Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Stabilizer Valve For The Audi V-8:

On fuel injected engines, the throttle closes completely and idle air is supplied through a passage around the throttle valve. The idle stabilizer valve regulates the amount of measured air (air which has passed through the air mass sensor) bypassing the throttle valve, to provide a fast idle during engine cold start and warm-up, and compensate for additional loads at idle such as A/C compressor and electrical loads from the alternator. The more air that flows through the valve, the faster the idle.

The idle stabilizer valve consists of a rotor with a vane, a return spring, and a small single winding DC electric motor. The motor is operated by a cycled DC current (duty cycle) which causes the armature to work against the return spring. The number of degrees the valve turns depends on the duty cycle. The greater the duty cycle (on time/off time) the greater the torque against the return spring, and the further the valve opens. The Engine Cotrol Module (ECM) monitors the coolant temperature, engine speed, throttle position (idle switch), and A/C compressor ON/OFF signal to determine the correct duty cycle for stable idle under different operating conditions. If the idle speed slows below a specified rpm, the ECM increases the duty cycle to compensate and bring the idle up to normal. If the idle speed is greater than normal, the duty cycle is reduced and the idle slows. With this system, no idle speed adjustments are necessary or possible. If the idle speed is incorrect, a problem is indicated. If the idle stabilizer or circuit fails, the return spring moves the valve to a fixed position so the engine will idle.