How To Test The Throttle Position Sensor (GM 3.1L, 3.4L)

This article will take you step by step thru' the testing and troubleshooting of the throttle position sensor on your GM 3.1L or 3.4L V6 equipped car (Buick, Chevrolet, Oldsmobile, Pontiac). You don't need an automotive scan tool, since all of the tests, in this article, are done with just a simple multimeter.

Before you start the TPS tests, read this section first. At the bottom, you'll find a complete list of GM makes and models this test info applies to. Also, here, you'll find important 'do's and don'ts' that'll make testing the TP sensor on your car a breeze.

You can find this tutorial in Spanish here: Cómo Probar El Sensor TPS (GM 3.1L, 3.4L) (at: autotecnico-online.com).

Symptoms Of A Bad GM TPS

The most common is the check engine light lit up on the instrument cluster, reminding you that you are not imagining there's something wrong. Here are a couple of others:

1. TPS diagnostic trouble codes (DTCs) stored in the vehicle's computer's memory.
   • P0120, P0121, P0122, P0123, P0125, P1121, P1122
2. Really bad gas mileage.
3. Transmission does not shift out of second gear.
4. No power as you accelerate the vehicle.
5. Hesitation when you step on the accelerator pedal.
6. Car does not idle right (unstable/rough idle).

What Tools Do I Need For The TPS Test?

The throttle position sensor is a very easy component to test with few simple tools. You'll need:

• A multimeter.
  • If you don't have a digital multimeter o need to buy one, check out my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.
• A back probe or a wire piercing probe.
  • You can see an example of this tool here: Wire Piercing Probe.

Also, you'll need someone to help you. This assistant will come in handy when doing TEST 3 of this tutorial.

Circuit Descriptions Of The GM 3.1L And 3.4L TPS

The throttle position sensor on your 3.1L (or 3.4L) GM vehicle has three wires connected to it. Each wire has a specific task to accomplish and with the photos and circuit descriptions below, you'll test each one with your multimeter.

All three circuits start or end at the PCM (that is whether you see the glass as half empty or half full), so you got to be very careful not to short any of these wires (circuits) to power (12 Volts). If this happens, you'll have a fried Computer on your hands.

• Circuit labeled A:
  1. 5 Volts from PCM.
• Circuit labeled B:
  1. Sensor Return (Ground) Circuit.
• Circuit labeled C:
  1. Throttle Position (TP) Signal Circuit.

An important testing tip I'm going to share with you, that may save you a big headache, is that you should avoid probing the front of the TP sensor connector's terminals to test for the 5 Volts and Ground. If you do probe the front of the female terminal, be very careful not to damage it with the multimeter's test lead.

How Does The GM 3.1L And 3.4L TPS Work?

In a nutshell, the throttle position sensor tells the computer how much throttle movement you are causing as you accelerate or decelerate going down the road. Now to be a little more specific, when you crank, start, and get your car moving:

1. The PCM supplies the TP sensor with 5 Volts and Ground.
2. Once the engines starts and is running (idle), the TPS reports a DC voltage signal of about 0.5V to 0.9V to the PCM. This is the base voltage that the computer needs to have.
3. Once you throw the car in drive and accelerate the car to get it moving, the throttle position sensor measures whatever movement of the throttle, is caused by your foot action on the accelerator pedal, into a rising DC voltage signal that the PCM uses to do its little song and dance (calculate fuel injection, ignition timing, transmission shift points, etc).
4. As you release foot pressure on the accelerator pedal to slow down, the throttle closes and of course the TP sensor sends the info to the PCM.
5. This happens continually the whole time you're driving.

Easy stuff, no? Well, the tests to diagnose the TP sensor's performance are as easy too. Now, since you'll be working in the engine compartment take all necessary safety precautions and use common sense. OK, enough of my yakking let's get this party started, for the first test go to: TEST 1: Testing The TP Signal.

