulic pressure.

NOTE:
Only the rear brakes are controlled by the EBD function.


ABS Braking Mode
If the ABS module determines that ABS braking is necessary, it energizes the inlet and outlet valves of the related brake and starts the hydraulic return pump. The inlet valve closes to isolate the brake from pressurized fluid; the outlet valve opens to release pressure from the brake into the accumulator, and the return pump circuit. The reduced hydraulic pressure allows the wheel to accelerate. The ABS module then operates the inlet and outlet valves to modulate the pressure in the brake to apply the maximum braking effort without locking the wheel. Control of the valves for each wheel takes place individually.

Active Braking Mode
The active braking mode is used to generate and control hydraulic pressure to the brakes for functions other than Normal and ABS braking, for example RSC, DSC, EBA, ETC, HDC.
For active braking, the ABS module energizes the pilot valves and priming valves, starts the return pump and energizes all of the inlet valves. Brake fluid, drawn from the reservoir through the master cylinder and priming valve, is pressurized by the return pump and supplied to the inlet valves. The ABS module then operates the inlet valves and outlet valves, as required, to modulate the pressure in the individual brakes. Some noise may be generated during active braking.

Service Information
The ABS module comprises an HCU and an Electronic Control Unit (ECU) that must not be separated. The ABS module and HCU assembly is supplied as a single component and arrives in a pre-filled state.

NOTE:
The ABS module, HCU and the sensor cluster are fragile components and must be discarded if dropped or damaged.


CONTROL DIAGRAM

NOTE:
A = Hardwired; D = High Speed CAN bus; N = Medium Speed CAN bus; U = Private CAN bus









PRINCIPLES OF OPERATION

Anti-Lock Brake System
ABS controls the speed of all road wheels to ensure optimum wheel slip when braking at the adhesion limit. The wheels are prevented from locking to retain effective steering control of the vehicle.
The front brake pressures are modulated separately for each wheel. The rear brake pressures are 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.

Corner Brake Control
CBC influences the brake pressures, below and within 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 brakes and powertrain torque control to assist in maintaining the lateral stability of the vehicle. While the ignition is energized the DSC function is permanently enabled, unless selected off using the DSC switch. Even if DSC is deselected, driving maneuvers with extreme yaw or lateral acceleration may trigger RSC activity to assist the vehicle stability.
DSC enhances driving safety in abrupt maneuvers and in under-steer or over-steer situations that may occur in a bend. The ABS module monitors the yaw rate and lateral acceleration of the vehicle, steering input, and then selectively applies individual brakes and signals for powertrain torque adjustments to reduce under-steer or over-steer.

In general:
- In an under-steer situation; the inner wheels are braked to counteract the yaw movement towards the outer edge of the bend.
- In an over-steer 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. Whenever the tracking stability deviates from the target data, the ABS module intervenes by applying the appropriate brakes.
The following interactions occur in an intervention situation:
- High speed CAN signal to the ECM, to reduce engine torque.
- High speed CAN signal to the Active On-demand Coupling module, to open the locking torque of the center coupling.
- Application of braking to the appropriate corner of the vehicle.

Electronic Brake Force Distribution
EBD limits the brake pressure applied to the rear wheels. When the brakes are applied, the weight of the vehicle transfers forwards, reducing the ability of the rear wheels to transfer braking effort to the road surface. This may 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 ABS is normally invoked.

NOTE:
Only the rear brakes are controlled by the EBD function.


Electronic Traction Control
ETC attempts to optimize forward traction by reducing engine torque, or by applying the brake of a spinning wheel until traction is regained.
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 brake is applied to the spinning wheel, 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.
When the DSC function is selected off using the DSC switch, the engine torque reduction feature is disabled.

Emergency Brake Assist
EBA assists the driver in emergency braking situations by automatically maximizing the applied braking effort. There are two situations when the ABS module will invoke EBA:
- When the brake pedal is rapidly pressed.
- When the brake pedal is pressed hard enough to bring the front brakes into ABS operation.
When the brake pedal is rapidly pressed, the ABS module increases the hydraulic pressure to all of the brakes until the threshold for ABS operation is reached. This action applies the maximum braking effort for the available traction. The ABS module monitors for the sudden application of the brakes, using inputs from the brake pedal switch 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, sufficiently for the hydraulic pressure in the HCU to drop below a 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.
- The sudden engagement of the clutch after a downshift on manual transmission vehicles.
- A downshift using the CommandShift function on automatic transmission vehicles.
When the ABS module detects the onset of wheel slip without the brakes being applied, the ABS module signals the ECM via the high speed CAN bus to request a momentary increase in engine torque.

Hill Descent Control
HDC uses engine braking and brake intervention to control the vehicle speed and acceleration during low speed off-road descents, and in low grip on-road conditions. Generally, equal pressure is applied to all 4 brakes, but pressure to individual brakes may be modified by the ABS and DSC functions to retain the vehicles stability. Selection of the HDC function is controlled by the HDC switch located on the floor console. HDC operates at vehicle speeds up to 50 km/h (31 mph).

