Diagnose and Replace throttle position sensor TPS
A throttle position sensor is just like a rotating volume control on your radio or TV. Instead of turning the knob by hand to “pump up the gas,” the throttle position sensor connects to the throttle plate shaft or to the throttle cable. Its job is to tell the powertrain control module (PCM) where your foot is on the pedal. In newer cars, the TPS is located right on the gas pedal. The manufacturers eliminated the cable to the throttle body and rely instead on stepper motors to open and close the throttle plate.
If you’ve ever experienced static with the volume control on your radio or TV, then you’ll know why your PCM gets so annoyed when it cannot get a “clean” signal from the TPS. If it hears too much static or dropouts on the line, it simply gives up and sets a trouble code. Then it’s your job to start troubleshooting. Here’s how to do it.
The PCM sends a constant 5 volt reference signal to the variable resistor inside the TPS case. To detect whether the reference signal arrived safely, it also monitors the return ground back at the computer. So all the tests you perform must be done with the connector still connected to the TPS and the key turned to the RUN position. How do you tap into the wires? You can either backprobe (stick the lead into the back of the connector) or stick a pin through the wires and seal the holes afterwards with nail polish.
If the PCM doesn’t see a return signal, it sets a trouble code for a bad reference signal. If that trouble code pops up, connect one lead of your digital voltmeter to the 5V reference in wire and the other to a good ground. The reading must be 5 volts. If it isn’t, you’ve either got a break in the wire between the PCM and the TPS, or you have a bad PCM. Find the 5v reference wire in the PCM connector and check for it there. If you get 5 volts there, that confirms a broken wire.
To check for reference return ground, disconnect the TPS connector and place one lead of your meter on the 5 volt reference signal IN pin and the other on the reference ground. It must read 5 volts. If it doesn’t, follow the same procedure at the PCM that you used to check for reference voltage.
Reconnect the connector to the TPS and then test the TPS signal. This one can be a bit tricky. Most pros use a lab scope for this check because it will show a gradual sweeping curve as you open the throttle. If there are any dead spots (or static) in the TPS signal, it shows up dramatically on the scope. A digital voltmeter, on the other hand, averages the voltage readings as you open the throttle. So a quick dropout it harder to detect. That’s why it’s important to perform the TPS voltage test slowly if you’re using a digital meter.
Some TPS sensors have a 4th wire. Manufacturers use the 4th wire to tell the computer when the TPS is at “0.” When the wiper returns to the “off” stop, it opens a switch, cutting off TPS voltage. If your TPS only has 3 wires and it tests as bad, you may have to follow a special alignment procedure when installing the new sensor. The alignment/adjustment procedure is designed to make sure the “off” voltage falls within a certain acceptable range for the PCM. If you don’t adjust it properly, the PCM may think that the throttle is partially open, when it is really closed.
What causes a TPS to fail? Simple. Plain old wear. If you do mostly city driving, you’ll discover that the “static” or “dropouts” occur when you are trying to accelerate from around 30mph. That’s the point where the wiper arm is constantly moving back and forth as you try to maintain speed in city traffic.
© 2012 Rick MuscoplatPosted on by Rick Muscoplat