Where To Buy The Throttle Position Sensor And Save

The following links will help you comparison shop for the original AC Delco (Delphi) throttle position sensor and after-market TPS:

Not sure if the above throttle position sensor fits your particular GM vehicle? Don't worry, once you get to the site they'll make sure it fits. If it doesn't, they find you the right part.

TEST 1: Testing The TP Signal

If the throttle position sensor (TPS) is working correctly, then as you open the throttle plate, the multimeter will register a smooth increase in the DC voltage until the maximum voltage is reached, which is about 4.5 Volts DC.

Then, as you slowly release the throttle plate back to its closed position, the multimeter will display a gradual decrease in voltage till the initial base voltage is achieved (which you recorded in the beginning of the test).

If the TPS is bad, its voltage signal will stay stuck in one value as you open/close the throttle plate. Or, the voltage signal will have sudden gaps as you open/close the throttle plate to its fully open or fully closed position, especially when you tap on the sensor.

You'll need a helper to assist you in this test step, since he or she will need to lightly tap on the throttle position sensor's body with a screw driver (or other appropriate tool) while you observe the multimeter and manually actuate the throttle.

IMPORTANT: The throttle position sensor (TPS) must remain connected to its electrical connector for this test to function properly. To be able to access the voltage inside the signal wire, you'll need to use either a back probe or a wire piercing probe. You can see an example of this tool here: Wire Piercing Probe.

OK, let's start testing:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

   Connect the red test lead to the wire labeled with the letter C of the TP sensor harness connector.

3. 3

   Ground the black multimeter test lead directly on the battery negative (-) post.

4. 4

   Manually rotate the throttle plate.
   
   You'll get the best results by opening and closing the throttle plate directly on the throttle body instead of stepping on the accelerator pedal.

5. 5

   The multimeter should show an increasing voltage as you (or your helper) open up the throttle plate.
   
   You'll get the best results by opening and closing the throttle plate directly on the throttle body instead of stepping on the accelerator pedal.

6. 6

   The multimeter should show a decreasing voltage as you begin to close the throttle plate.

7. 7

   Using a screwdriver's handle, gently tap the TP sensor as you open and close the throttle plate and observe the multimeter.
   
   The purpose (of tapping the TP sensor with the screwdriver's handle) is to see if the TP sensor shows gaps in the voltage signal. Why? Because a good TP sensor will show a continuous increasing or decreasing voltage signal even while getting tapped by the screw-driver's handle.

Let's interpret your test result:

CASE 1: The multimeter registered a smooth increase or decrease in voltage. This is the correct test result and tells you that the TP sensor is working OK and is not the cause of the TPS fault code issue.

If the fuel injection computer is still setting a TPS trouble code, consult the suggestions found here: The TPS Code Won't Go Away.

CASE 2: The multimeter DID NOT register a smooth increase or decrease in voltage. Then the throttle position sensor (TPS) is bad only if you have confirmed that:

• The TPS sensor is not producing a voltage signal that increases/decreases as you open/close the throttle plate (TEST 1).
• The TPS sensor is receiving 5 Volts DC (TEST 2).
• The TPS sensor is receiving Ground (TEST 3).

If your TPS sensor tests produced the above 3 results, then replacing the throttle position sensor will solve the TPS diagnostic trouble code (DTC) lighting up your check engine light (CEL) on your GM car or mini-van.

CASE 3: The multimeter DID NOT register any voltage. Then the throttle position sensor (TPS) is bad only if you have confirmed that:

• The TPS sensor is not producing a voltage signal that increases/decreases as you open/close the throttle plate (TEST 1).
• The TPS sensor is receiving 5 Volts DC (TEST 2).
• The TPS sensor is receiving Ground (TEST 3).

If your TPS sensor tests produced the above 3 results, then replacing the throttle position sensor will solve the TPS diagnostic trouble code (DTC) lighting up your check engine light (CEL) on your GM car or mini-van.