Antilock Brakes / Traction Control Systems: Description and Operation: Description and Operation

Anti-Lock Control - Traction Control








COMPONENT LOCATIONS - SHEET 1 OF 2








COMPONENT LOCATIONS - SHEET 2 OF 2

GENERAL
The anti-lock control - traction control system is based on the 4 channel Bosch 8.0 system and provides the following brake functions:
^ Anti-lock Braking 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).
^ Hill Descent Control (HDC).
The system consists of the following components:
^ A DSC switch.
^ An HDC switch.
^ An HDC relay.
^ A stoplamp switch.
^ Four wheel speed sensors.
^ A yaw rate and lateral acceleration sensor.
^ A steering angle sensor.
^ Warning indicators, four on vehicles with a high line instrument cluster and five on vehicles with a low line instrument cluster.
^ A Hydraulic Control Unit (HCU) with attached ABS module.

DSC SWITCH
The DSC switch allows the DSC function to be selected off. Although Land Rover recommend that DSC is selected on for all normal driving conditions, it may be beneficial to de-select DSC, to maximize traction, under the following conditions:
^ If the vehicle needs to be rocked out of a hollow or a soft surface.
^ Driving on loose surfaces or with snow chains.
^ Driving in deep sand, snow or mud.
^ On tracks with deep longitudinal ruts.

The DSC switch is a non-latching switch installed in the center switch pack on the instrument panel. 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 s for the ABS module to react. The DSC function is re-enabled at the beginning of each ignition cycle.

The status of the DSC switch selection is shown by the DSC warning indicator. The DSC warning indicator is off while DSC is selected on, and continuously illuminated while DSC is selected off.

A DSC switch request to disable DSC is ignored if the air suspension system has failed, or is in off-road height at speeds above 60 km/h (37.5 mph).

To guard against incorrect operation or a broken switch, if the input from the DSC switch is held high for more than one minute, a failure is stored in the ABS module.

Even if DSC is deselected, driving maneuvers with extreme yaw or lateral acceleration may trigger DSC activity to assist vehicle stability.

HDC SWITCH
The HDC switch controls the selection of the HIDC function.

The HDC switch is a non-latching switch installed on the center console, to the rear of the gear shift lever. Pressing and releasing the HIDC switch momentarily connects an ignition power feed to the ABS module. With the first press and release of the HIDC switch, the ABS module enables operation of the HIDC function. When the HDC switch is pressed and released again, the ABS module disables operation of the HIDC function.

To guard against incorrect operation or a broken switch, if the switch is pressed for more than 10 seconds no change of state occurs. If the input from the HIDC switch is held high for more than one minute, a failure is stored in the ABS module.

HDC RELAY
The HDC relay is used to illuminate the stoplamps when the brakes are activated during HIDC operation and during dynamic application of the parking brake.

The HDC relay is a non-serviceable, solid state relay on the circuit board of the Central Junction Box (CJB). Operation of the HIDC relay is controlled by the ABS module switching the coil to ground. The ABS module monitors brake system hydraulic pressure and energizes the HIDC relay during active braking. A pressure threshold and time filter prevent the stoplamps from flickering when HIDC is braking.

STOPLAMP SWITCH
The stoplamp switch is mounted in the brake pedal bracket and operated by the brake pedal. The stoplamp switch is a two pole switch: The Brake Switch (BS) pole supplies a brake pedal status signal to the ABS module, the Brake Lamp Switch (BLS) pole operates the stoplamps and also supplies a brake pedal status signal to the ABS module and to the Engine Control Module (ECM).

While the brake pedal is released:
^ The BS contacts are closed, and connect an ignition power feed from the Central Junction Box (CJB) to the ABS module.
^ The BLS contacts are open.
When the brake pedal is pressed:
^ The BS contacts open.
^ The BLS contacts close, and connect an ignition power feed from the CJB to the three stoplamps, the ABS module and the ECM.

The ABS module monitors the status inputs from the stoplamp switch and broadcasts the brake pedal status and an associated quality factor on the high speed Controller Area Network (CAN) bus.

WHEEL SPEED SENSORS








An active wheel speed sensor is installed in each wheel hub to provide the ABS module with a rotational speed signal from each road wheel. The head of each wheel speed sensor is positioned close to a 48 tooth sensor ring on the outer diameter of the constant velocity joint of the halfshaft. A flying lead connects each sensor to the vehicle wiring.

The wheel speed sensors each have a power supply connection and a signal connection with the ABS module. When the ignition switch is in position 11, the ABS module supplies power to the wheel speed sensors and monitors the return signals. Any rotation of the halfshafts induces current fluctuations in the return signals which are converted into individual wheel speeds and the overall vehicle speed by the ABS module.

