Cruise Control Master Switch: Input/Output
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 2004 Land Rover Discovery, 2003 Land Rover Discovery, 2002 Land Rover Range Rover, 2002 Land Rover Discovery, and 2001 Land Rover Range Rover. However, it is still accessible from the selected car via links, so may be relevant.
The input from the cruise control master switch to the cruise control ECU is either a 12 Volts ignition feed or an open circuit. 12 Volts indicates that the cruise control master switch is on and the system can be activated. An open circuit indicates that the cruise control master switch is off and cruise control cannot be activated.
Textbook/T4 will not communicate with the cruise control ECU if the cruise control master switch is off.