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Q: Excessively high voltage in Throttle Position Sensor

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My car is cranking but not starting, I lack a fuel pressure gauge so I was testing other, related stuff until I could get my hands on one. I got to the Throttle Position Sensor and started testing it. I started with the power supply. My repair guide says it should be about 5 volts, I got 5.1 so I figure that's pretty close so I chalked it up as good. Then I went to the ground return which my repair guide says it should be 12 volts. And I got 12 volts. So I chalk that up as good. Then I move on to what it's reading from the actual throttle position.

My repair guide says when closed (@ screw) should be 0.5 ~ 0.9v. I got an astonishing 25.7 volts. I had exposed a tiny portion of wiring so I could get my multimeter lead onto it (because it needs to be plugged in to read the throttle position) and I hooked the other end to my negative battery post. Did I do that wrong? (repair guide says to use an inline lead which I don't have) Or is that the actual reading? If so what does that mean?

My car has 185000 miles.
My car has an automatic transmission.

A: I'm not sure why you are getting such a...

I'm not sure why you are getting such a high voltage, but you are making one mistake when checking TPS voltage. The ECM sends a 5 volt reference voltage to the TPS which passes through a variable resistor of the sensing circuit. This of course is connected to your throttle pedal. When the 5 volts passes through the resistor of the TPS, the voltage will vary based on the position of the throttle plate. This is how the ECM knows what you are doing with the gas pedal.

You can test a TPS in two ways, You can simply test the resistance with an Ohm meter or you can test the voltage change that occurs in the reference circuit. The mistake you are making is connecting the multimeter to the negative battery post. You need to connect one end to the 5 volt reference sent to the TPS from the ECM and the other lead to the center wire coming out out of the TPS. This is what they mean by inline. While doing this, with the key on, move the throttle plate and the voltage should change smoothly. It is the smooth change in voltage or resistance we are looking for. If it glitches, jumps or just doesn't change, you need a new TPS.

To add, a TPS won't make a car not start. You need to check the fuel pressure, check for spark and injector pulse.

If you should require further assistance, I recommend having the starting issue inspected by a qualified technician, such as one from YourMechanic.

Good luck!

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