Throttle Position Sensor

Most (TPI) throttle position indicator sensors have extra pins for extra functionality; for this discussion, the three-wire design will be discussed. To help explain this, these three leads and their function as it relates to engine and transmission performance are analyzed.

Throttle Position Sensor

One of the leads is connected directly to the computers 5-volt voltage supply. This voltage pin is connected to one end of the resistance strip inside of the TP sensor. Another wire is connected to the other end of this internal resistance strip and connected to the ground. The third wire is connected to the movable arm that swipes across an internal resistance strip sending a varying voltage signal back to the PCM, indicating the driver's intentions. This is called the signal circuit of the sensor because it's the lead that sends the varying or changing voltage PCM indicating the changes that are occurring to throttle plate's angle.

Four different views of a throttle position sensor.

It's attached to the throttle plate shaft and changes output voltage as the plate opens and closes. The output voltage will be low or around 0.45 volts when 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, the PCM will register this 4.5 volt signal and increase the fuel injector pulse width to compensate.

The TP sensor is also used by the (TCM) Transmission Control Module to indicate load and the driver's intentions. For instance, this helps the TCM determine the shifting schedule. If the throttle plate were to approach WOT (4.5 volts) suddenly, this would indicate that it's time for a downshift. The PCM and the TCM share this sensor's output. A faulty TP sensor affects engine performance and causes erratic shifting and torque converter engagement.