How To Test The 1996-1998 3.8L Ford Mustang Throttle Position Sensor (TPS)

TEST 2: Testing The 5 Volt Reference Signal

For our next test we're going to make sure that the throttle position sensor is getting 5 Volts DC.

The TPS gets these 5 Volts on the wire labeled with the number 1 in the photo above.

If 5 Volts are present, then our next step is to check that the TPS is getting Ground in TEST 2.

Let's get started:

1. 1

   If the engine isn't already at normal operating temperature, start it and let it warm up.

2. 2

   Once it's warmed up, turn it off and remove any thing that may be blocking access to the throttle position sensor on the intake manifold's throttle body.

3. 3

   Disconnect the TPS from its electrical connector.

4. 4

   Set the multimeter function to Volts DC mode
   
   Don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing.

5. 5

   Probe the wire identified with the number 1 in the photo, with the red multimeter test lead and an appropriate tool (like a wire-piercing probe or a back probe).

6. 6

   Connect the black multimeter test lead to a good and clean Ground point on the engine or directly on the negative (-) battery terminal.

7. 7

   Rotate the key to its ON position but don't start the engine.

8. 8

   The multimeter should display 4.5 to 5 Volts on its screen.

Let's analyze your test results:

CASE 1: The multimeter registered 4.5 to 5 Volts. This is the correct test result and it confirms that the fuel injection computer and the circuit is supplying the TPS with power.

The next step is to test the Signal Return Circuit, go to: TEST 3: Testing The Sensor Return (Ground) Circuit.

CASE 2: The multimeter DID NOT register 4.5 to 5 Volts. This test result tells you that the computer or the circuit are NOT providing the voltage that the TPS needs to operate.

The two most likely reasons for this are: 1) an open-circuit problem in the circuit or 2) the PCM may be fried.

Altho' it's beyond the scope of this article to test these two conditions, you have now eliminated the throttle position sensor (TPS) on your Mustang as being the cause of the problem and/or the TPS diagnostic trouble code (DTC) lighting up the check engine light (CEL).

TEST 3: Testing The Sensor Return (Ground) Circuit

Besides providing the TPS with power (5 Volts), the PCM also has to provide a Ground for the throttle position sensor, and in this test step you're gonna' check that it is.

The wire that supplies the TPS with Ground is the wire labeled with the number 3 in the photo above.

NOTE: The fuel injection computer, on your Ford Mustang, is the component that supplies Ground to the throttle position sensor. For this reason, you must be careful not to short-circuit this wire to battery 12 Volts, or you'll fry the fuel injection computer. The multimeter voltage test I'm suggesting in the test instructions is a safe way to test for the presence of Ground in the wire.

Alright, let's start:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

   Disconnect the TPS from its electrical connector.

3. 3

   Probe the circuit labeled with the number 3 in the photo, with the black multimeter test lead.

4. 4

   Connect the red multimeter test lead to the battery positive (+) terminal.

5. 5

   Turn the key to its ON position but don't start the engine.

6. 6

   The multimeter should display 10 to 12 Volts if Ground is present.

Let's analyze your test results:

CASE 1: The multimeter displayed 10 to 12 Volts. This is the correct test result.

You can conclude that the PCM and the wire/circuit (that supply this Ground) are OK. You can also conclude that the TPS is bad and needs replacement if you have:

• Confirmed that the TPS signal voltage does not increase/decrease as you open/close the throttle plate (TEST 1).
• Confirmed that the TPS is getting 5 Volts DC (TEST 2).
• Confirmed that the TPS is getting Ground (this test section).

CASE 2: The multimeter DID NOT display 10 to 12 Volts. This test result indicates a problem with either the PCM (internal fault/problem) or an open-circuit problem in the wire between the TPS and the PCM itself.

Altho' testing these two conditions are beyond the scope of this article, you have now eliminated the throttle position sensor (TPS) on your Ford vehicle as being the cause of the problem and/or the TPS diagnostic trouble code (DTC) lighting up the check engine light (CEL).

TPS Code Won't Go Away

So you've tested your Ford Mustang TPS and according to the test results, the TPS is good. But the check engine light keeps coming back on even after you erased the diagnostic trouble code (DTC) from the computer's (PCM) memory. Well, here are a couple of suggestions that might inspire your next diagnostic move:

1. The throttle plate's idle-stop screw's factory adjustment has been altered so that the engine could be idled up and mask a miss/misfire and/or rough idle. This increases the TP Sensor's signal to the PCM. The PCM doesn't like it and lights up the check engine light (CEL).
2. The throttle cable is binding and causing the throttle plate to not fully close.
   • This can be verified by simply having someone inside the vehicle pushing the accelerator pedal to the floor and releasing it, with the engine OFF, while you visually check that the throttle plate and cable are not getting stuck somewhere in their travel.
3. The TPS is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:
   • I have found that the best way to test these intermittent problems is to road-test the vehicle with the multimeter hooked up to the TP signal wire with a long wire so that I can comfortably observe the signal going up and down as I or someone else drives.
4. The TP sensor's connector is bad, usually the locking tab is broken and the connector has worked itself loose, causing an intermittent false connection.