Engine management system - EDC (Throttle Potentiometer): Description and operation

Throttle Potentiometer

The throttle potentiometer is located on the pedal box in the driver's footwell. The throttle potentiometer consists of two resistance tracks and two sliding contacts, effectively a pair of potentiometers, connected to the accelerator pedal assembly. The use of a pair of identical sensing elements ensures a position signal is still provided even if one of the sensing elements develops a fault; this is required because there is no mechanical linkage between the accelerator pedal and the ECM. As the accelerator pedal is depressed, the sliding contacts move along the resistance tracks to change the output voltage of the sensor.

By monitoring the voltage outputs from the throttle potentiometer, the ECM is able to determine the position, rate of change and direction of movement of the accelerator pedal. It will also store the voltages which correspond with closed 'throttle' and wide open 'throttle' and will adapt to new ones in the event of component wear or replacement.

The ECM uses the throttle potentiometer voltage to determine closed 'throttle' position to instigate idle speed control, and to enable the overrun fuel reduction strategy.

The throttle potentiometer signal is also broadcast on the CAN bus, where it is used by the EAT ECU to determine the correct point for gearshifts and kickdown.

The connector and sensor terminals are gold plated for corrosion resistance and temperature stability, care must be exercised when probing the connector and sensor terminals

The ECM supplies the throttle potentiometer with a regulated 5 volts supply and an earth path for the resistive tracks. The output signals vary according to the position of the accelerator pedal. The throttle potentiometer earth also acts as a screen to protect the integrity of the signal.

If the throttle potentiometer signal fails, the ECM increases the idle speed to 1,250 rev/min, and the engine speed will not increase when the accelerator is depressed.

In the event of an throttle potentiometer signal failure, the following symptoms may be observed (unless the brake is activated): l No accelerator response. l Failure of emission control. l Automatic gearbox kickdown inoperative.