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. To help explain this we will take a look at three leads and the function of these three leads as they relate to engine performance.
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. The other end is connected to ground. The third wire is connected to the movable arm that will swipe across this internal resistance strip indicating the drivers intentions. This is called the signal circuit because it is the lead the sends the varying or changing voltage signal to the PCM indicating throttle opening during engine 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 wide open throttle. Fuel injection and ignition timing are adjusted by the PCM as the throttle plate is moved. If the driver suddenly accelerates the engine 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.
It’s best to diagnose a TP sensor with a scanner or scope and would be displayed as a percentage. The PCM coverts the voltage to a percentage on the display screen of a scanner. Just plug the scanner in, locate the TP sensor on the screen, and with key on engine off steadily depress the accelerator to the floor board. The percentage should increase steadily from 0 to 100%. Check with the manual for the appropriate specifications. 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 and may draw more voltage than the system can handle. Use a high impedance digital ohm/voltage meter to diagnose automotive circuits.
First select the Ohm setting and connect the leads to the signal and ground pins of the sensor. As you manually open the throttle to fully open and fully closed you should see a smooth movement in the resistance reading. Any infinity readings or skips in the sweep indicate a bad sensor. Set the DMM to voltage and install test leads between the pins in the connector and sensor. With the KOEO “Key on engine off” move the throttle between fully open and fully closed and insure the reading is smooth with a low reading of 0.45 volts closed to 4.5 volts open.
