Land Rover Discovery Owners & Service Manuals

Land Rover Discovery: Speed Control Switches

Speed Control Switches

  1. Vehicles with standard speed control
  2. Vehicles with adaptive speed control
  1. Set speed '+' switch (Engage speed control/increase speed)
  2. Resume 'RES' (Resume previous set speed)
  3. Set speed '-' switch (Decrease speed)
  4. Cancel 'CAN' (Disengage speed control)
  5. Time gap decrease switch (Vehicles with adaptive speed control only)
  6. Time gap increase switch (Vehicles with adaptive speed control only)

The speed control switches are located on the right steering wheel switchpack. The switches are connected via fly leads to the clockspring. The speed control switches are resistive ladder type switches which vary the resistance of a 5 volt signal supplied from the Central Junction Box (CJB).

The switch information is transmitted on the Local Interconnect Network (LIN) bus by the Steering Wheel Module (SWM) to the Body Control Module (BCM), the BCM then transmits the data on the High Speed (HS) Controller Area Network (CAN) powertrain systems bus for use by the Engine Control Module (ECM) and to the Adaptive Speed Control Module (ASCM) on vehicles with adaptive speed control.

The adaptive speed control switches are the same as used for standard speed control, with the addition of two time gap switches. The time gap switches allow the driver to adjust the follow mode and forward alert functions to one of four pre-set time gaps (gap 1 = 0.8 second; gap 2 = 1.3 seconds; gap 3 = 1.8 seconds; gap 4 = 2.3 seconds). The selected time gap is displayed in the message center when the time gap switches are operated.

When adaptive speed control is engaged, the switches adjust the follow mode time gap. When adaptive speed control is disengaged, the switches adjust the forward alert time gap (provided forward alert is enabled in the instrument cluster menu). The follow mode function defaults to the gap 3 setting each time the ignition is switched on. The forward alert function recalls the last selected time gap when the ignition is switched on.



  1. Adjuster screw
  2. Mounting bracket
  3. Adaptive Speed Control Module (ASCM)

The ASCM is mounted on a bracket which is attached with three screws to the front armature.

The ASCM contains a forward looking radar transceiver together with related controlling hardware and software. A HS CAN Powertrain bus connection allows the ASCM to communicate with other system control modules. Power supplies to the ASCM are from the ignition relays in the Battery Junction Box (BJB) and the Central Junction Box (CJB).

The ASCM is active whenever the ignition is on, even if speed control is not engaged, in order to operate the forward alert, advanced emergency brake assist (AEBA) and intelligent emergency braking (IEB) functions. With the ignition on, the ASCM is electrically powered, but no radar transmissions are emitted until the vehicle is in motion.

The ASCM transmits a radar beam forward of the vehicle and detects the returning signals reflected off other vehicles and objects ahead. The radar beam is electronically scanned at a rate of 20 sweeps/second across a total arc of 20º centered on the longitudinal axis of the vehicle. The radar operates at millimetric wavelengths (76 - 77 GHz) and transmits a frequency modulated continuous wave signal at a relatively low power level (no high power pulses). The control module detects the range, relative velocity and angle of objects within the scanned arc for up to a distance of approximately 130 meters (426.5 feet).

The ASCM compares vehicle speed data from the Anti-lock Brake System (ABS) control module with the relative speed of an external object as detected by the radar to determine if the object is stationary or not. If tires are fitted which are different in diameter from those specified for the vehicle, the vehicle speed, calculated by the ABS control module, will not be the true road speed. This situation may cause stationary objects to be falsely identified as moving vehicles and result in automatic deceleration on a clear road.

The ASCM continuously monitors both moving vehicles and stationary objects to determine if it can 'see' normally. If the ASCM can detect only a few objects, because it is physically blocked (for example by an accumulation of snow or mud on the lower grill of the front bumper, or an incorrectly located licence plate), or there are few roadside objects or other vehicles in the area, it may determine that it is blocked. The ASCM then inhibits adaptive speed control and records a Diagnostic Trouble code (DTC). The system will reset after an ignition cycle. In this case the ASCM does not have a fault and should not be replaced.

If the ASCM is replaced in service it must be mechanically aligned vertically.

Horizontal alignment is achieved by putting the ASCM into service mode using approved Land Rover diagnostic equipment. The vehicle then needs to be driven for a short period while the ASCM performs a calibration routine. Calibration is complete when the follow mode warning indicator in the Instrument Cluster (IC) stops flashing.

For additional information, refer to: Speed Control Sensor Adjustment (310- 03 Speed Control, General Procedures).



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