WARNING: Incorrect use of the HDC function may compromise the stability of the vehicle, resulting in a dangerous and uncontrolled hill descent. Pressing the clutch pedal and/or 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 clutch pedal is pressed or the transmission is in the 'Neutral' position. It is not recommended to drive the vehicle further than is absolutely necessary with HDC selected, and the clutch pedal pressed/ or the transmission in 'Neutral'.
On manual transmission vehicles, HDC may be used in first and reverse gears only. Once the vehicle is moving, the clutch pedal is to be fully released. The vehicle is not recommended to be driven with HDC active and the transmission in 'Neutral'.
On automatic transmission vehicles, HDC may be used in 'D' drive, 'R' reverse and CommandShift '1' only. When in 'D', the TCM will automatically select the most appropriate gear. The vehicle is not recommended to be driven with HDC active and the transmission in 'Neutral'.
HDC is able to be selected at speeds up to 80 km/h (50 mph), but will only be enabled at speeds below 50 km/h (31 mph).

When HDC is selected:
- At speeds up to 50 km/h (31 mph), the HDC information indicator lamp is permanently illuminated if a valid gear is selected.
- At speeds between 50 to 80 km/h (31 to 50 mph), the HDC information indicator lamp 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 the vehicle speed is greater than 80 km/h (50 mph), the HDC information indicator lamp will not illuminate and HDC will not be selected.
- If the speed reaches 80 km/h (50 mph) or more, a warning chime is sounded, the HDC function is switched off, the information indicator lamp is extinguished and, on vehicles with the high line instrument cluster, a message advising that HDC has been switched off is displayed in the message center.
For additional information, refer to Instrument Cluster Initial Inspection and Diagnostic Overview
For additional information, refer to Information and Message Center Description and Operation
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. Operation of the vehicle stoplamps during HDC is controlled by the CJB.
Applying the foot brake during active braking may result in a pulse being felt through the brake pedal.
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:





The target speed is varied between the minimum and maximum values using the accelerator pedal, or by pressing the speed control '+' and '-' buttons (dependant on vehicle specification). The HDC Target speed will also vary depending on the Terrain Response mode selected.
During changes of target speed, the ABS module limits vehicle deceleration and acceleration to -0.5 meters per second squared (m/s2) (-1.64 feet per second squared (ft/s2)); and +0.5 m/s2 (+1.64 ft/s2) respectively.
To provide a safe transition from active braking to brakes off, the ABS 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 is detected during active braking:
- HDC is selected off using the HDC switch.
- Failure of a component used by HDC, but not critical to fade out function.
- Accelerator pedal pressed when transmission is in neutral.
- Brakes overheat.
If fade out is invoked because of HDC de-selection 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/s2 (1.64 ft/s2), until the maximum target speed is reached, or until no active braking is required for 0.5 second. 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 and the HDC information indicator lamp is extinguished. The HDC warning indicator lamp is then illuminated (on low-line instrument cluster), or a message advising of the fault is displayed in the message center (on high-line instrument cluster).
For additional information, refer to Instrument Cluster Initial Inspection and Diagnostic Overview
For additional information, refer to Information and Message Center Description and Operation
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 lamp flashes. At the end of fade out, the HDC information indicator flashes. The flashing warning and information indicator lamps, and/or displayed message continue while HDC remains selected, until the brakes have cooled.
For additional information, refer to Instrument Cluster Initial Inspection and Diagnostic Overview
For additional information, refer to Information and Message Center Description and Operation
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 preset temperature limit.

Roll Stability Control
The RSC function uses the brakes and the engine to attempt to restore vehicle stability, if the vehicle is forced into a harsh maneuver that risks it tipping over.
The ABS module monitors driver inputs and vehicle behavior using various powertrain signals and the inputs from the wheel speed sensors, the steering angle sensor, the roll rate sensor, yaw rate and lateral acceleration sensors. These inputs are compared with modeled behavior and, if the vehicle behavior reaches a given risk level, the ABS module cuts the engine power, or brakes one or more wheels sufficiently to allow the vehicle to regain stability and help the driver remain in control.
While the ignition is energized, RSC is permanently enabled even if the DSC function is selected off.

Terrain Response System Integration
The Terrain Response function integrates the ABS and other vehicle system control modules to assist the vehicle when driving off-road or during difficult surface conditions. Terrain Response is activated when the rotary control switch is set to a Terrain Response special program.
When a Terrain Response special program is activated, the ABS module along with other vehicle system control modules will operate in accordance with programmed software maps. The software maps allow the ABS system to function with a threshold that will assist the selected Terrain Response special program. For additional information, refer to Ride and Handling Optimization Ride and Handling Optimization
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