# Throttle Position Sensor (TPS) The Throttle Position Sensor is located in / on the throttle body. This sensor is used to inform the Power train Control Module (PCM or Engine Control Unit (ECU)) about the amount of throttle valve opening.
A General Motors TP-Sensor.
This sensor is usually designed as a potentiometer. One end of the resistive element is connected to ground. (Usually a sensor ground internal to the PCM). The other end of the resisitve element is connected to a stable voltage. This voltage is often called reference voltage or voltage reference and is usually +5V. A movable contact or "wiper" moves along the resistive track. This movable contact is attached to the throttle valve shaft. As the contact moves along the resistive track, the voltage will vary. Usually the voltage returned to the PCM is low with closed throttle and high with open throttle. The voltage is never 0V, nor 5V. Often the value is between 0.5 and 4.8V. This is done so the PCM can detect circuit faults such as opens and shorts.
| Techniques of diagnostics of the Throttle Position Sensor can be seen in the Flash movie. To see Flash movie »
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The signal from the TP-Sensor is used by the PCM for fuel and ignition timing calculations. The signal is also often used for determining the proper gear in the transmission and to operate certain emission control devices such as Exhaust Gas Recirculation (EGR). The signal from the TP-sensor is also used to determine if the engine is at idle, cruise, or at Wide Open Throttle (WOT). At idle, the PCM will act to keep the idle stable, by controlling how much air is allowed to bypass the throttle plate and by controlling Air / Fuel Ratio (AFR) as well as ignition timing. At cruise or off-idle the PCM will act to maximize fuel economy as well as minimize emissions, by controlling the AFR and ignition timing. (Certain emissions control devices such as the EGR valve may only operate off-idle as well). When the voltage from the TP-sensor indicates WOT (Often defined as more than 80% of the maximum voltage from the sensor), the PCM will act to maximize engine power and fuel economy as well as emissions become secondary. The PCM will richen the AFR and may, depending to the manifold pressure, retard the ignition timing. The EGR valve will often be disabled. The PCM may disregard the signal from the Oxygen Sensor, or it may allow the sensor signal to indicate rich without adjusting the AFR, at least for a period of time. If the signal from the TP-Sensor is indicating WOT and the engine is being cranked, the PCM may cease to inject fuel (Clear Flood mode).
Checking the TP-Sensor signal. To view the waveform from the TP-sensor, connect the probe to the signal return (Center or wiper wire) from the sensor and to one of the analog oscilloscope inputs (1-4). The ground lead (Black with an alligator clip attached to the end) should be connected to chassis ground.
Hook up diagram for obtaining waveforms from a TP-Sensor. 1 – Ground connection. The lead with the alligator clip goes here. 2 – Connection point for the oscilloscope probe or test lead.
In the "USB Oscilloscope" program, the user preset "Potentiometer" can be selected "Control => Load user preset => Potentiometer". Because the waveform should be obtained from the whole sweep range of the sensor, it is best to perform this test with the ignition key in the "run" position, but the engine "off". It is also best to record the waveform so it can be studied in more detail after the test. Set the USB Oscilloscope in the record mode by clicking on the record button. Slowly and smoothly open the throttle all the way to full open, then slowly close the throttle. When done, click the record button again, to stop the recording and "freeze" the waveform. When the throttle is closed, the voltage obtained should be between 0.25 to 0.65V. (Verify manufacturers specifications using a reputable service manual or similar). As the throttle is opened, the voltage should increase smoothly.
This waveform is from a good TP-Sensor. Ignition is on, the engine is stopped and the throttle is slowly opened. (In this case the throttle is closed rather abruptly, the throttle should be closed slowly as well).
When the throttle is completely open, the voltage should be approximately 3.9…4.7V. (Verify specifications).
Typical malfunctions. The wiper or movable contact of the TP-Sensor, which is a Potentiometer type mechanically moves on a resistive track or layer. The sliding contact cause wear which eventually will lead to signal degradation and / or drop-outs. The symptoms of a failing TP-sensor depends somewhat on where (at the bottom, middle or top) of the resistive layer the damage has occurred. If at the lower end, idle may be affected. The engine may also possibly stall. If at cruise, the engine may run erratic, along with erratic transmission shifting. (Incidentally, the sensor is most likely to fail at the point where it spends the most time moving, which will be in some cruise or intermediate position. If failing at WOT, the engine may lack power.
This waveform is typical of a failing TP-Sensor. Notice the glitches about half-way up the ramp.
When these glitches are encountered, the signal voltage will signal the PCM of abrupt throttle changes and the vehicle will buck or jerk.
This waveform is another example of a damaged TP-Sensor. In this case, the damage is throughout the movement.
With this type of signal going to the PCM, the vehicles behavior may very well be unpredictable.