Anti-Lock Control - Stability Assist 2 without Hill Descent Control (HDC)

Anti-Lock Control - Stability Assist

COMPONENT LOCATION

NOTE:
Left-Hand Drive (LHD) shown; Right-Hand Drive (RHD) similar.









OVERVIEW
The vehicle ABS and stability control system features a control module, with an integral 4-channel HCU. The integrated ABS module and HCU is located on the rear LH side of the engine compartment, and is installed in the brake hydraulic circuit between the brake master cylinder and the 4 brake calipers.
The ABS module is connected to the high speed Controller Area Network (CAN) bus, and actively interacts with other vehicle system control modules and associated sensors to receive and transmit current vehicle operating information.
When required, the ABS module will actively intervene and operate the HCU during braking or vehicle maneuvers to correct the vehicle attitude, stability, traction or speed. During severe incidents of vehicle correction, the ABS module will also request the Engine Control Module (ECM) to reduce engine power in order to further stabilize and correct the vehicle.
To provide full system functionality, the ABS and DSC systems comprise the following components:
- DSC switch
- 4 wheel speed sensors
- Sensor cluster
- Steering angle sensor
- Instrument cluster indicator lamps
- Integrated ABS module and HCU.
The ABS system also provides brake functions that are designed to assist the vehicle or aid the driver. The following functions are detailed in the section 'Principles of Operation':
- Anti-lock Brake System (ABS)
- Corner Brake Control (CBC)
- Dynamic Stability Control (DSC)
- Electronic Brake Force Distribution (EBD)
- Electronic Traction Control (ETC)
- Emergency Brake Assist (EBA)
- Engine Drag-Torque Control (EDC)
- Roll Stability Control (RSC)
All the brake functions listed, are automatically active when the ignition is in power mode 6 (ignition) and the engine is running.

DYNAMIC STABILITY CONTROL SWITCH
The DSC switch allows the DSC function to be selected off. The DSC switch is non-latching and is installed in the floor console, forward of the gear selector lever. Pressing the DSC switch connects an ignition power feed to the ABS module. With the first press of the DSC switch, the ABS module disables the DSC functions. When the DSC switch is pressed again, the ABS module re-enables the DSC functions. The DSC switch must be pressed for a minimum of 0.3 second for the ABS module to react. The DSC function is re-enabled at the beginning of each ignition cycle.
To confirm that the DSC function is selected off, the amber colored DSC warning indicator lamp located in the instrument cluster, is continuously illuminated. On vehicles installed with a high-line instrument cluster, a message is also displayed in the message center to confirm that DSC is selected off.
For additional information, refer to Instrument Cluster Description and Operation
For additional information, refer to Information and Message Center Description and Operation
When the DSC function is selected on, the DSC warning indicator lamp is extinguished.
Although Land Rover recommend that DSC is selected on for all normal driving conditions, it may be beneficial to de-select DSC in order to maximize traction under the following conditions:
- To rock the vehicle out of a hollow or a soft surface.
- When driving on loose surfaces or with snow chains installed.
- When driving in deep sand, snow or mud.
- When driving on tracks with deep longitudinal ruts.
Even when DSC is deselected, driving maneuvers with extreme yaw or lateral acceleration may trigger RSC activity to assist the vehicle stability.
To prevent mis-use of, or in the event of a broken DSC switch, a Diagnostic Trouble Code (DTC) is stored in the ABS module memory if the input from the DSC switch is held high for more than 1 minute.

WHEEL SPEED SENSORS









An active wheel speed sensor is installed in each wheel knuckle, and provides the ABS module with a rotational speed signal from each road wheel. The head of each wheel speed sensor is positioned close to a magnetic encoder incorporated into the seal of the wheel bearing. Each front axle bearing encoder contains 44 north and south poles; each rear axle bearing encoder contains 48 pole pairs. A fly lead connects each sensor to the vehicle harness.
The wheel speed sensor is supplied with a power supply and a signal connection from the ABS module. When the ignition switch is in power mode 6 (ignition), the ABS module supplies power to the wheel speed sensors and monitors the return signals. Rotation of the wheels induces current fluctuations in the speed sensor return signals. The ABS module subsequently converts the return signals into individual wheel speeds, and the overall vehicle speed.
The ABS module outputs the individual wheel speeds, and vehicle speed on the high speed CAN bus for use by other systems. The quality of the vehicle speed signal is also broadcast on the high speed CAN bus. If all wheel speed signals are available to calculate vehicle speed from, the quality of the vehicle speed signal is set to 'data calculated within specified accuracy'. If one or more wheel speed sensors are inoperative, the quality of the vehicle speed signal is set to 'accuracy outside specification'.
The ABS module monitors the wheel speed sensor circuits for faults. If a fault is detected the ABS module stores a related DTC in memory and illuminates the appropriate warning indicator lamps, depending on the system functions affected (DSC/ETC, ABS, EBA/EBD). A warning chime is also sounded to alert the driver to the fault condition.
For additional information, refer to Instrument Cluster Description and Operation
For additional information, refer to Information and Message Center Description and Operation
As the wheel speed sensors are active devices, a return signal is available when the road wheels are not rotating. This enables the ABS module to check the condition of the speed sensors while the vehicle is stationary.