The ABS module outputs the individual wheel speeds and the 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 is faulty, 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 fault code and illuminates the appropriate warning indicators, depending on the system functions affected (DSC/ETC, ABS, EBA/EBD, HDC). A warning chime sounds and, on vehicles with the high line instrument cluster, a related message is shown in the message center.

Since the wheel speed sensors are active devices, a return signal is available when the road wheels are not turning, which enables the ABS module to check the sensors while the vehicle is stationary. In addition, the direction of travel of each wheel can be sensed. This information is broadcast on the high sped CAN bus for use by other systems.

YAW RATE AND LATERAL ACCELERATION SENSOR
The yaw rate and lateral acceleration sensor provides the ABS module with inputs of yaw rate and lateral acceleration.

The yaw rate and lateral acceleration sensor is installed under the center console and secured to the transmission tunnel with two bolts.

When the ignition switch is in position 11, the yaw rate and lateral acceleration sensor receives an ignition power feed from the CJB. The sensor is self diagnosed by the ABS module and can be interrogated using T4. The ABS module broadcasts the yaw rate and lateral acceleration values, on the high speed CAN bus, for use by other systems.

The ABS module monitors the yaw rate and lateral acceleration sensor for faults. If a fault is detected the ABS module stores a related fault code and illuminates the DSC warning indicator and, on vehicles with the low line instrument cluster, the HDC warning indicator. A warning chime sounds and, on vehicles with the high line instrument cluster, a message advising of an HDC fault is shown in the message center.

STEERING ANGLE SENSOR





The steering angle sensor measures the steering wheel angle and the rate of change of the steering wheel angle (known as the steering wheel angle speed). These measurements are output on the high speed CAN bus, together with a quality factor signal, and used by the ABS module for CBC and DSC operation.

The steering angle sensor is fixed to the pivot bracket of the steering column by three screws. A gear wheel in the steering angle sensor engages with a plastic drive collar fixed onto the lower shaft of the column. Inside the steering angle sensor, the gear wheel meshes with a gear train containing magnets. An eight pin electrical connector provides the interface between the vehicle wiring and integrated circuits in the steering angle sensor.

The steering angle sensor uses the Magneto Resistive (MR) effect, which evaluates the direction of magnetic fields, to measure the angular position of the lower shaft, and thus the steering wheel angle. When the steering wheel turns, the steering column lower shaft rotates the gear wheel in the steering angle sensor, which drives the gear train and rotates the magnets on the gears. The direction of the magnetic fields is constantly monitored by the steering angle sensor and converted into a steering wheel angle and steering wheel angle speed.

The steering angle sensor performs a plausibility check of the steering wheel angle each time the following conditions coexist:
^ The vehicle is traveling in a straight line.
^ The vehicle speed is between 20 and 25 km/h (12.5 and 15.6 mph).
^ The transfer box is in high range.
^ The brake pedal is not pressed.
^ There is no ABS, DSC or ETC activity.

The steering angle sensor uses inputs of wheel speed, yaw rate and lateral acceleration to determine when the vehicle is traveling in a straight line. When all of the conditions co-exist, the steering angle sensor checks the steering angle, which should be 0 � 15'. If the steering angle is outside the limits on two successive checks, the steering angle sensor changes the quality factor signal to 'outside specification' for the remainder of the ignition cycle and stores a fault code. At the beginning of each ignition cycle the quality factor signal is reset to 'within specified accuracy'.

The status of the steering angle sensor can be determined using T4.

If the steering angle sensor is replaced, the new sensor must be calibrated using T4. The steering angle sensor must also be re-calibrated any time it is disturbed from the steering column, or if the upper and lower steering columns are separated.





Steering Angle Sensor Harness Connector C0862





Steering Angle Sensor Harness Connector C0862 Pin Details

WARNING INDICATORS
The following anti-lock control - traction control indicators are installed in the instrument cluster:

ABS Warning Indicator
The ABS warning indicator is an amber colored indicator located between the coolant temperature gage and the fuel level gage.

The ABS warning indicator is continuously illuminated if there is a fault that affects ABS performance or causes the ABS function to be disabled.

Operation of the ABS warning indicator is controlled by a high speed CAN bus message from the ABS module to the instrument cluster.

