Idle/Throttle Speed Control Unit: Description and Operation

Fig. 18 Idle Control Valve:

An electrically operated idle rpm control valve keeps the idle speed stable under various operating conditions. Measured intake air from the airflow meter bypasses the throttle plate through a variable orifice in the idle control valve.
An additional electronic control unit (separate from the Motronic ECU) controls the orifice opening according to the engine speed and operating conditions as specified by various sensors and switches on the engine and transmission.