SENSOR CLUSTER









The sensor cluster is installed beneath the center console and is secured to the transmission tunnel with 2 studs and nuts. The sensor cluster is a compact unit that provides the ABS module with inputs of yaw rate, roll rate, longitudinal and lateral acceleration. The ABS module broadcasts the input values on the high speed CAN bus for use by other systems.
When the ignition is in power mode 6 (ignition), the sensor cluster receives an ignition power feed from the ABS module. The sensor cluster is also connected to the ABS module via a private CAN bus.
The sensor cluster is diagnosed by the ABS module. If a sensor fault is detected the ABS module stores a related DTC in memory and illuminates the appropriate warning indicator lamps, depending on the system functions affected (DSC/ETC, ABS, EBA/EBD). A warning chime is also sounded to alert the driver to the fault condition.
For additional information, refer to Instrument Cluster Description and Operation
For additional information, refer to Information and Message Center Description and Operation

STEERING WHEEL MODULE









The steering wheel module contains the steering angle sensor, clockspring and steering column multifunction switches to form an integral component. The steering angle sensor is connected directly to the clockspring.
The steering wheel module is mounted to the upper steering column with 2 screws, and receives an electrical supply from the main harness via a 4-pin connector located on the side of the unit. The module provides the location and electrical connection for the 2 multifunction switches.
Input signals from the steering angle sensor are received and processed by the steering wheel module to calculate the steering wheel angle, and steering wheel angle speed. The information is transmitted on the high speed CAN bus for use by the ABS module.

Steering Angle Sensor
The steering angle sensor is connected to the steering wheel with 2 dowels. The steering angle sensor monitors steering wheel rotation, and provides information of the steering wheel angle and rate of speed of steering wheel movements.
A code wheel and 16 optical digital sensors are installed inside the steering wheel angle sensor. Rotation of the code wheel is read by the optical-digital sensors to produce steering wheel rotational speed signals. The steering angle sensor is able to measure a rotation range of +/- 720 degrees, although the steering mechanism will only allow the steering wheel to rotate a maximum of +/-540 degrees.
Signals from the steering angle sensor are transmitted on the vehicle high speed CAN bus, and received and processed by other systems such as DSC. The transmitted information includes details of steering wheel angle and steering wheel rotational speed, along with signal integrity information.
If a fault occurs within the steering angle sensor, a DTC will be set and stored in the steering angle sensor memory. The steering angle sensor fault is also stored in the ABS module memory that illuminates the appropriate warning indicator lamps, depending on the system functions affected (DSC/ETC, ABS, EBA/EBD). A warning chime is also sounded to alert the driver to the fault condition.
For additional information, refer to Instrument Cluster Description and Operation
For additional information, refer to Information and Message Center Description and Operation
The steering angle sensor and ABS module are able to be interrogated using the Land Rover approved diagnostic equipment.

Clockspring
The clockspring is a rotary coupling that provides the electrical connection between the fixed steering wheel module, and the steering wheel mounted controls and Supplemental Restraint System (SRS) driver's airbag.
A colored indicator is provided within the clockspring and is visible through a transparent perspex cover when the steering wheel is removed, and the steering angle sensor is aligned to the central position. The indicator makes sure the steering angle sensor and steering system are correctly aligned following repairs to the steering mechanism.

Service Information
Before separating the steering components from the connecting steering column, the clockspring must be aligned to the center position (indicator visible) and the wheels pointing straight ahead. The clockspring is retained in the central position with a locking screw.

INSTRUMENT CLUSTER INDICATOR LAMPS









The instrument cluster contains 2 types of indicator lamps to display the operating status of the selected anti-lock control - stability assist functions. The indicator lamps provide a visual notification of either a system warning or information indication to the driver. There are 4 indicator lamps on vehicles with a high-line instrument cluster; 5 indicator lamps on vehicles with a low-line instrument cluster.
The following anti-lock control - stability assist indicator lamps are installed in the instrument cluster:
- ABS warning indicator lamp
- Brake warning indicator lamp
- DSC warning indicator lamp
For additional information, refer to Instrument Cluster Description and Operation

ANTI-LOCK BRAKE SYSTEM MODULE
The ABS module controls the brake functions by operating the HCU to modulate hydraulic pressure to the individual wheel brakes.
The ABS module is attached to the HCU and forms an integral component. A 47-pin connector provides the electrical interface between the ABS module and the vehicle wiring. The ABS module may be interrogated using the Land Rover approved diagnostic equipment.

HYDRAULIC CONTROL UNIT
The HCU is a 4 channel unit that modulates the supply of hydraulic pressure to the brakes, under the control of the ABS module.
The master cylinder primary and secondary circuit outlets are connected to the HCU primary and secondary circuits. For additional information, refer to Hydraulic Brake Actuation Description and Operation Each of the HCU circuits contains the following components to control the supply of hydraulic pressure to the brakes:
- A normally open, solenoid-operated pilot valve, to enable active braking.
- A normally closed, solenoid-operated priming valve, to connect the brake fluid reservoir to the dual circuit hydraulic pump during active braking.
- A hydraulic pump, to generate hydraulic pressure for active braking and return brake fluid to the reservoir.
- Normally open, solenoid-operated inlet valves and normally closed, solenoid-operated outlet valves, to modulate the hydraulic pressure in the individual brakes.
- An accumulator and a relief valve, to allow the fast release of pressure from the brakes.
- Filters, to protect the internal components from contamination.
The primary circuit also incorporates a pressure sensor to provide the ABS module with a hydraulic pressure signal.
Contact pins on the HCU mate with contacts on the ABS module to provide the electrical connections from the ABS module to the dual circuit hydraulic pump motor, and the pressure sensor. The solenoids that operate the valves are installed within the ABS module.

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.
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.
- 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.

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.