Fuel Delivery and Air Induction: Description and Operation

Function of the electronic throttle control system
In the electronic throttle control system, the throttle valve is not actuated by a cable from the accelerator. There is no mechanical connection between the accelerator and throttle valve.
The accelerator position is communicated to the engine control unit by two throttle position sensors (variable resistance, accommodated in a casing), which are connected to the accelerator.
The accelerator position (determined by the driver) is a main input variable for the engine control unit.
An electric motor (throttle actuator) in the throttle valve control unit (J338) actuates the throttle valve over the entire range of engine speeds and load.
The throttle valve is actuated by the throttle actuator under instructions from the engine control unit.
When the engine is off and the ignition is turned on, the engine control unit controls the throttle actuator precisely, following specifications from the throttle position sensor. That is, if the accelerator is half-way to the floor, then the throttle actuator opens the throttle valve to the same angle; the throttle valve is then opened approximately half-way.
When the engine is running (under load), the engine control unit can open or close the throttle valve independently of the sensor.
As an example, the throttle valve can already be completely open, even though the accelerator has only been pressed halfway down so far. This has the advantage of preventing throttling loss at the throttle valve.
Under certain load conditions, this also results in significantly reduced emissions and improved fuel consumption.
It is wrong to believe that electronic throttle control only consists of two components. It is a system which contains all components needed to help determine, regulate, and supervise the throttle valve position (e.g. the throttle position sensor which measures the accelerator position, the throttle valve control unit, the EPC indicator lamp, the engine control unit).