Land Rover Discovery Owners & Service Manuals

Land Rover Discovery: Power Steering - Operation

For the electric power steering system to provide assistance the engine must be running - with one exception - see: ASSISTANCE CHANGES DURING ECO STOP/START FUNCTIONS, below.

If a fault occurs which affects control of the electric motor, the PSCM (Power Steering Control Module) disables the electric motor. The driver no longer benefits from power assisted steering, but incorrect control of the electric motor is prevented and the vehicle remains fully steerable, although with greater physical effort.

VARIABLE POWER ASSISTANCE

At low speeds, when tire resistance to steering inputs is highest, more assistance is applied to reduce the steering wheel torque to comfortable levels. At higher speeds, when tire resistance to steering inputs is less, less assistance is applied.

PULL DRIFT COMPENSATION

NOTE:

This is not a lane keeping feature.

This feature aids the driver by compensating for the accumulated effects of various factors that contribute to the pull/ drift of the vehicle. The feature reduces the steering wheel torque offset that the driver feels while driving in a straight line, by applying a counter torque. The vehicle will drift less if the driver's hands are taken off the steering wheel, and steering effort in general is reduced.

ASSISTANCE REDUCTION IF SYSTEM OVERHEATS, VOLTAGE IS TOO HIGH OR LOW, OR STEERING VIBRATION OCCURS

This feature ensures that the driver's steering requirements continue to be supported during various extreme conditions including:

  • Extreme voltage fluctuations (over and under voltage).
  • Extreme high temperature.
  • Steering system vibration optimization, detected for example when steering on a low friction surface.

A temperature sensor is installed in the Power Steering Control Module (PSCM) for overload protection of the electric motor. The PSCM reduces the level of power assistance if the temperature of the electric motor is too high, to reduce the amount of heat generated.

Temperature overload can occur at high ambient temperatures combined with high levels of steering activity, especially when stationary. A temperature overload can also occur if an attempt is made to repeatedly turn the front wheels against a solid object such as a kerb. Reducing power assistance in this situation protects the power steering components against excessive mechanical stresses and helps to give an indication to the driver that there is a solid object preventing the wheels turning.

Power assistance begins to reduce at a PSCM temperature of approximately 110 ºC (230 ºF) and reduces to zero at a temperature of 125 ºC (257 ºF). At a given level of reduction in the power assistance, the PSCM records a Diagnostic Trouble Code (DTC) and signals the IC to produce a warning that assistance has been reduced. When the PSCM has cooled, normal power assistance is restored and the related IC warnings are extinguished.

ASSISTANCE CHANGES DURING ECO STOP/START FUNCTIONS

This feature actively controls the driver's steering torque feedback during the engine Eco stopping and starting activities. It integrates the main steering assistance feature with micro hybrid or enhanced starter motor powertrain events, ensuring smooth, progressive, yet timely decreases and increases in power assistance. It is also designed to remove unwanted steering wheel movement during these events, which are not directly instigated by the driver.

For vehicles fitted with an Automatic transmission, a reduced level of assist is maintained throughout the Eco stop and the driver will be able to request an engine restart by turning the steering wheel. The restart request in this case is based on the amount of current drawn by the steering during the Eco stop.

For vehicles fitted with a Manual transmission, assist is reduced completely during an Eco stop and the driver will not be able to request the engine to restart through use of the steering.

SOFT END STOPS

The soft end stop feature reduces the level of power assistance shortly before the mechanical end stops are reached. Although the driver will perceive this as increased steering resistance towards full lock, the feature improves the refinement of the system at extremes of lock. In addition, this feature reduces the stresses on the mechanical and electrical components of the steering system.

PARK ASSIST SYSTEM INTERFACE (WHERE FITTED)

When the park assist system is active, steering trajectory requests from the parking aid module are processed by the PSCM and applied to the electric motor. This feature assists the driver, during parallel parking manoeuvres, by selectively applying turning forces to the steering system to automatically steer the vehicle.

For additional information, refer to: Parking Aid (413-13 Parking Aid, Description and Operation).

LANE DEPARTURE WARNING (WHERE FITTED)

When the Lane Departure Warning (LDW) system is active, haptic feedback (vibration) warning requests from the Auto High Beam Control Module (AHBCM) are processed by the PSCM and applied to the electric motor.

This feature assists the driver by providing a haptic feedback warning when the vehicle drifts towards the edge of the lane.

For additional information, refer to: Warning Devices (413-09 Warning Devices, Description and Operation).

CONTROL DIAGRAM

A = Hardwired, AM = High speed CAN (Controller Area Network) Chassis bus, AN = High speed CAN Powertrain bus, AO = Medium speed CAN Body bus.

CONTROL DIAGRAM

  1. Power Steering Control Module (PSCM)
  2. Crankshaft Position (CKP) sensor
  3. Engine Control Module (ECM)
  4. All Wheel Drive Control Module (AWDCM)
  5. Transmission Control Module (TCM)
  6. Instrument Cluster (IC)
  7. Central Junction Box (CJB)
  8. Reverse gear sensor (vehicles with manual transmission)
  9. Parking aid sensor
  10. Terrain Response switchpack and control module
  11. Steering Angle Sensor Module (SASM)
  12. Parking Aid Control Module (PACM) (if park assist system fitted)
  13. Anti-lock Brake System (ABS) control module
  14. Gateway Module (GWM)
  15. Auto High Beam Control Module (AHBCM)
  16. Wheel speed sensor
  17. Ground wire
  18. Power supply
  19. Torque sensor

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