Idle Speed Control: Function
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 idle air control valve utilizes two coils that use opposing pulse width modulated (PWM) signals to control the position of a rotary valve. If one of the circuits that supplies the PWM signal fails, the ECM closes down the remaining signal preventing the idle air control valve from working at its maximum/minimum setting. If this should occur, the idle air control valve assumes a default idle position at which the engine idle speed is raised to 1200 rev/min with no load placed on the engine.