Throttle Position Sensor
The Throttle Position (TP) sensor is a potentiometer located on the throttle body of a fuel injected engine. In it's simpist form it is a potentiometer. A three wire resistor with a sliding armthat swipes across a resistance strip in turn changing the resistance. This dynamic resistance increases or decreases the signal voltage back to the PCM. In a nutshell a TP sensor is a devicethat changes voltage by varying internal resistance.
Some TP sensors have extra pins for extra internal functionality but for this discussion we will stick to a simple three wire design. To help explain this we will look at the three leads and each of these leads/wire/PCM or TCM function as it relates to either engine or transmission 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. Another wire is connected to the other end of this internal resistance strip and in turn 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 PCM or TCM indicating the drivers intentions. This is often called the signal circuit of a sensor because it is the lead the sends the varying or changing voltage signal to the various computers 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 is 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.
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 steadilly depress the accelerator to th efloor board. The percentage should increase steadily from 0 to 100% or whatever the specifications dictate. Check with the manual for the appropriate specifications. You may also use a DMM to diagnose this sensor using the ohm and voltage settings.
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.
