Lots of engine management systems contain an idle speed control motor. In older versions the idle speed is controlled with an external valve which bypasses the throttle valve. As a replacement for the external version, a throttle valve controlling the main throttle valve is used more and more. It is possible that the throttle valve control motor is used by a "drive by wire" system. The throttle valve control motor is then not only used for the idle speed regulation but also for an engine under (high) load.
The throttle valve control motor in this measurement example is a DC motor of which both terminals are connected to an H bridge in the ECU. The duty cycle and frequency of the H bridge signals vary. On other cars the signal might only be duty cycle varying.
A motor controlled throttle valve is kept in its neutral position by a spring. In this position the engine runs at a speed of about 1000 to 1500 RPM. When the engine needs to run at a lower speed, the ECU applies a positive duty cycle signal to the motor to nearly close the throttle valve. When the engine needs to run faster, a negative duty cycle signal is applied to further open the throttle valve. The polarity and duty cycle of the signal determines the direction and amount of opening of the throttle valve from its neutral position.
The ECU used the signal from the throttle valve position sensor to determine the exact position of the throttle valve.
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