Throttle Position Sensor

Four different views of a throttle position sensor.

The Throttle Position (TP) sensor is located on the throttle body of a fuel injected engine. This type of sensor is known as a potentiometer. A variable resister used to control voltage in a circuit. The TP sensor is used to detect engine load. The PCM and the TCM share its output. This sensor's signal, along with the vehicle speed sensor, is used by the TCM to provide optimal shift timing.

Most TP sensors have extra pins for extra internal functionality, but for this discussion we look at three terminals and this sensors relation to automatic transmission performance. This is because a faulty TP sensor will cause erratic shift timing and affect torque converter engagement.

The Throttle Position (TP) sensor.

One lead is connected directly to the computer's 5-volt reference voltage supply. The opposite end of this internal resistance strip is connected to ground. Another is connected to the PCM. This wire is connected to the movable arm that swipes across an internal resistance strip sending a varying voltage signal back to the PCM. This signal indicates the engine's throttle angle. This is called the signal circuit of the sensor, because it's the lead the sends the varying voltage signal to the controller indicating the changes in throttle angle.

The sensor is attached to the throttle plate shaft and changes output voltage as the throttle plate opens and closes. The output voltage will be low or around 0.45 volts if the throttle is closed and will be high or 4.5 volts at (WOT) Wide Open Throttle. The fuel injection and ignition timing are adjusted by the PCM as the throttle plate and TP sensor are moved. If the driver suddenly accelerates their vehicle, the PCM would receive this 4.5 volt signal and increase the fuel injection timing to compensate. This would also indicate to the TCM that a downshift is needed.