When the ignition switch is first turned to position 11, the ABS warning indicator illuminates for approximately 3 seconds as a bulb check. During the bulb check, if a fault is stored in the memory of the ABS module, the ABS warning indicator goes off for 0.5 second, 0.5 second after the start of the bulb check. If a fault during the previous ignition cycle caused the ABS warning indicator to be illuminated, the ABS warning indicator may remain illuminated after the next bulb check, even if the fault has been rectified and cleared from the ABS module; the ABS warning indicator remains illuminated until vehicle speed reaches 15 - 20 km/h (9.5 - 12.5 mph) while additional checks of the related inputs are performed.

Brake Warning Indicator
The brake warning indicator is a dual colored indicator, located in the coolant temperature gage, that illuminates amber for EBA faults and red for EBD faults. The brake warning indicator is also used to give warnings of:
^ Low brake fluid level (illuminates red).
^ Brake pad wear (illuminates amber).
Operation of the brake warning indicator is controlled by a high speed CAN bus message from the ABS module to the instrument cluster.

When the ignition switch is first turned to position 11, the brake warning indicator illuminates amber for approximately 1.5 seconds then red for approximately 1.5 seconds, as a bulb check.

DSC Warning Indicator
The DSC warning indicator is an amber colored warning indicator located in the tachometer.

Each time the DSC or the ETC function is active, the DSC warning indicator flashes at 2 Hz. If DSC has been selected off, or there is a fault that disables the DSC or the ETC function, the DSC warning indicator is continuously illuminated. If DSC has been selected off, vehicles with the high line instrument cluster also display a message, advising that DSC is switched off.

Operation of the DSC warning indicator is controlled by a high speed CAN bus message from the ABS module to the instrument cluster.

When the ignition switch is first turned to position 11, the DSC warning indicator illuminates for approximately 3 seconds as a bulb check. If a fault during the previous ignition cycle caused the DSC warning indicator to be illuminated, the DSC warning indicator may remain illuminated after the next bulb check, even if the fault has been rectified and cleared from the ABS module, the DSC warning indicator may remain illuminated during vehicle operation while additional checks of the related inputs are performed.

HDC Information Indicator
The HDC information indicator is a green colored indicator located in the tachometer.

The HDC information indicator is continuously illuminated while the HDC function is selected on and the vehicle is within the parameters for HDC operation, when the vehicle is outside the parameters for HDC operation, the HDC information indicator is flashed at 2 Hz.

Operation of the HDC information indicator is controlled by a high speed CAN bus message from the ABS module to the instrument cluster.

HDC Warning Indicator
On the low line instrument cluster, the HDC warning indicator is an amber colored indicator located between the tachometer and the speedometer. On vehicles with the high line instrument cluster, the HDC warning indicator consists of a message in the message center.

On the low line instrument cluster the HDC warning indicator is continuously illuminated if there is a fault that affects the HDC function, and flashed at 2 Hz if the HDC function is temporarily unavailable because of brake overheat.

On the high line instrument cluster, appropriate messages are displayed in the message center if there is a fault that affects the HDC function, or if the HDC function is temporarily unavailable because of brake overheat.

Operation of the HDC warning indicator is controlled by a high speed CAN bus message from the ABS module to the instrument cluster.

On the low line instrument cluster, when the ignition switch is first turned to position 11, the HDC warning indicator illuminates for approximately 3 seconds as a bulb check.

HCU
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 HCU is attached by three mounting bushes to a bracket in the plenum box on the driver side of the engine compartment. Hydraulic pipes connect the HCU to the master cylinder and the brakes.

The primary and secondary outlets of the master cylinder are connected to primary and secondary circuits within the HCU. The primary circuit in the HCU has separate outlet ports to the front brakes. The secondary circuit in the HCU has separate outlet ports to the rear brakes. Each of the circuits in the HCU contain 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 return pump during active braking.
^ A return 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 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 return pump motor and the pressure sensor. The solenoids that operate the valves are installed in the ABS module.

Replacement HCU are supplied pre-filled. After installation on the vehicle, T4 must be used to operate the solenoid valves and the return pump to ensure correct bleeding of the HCU and brake circuits.

Schematic of HCU








The HCU has three operating modes: Normal braking/EBD, ABS braking and 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 the brakes of the trailing axle, to isolate the brakes from any further increase in hydraulic pressure.

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 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 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 ABS braking, e.g. DSC EBA, ETC, HIDC and dynamic application of the parking brake.

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.

ABS MODULE
The ABS module controls the brake functions using the HCU to modulate hydraulic pressure to the individual wheel brakes.

The ABS module is attached to the HCU, in the plenum box on the driver side of the engine compartment. A 46 pin connector provides the electrical interface between the ABS module and the vehicle wiring.





ABS Module Harness Connector C0506








ABS Module Harness Connector C0506 Pin Details