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. This sensor along with the vehicle speed sensor are used by the TCM to provide optimal shift timing.
Most TP sensors have extra pins for extra internal functionality but for this discussion we will stick to a three wire design. To help explain this we will look at the three leads and their function as it relates to transmission performance. A faulty TP sensor will cause erratic shifting and effect torque converter engagement.
One of the leads is connected directly to the computers 5 volt voltage supply. This voltage pin is in turn 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 ground. The third wire is connected to the movable arm that will swipe across this internal resistance strip sending a varying voltage signal back to the TCM indicating the drivers intentions. This is called the signal circuit of a sensor because it's the lead the sends the varying or "changing" voltage signal to the appropriate computer indicating the changes that are occurring during vehicle operation.
This device 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 / TP sensor are moved. If the driver suddenly accelerates their vehicle the PCM would read this 4.5 volt signal and increase the fuel injection timing to compensate. The TP sensor is also used by the TCM to indicate load and drivers intentions. For instance this helps the TCM determine the shifting schedule. If the throttle plate were to suddenly approach WOT (4.5 volts) this would indicate that its time for a down shift. This sensors output is often shared by the PCM and the TCM.
You may also use a DMM to diagnose this sensor using the ohm and voltage settings. This is because analog meters have low impedance meaning that there is little resistance. They may draw more voltage than the system can handle. Use a high impedance digital ohm/voltage meter to diagnose automotive circuits.
