Land Rover Discovery Owners & Service Manuals

Land Rover Discovery: Standard Speed Control

The speed control system is integrated with the engine management system and uses fueling intervention to automatically maintain a set vehicle speed.

Once engaged, the system can also be used to accelerate the vehicle without using the accelerator pedal. The speed control system comprises the following main components:

  • '+' and '-' (SET/increase speed and decrease speed) switches - located on right steering wheel switchpack
  • 'RES' (resume) switch - located on right steering wheel switchpack
  • 'CAN' (cancel) switch - located on right steering wheel switchpack
  • Clockspring
  • Speed control warning indicator in Instrument cluster (IC).

The speed control system also uses inputs from the brake pedal switch, clutch pedal position sensor (manual transmission only), the Accelerator Pedal Position (APP) sensor, the Engine Control Module (ECM) and the Antilock Brake system (ABS) control module.

The speed control is operated by the driver using only the switches on the right steering wheel switchpack. When speed control is active, the ECM regulates the output signals to the fuel injectors to adjust the fuel supply as required to maintain the set speed.

During speed control operation, the ECM controls vehicle speed by adjusting fuel injection duration and timing. When the accelerator pedal is pressed with speed control active, the ECM outputs a calculated throttle angle signal in place of the actual throttle angle signals produced by the APP sensor. The calculated throttle angle is derived from fuel demand based on the torque required to overcome the rolling resistance (vehicle motion drag).

The minimum set speed for speed control is 32 km/h (20 mph). Speed control is automatically suspended if the following conditions apply:

  • Vehicle speed falls below 24 km/h (15 mph)
  • The brake pedal is pressed
  • The clutch pedal is pressed (manual transmission only)
  • The cancel button is pressed
  • Neutral, park or reverse gear is selected
  • The Electric Park Brake (EPB) is applied
  • The difference between actual speed and the set speed is too great
  • If the accelerator pedal is used to accelerate beyond the set speed for too long (more than 5 minutes)
  • System error causes shut-off
  • Hill Descent Control (HDC) is selected
  • All Terrain Progress Control (ATPC) is selected
  • Terrain Response program 'Sand', 'Mud' or 'Rock Crawl' is selected.

There are four standard speed control switches located on the right steering wheel switchpack:

  • Set speed '+' (increase speed) switch
  • Decrease speed '-' switch
  • Cancel 'CAN' switch
  • Resume 'RES' switch.

The standard speed control switches are located on the right steering wheel switchpack. The switches are resistive ladder type switches which vary the resistance of a voltage signal sent to them. The resistive ladder is wired to the Steering Wheel Module (SWM) within the right steering wheel switchpack. The SWM is a Local Interconnect Network (LIN) node, which transmits the data to the Body Control Module (BCM) which passes the data onto the High Speed (HS) Controller Area Network (CAN) Powertrain systems bus. In the event of the ECM logging a speed control switch fault Diagnostic Trouble Code (DTC), fault finding should start at the right steering wheel switchpack and clockspring. The ECM diagnostic detects that the right steering wheel switchpack has not provided an updated live counter.

Speed Control Warning indicator

Speed Control Warning indicator

The speed control warning indicator illuminates when speed control is engaged.

Engage Speed Control

Speed control is engaged by pressing the SET '+' switch. On receipt of the switch input, the ECM adopts the current vehicle speed as the speed control set speed. When speed control is engaged, the Instrument Cluster (IC) illuminates the speed control warning indicator and the set speed is displayed in the message center. Once engaged the speed can be increased by one of 3 methods:

  • Press and hold the SET '+' switch which will cause the vehicle to accelerate. When the switch is released the attained speed will be set as the cruise speed
  • Repeatedly pressing the SET '+' switch. Each press increases the vehicle speed by 2 km/h (1 mph)
  • Use the accelerator pedal to increase the vehicle speed. Once the required speed is reached, a single press of the SET '+' switch will set the cruise speed.

Speed control adjustment is limited around the current vehicle speed. This means speed adjustment will not occur when the 'RES' switch is pressed or if the vehicle speed is noticeably different to set speed. This is noticeable when resuming speed control and a large difference exists between the set speed and the current speed. The driver can set a new speed, but cannot adjust the old speed.

Reduce Cruising Speed

The set speed can be reduced by pressing and holding the decrease speed '-' switch until the required speed is reached. When the switch is released the speed is reset at that value. The set speed can be reduced incrementally by pressing and releasing the decrease speed '-' switch. Each press will reduce the speed by 2 km/h (1 mph).

Suspend Speed Control

Suspending speed control means speed control has been deactivated without loss of the memory speed. In all suspend events, the memory speed is retained. Speed control can be suspended in a number of ways, as described previously in this section.

A single press of the resume 'RES' switch will resume speed control at the previously set speed.

NOTE:

In the event that a memory speed has been lost during a driving cycle, confirm with the customer if the "Cruise Not Available" message was observed. Loss of memory speed implies a fault has occurred. The message 'Cruise Not Available' may only be displayed on demand for some errors, so speed control needs to be requested via the SET '+' switch to drive the message.

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