How To Test The GM 3.8L Throttle Position Sensor (TPS)

Troubleshooting a bad throttle position sensor can be easily done with only a multimeter. The TPS is one of those sensors that you can test, to see if it's good or bad, without a scan tool and in this article I'm gonna' show you how to do it in three easy tests.

You can find this tutorial in Spanish here: Cómo Probar El Sensor TPS (GM 3.8L) (at: autotecnico-online.com).

NOTE: I've written several 'how to test' tutorials that are 3.8L GM specific, to see all GM 3.8L V6 test articles, go here: GM 3.8L Index Of Articles.

Important Tips And Suggestions

TIP 1: The 1988 thru' 1992 TP sensor does not look like the one in the photo, but you can still apply the testing info (in this article to it).

The 1988-1992 TPS is the adjustable type and the TPS in the photo is the non-adjustable type used from 1993 onward.

TIP 2: Since the1988-1992 TP sensors are the adjustable type, these need to be calibrated to a certain output voltage (when they're replaced with a new one). If this is the case in your vehicle, please read section: How To Calibrate the 1988-1992 TPS.

Symptoms Of A Bad TPS

You'll have the check engine light on, for sure, on your instrument cluster and one of several of the following symptoms:

• TPS diagnostic trouble codes (DTCs) stored in the vehicle's computer's memory.
• Really bad gas mileage. You know that it's not the price of gasoline that has you thinking that your 3.8L V6 equipped GM vehicle is costing you more at the pump.
• Transmission does not shift out of second gear. Now, this doesn't happen very often, but it happens.
• No power and/or hesitation as you accelerate the vehicle. It feels like all of a sudden someone cut the power out momentarily as you step on the gas to get the vehicle moving.

How The Throttle Position Sensor Works

The throttle position sensor's job is to measure the angle of the throttle. So here, in a nutshell, is how the throttle position sensor works when you crank and start your 3.8L GM V6 equipped vehicle:

1. The fuel injection computer supplies 5 Volts and Ground to the throttle position sensor.
2. Now, since the throttle is closed, the TPS (with power and Ground supplied) sends the PCM (Powertrain Control Module=Fuel Injection Computer) a DC voltage signal of about 0.5 to 0.8 Volt. This value is what the PCM associates with a closed throttle.
3. Once you throw the car in drive and accelerate the car, the throttle opens and the throttle position sensor immediately sends this change of the throttle angle as an INCREASING voltage signal to the PCM.
4. With this increasing voltage signal, the PCM knows it's time to inject more fuel, advance ignition timing, and a host of other things it has to do to keep your 3.8L GM equipped vehicle running optimally.
5. As you step off the accelerator pedal to slow down, the throttle plate closes and of course the TP sensor sends the info to the PCM as it returns to its base voltage signal, till the whole cycle begins again.

Pretty easy stuff? The cool thing is that the tests to check out 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 show on the road, go to: TEST 1: Testing The TPS Signal.

Where To Buy The TPS And Save

The following links will help you comparison shop for the throttle position sensor:

Not sure if the above TPS fits your particular 3.8L V6 GM vehicle? Don't worry. Once you get to the site they'll ask you for your vehicle's info. If it doesn't fit, they'll find you the right one.

TPS TEST 1: Testing The TPS Signal

We're gonna' jump right into the first test, which is to verify, with your trusty multimeter, that the throttle position sensor (TPS) is creating a viable throttle position signal that the fuel injection computer can use.

Before you actually practice the test on your car, I suggest you read the entire article first and especially take a look at the section: How The Throttle Position Sensor Works. Besides, there's a quiz at the end of the article.

One last thing, to properly test the throttle position sensor on your 3.8L GM car, you'll need a helper.

IMPORTANT: The throttle position sensor must remain connected to its electrical connector. You'll need to use a back probe on the connector or a wire piercing probe on the wire.

NOTE: If your car starts, crank it up and let it warm up. This test will be more effective at diagnosing a bad TPS if the engine and the TPS are at normal operating temperature. In some cases, a bad throttle position sensor (TPS) will make the car not start. So, if your car doesn't start, don't worry about the engine being warm, continue with the test steps.

OK, this is what you'll need to do:

PART 1:

1. 1

   Select Volts DC mode on your multimeter.

2. 2

   Probe the sensor's dark blue wire with the red multimeter test lead.
   
   This is the wire that connects to the terminal labeled with the letter B (in the illustration above).
   
   NOTE: The throttle position sensor must remain connected to its electrical connector.

3. 3

   Connect the black multimeter test lead to the battery negative (-) terminal.

4. 4

   Turn the key ON but the engine OFF.

5. 5

   The multimeter should register about 0.5 to 0.8 Volts DC.
   
   You'll usually see something between 0.38 and .042 Volts and this is OK too. If it doesn't, don't worry about it just yet, continue with the other steps.

PART 2:

1. 6

   Open the throttle very slowly and by hand, till it opens to its maximum open position.

2. 7

   The voltage reading, on your multimeter, should increase and stop at about 4.5 to 4.9 Volts DC (once the throttle is fully open).

3. 8

   Slowly close the throttle plate.

4. 9

   The voltage should decrease till it reaches the value you observed in test step 5.

PART 3:

1. 10

   Lightly tap on the throttle position sensor (TPS) with the handle of a screw-driver, while you rotate the throttle open and then rotate it back to its closed positions.
   
   I want to emphasize the words 'lightly tap', since the TPS sensor is made out of plastic and could break if beaten.
   
   The purpose of this screw-driver tapping is to see if it (the tapping) messes up the voltage readings your multimeter.

2. 11

   The tapping SHOULD NOT have any effect on the voltage readings.

3. 12

   Repeat step 10-11 several times to make sure of your multimeter test results.

Let's analyze your test results:

CASE 1: The multimeter registered a smooth rise and fall in the voltage reading. This tells you that the TPS is good. No further tests are needed.

Now, if the throttle position sensor code keeps coming back, take a look at the heading: TPS Code Will Not Go Away for a few more suggestions as to what could be causing the TPS Diagnostic Trouble Code (DTC).

CASE 2: The multimeter DID NOT register a smooth rise and fall in voltage and the voltage reading skipped or went dead when tapping the TPS. This test result tells you that the throttle position sensor (TPS) is bad. Replace the throttle position sensor.

CASE 3: The multimeter DID NOT register any voltage. This test result is not good but does not condemn the TPS as bad yet. Two more things need to be verified first, which are power and Ground to the sensor. Go to: TPS TEST 2: Testing The 5 Volt Reference Signal.