How To Test The Throttle Position Sensor (1994-1995 5.0L V8 Ford Mustang)

The throttle position sensor (TPS) on your 5.0L V8 Ford Mustang is a potentiometer-type sensor. And let me tell you, it's one of the easiest sensors to test on your Mustang's V8 engine.

In this tutorial, I'm going to show you step-by-step how to check it with a multimeter so you can find out if it's bad or if it's good and doing its job.

In total, there are three tests: checking the throttle position sensor's output signal (TPS), and making sure it's getting power (VREF) and Ground (SIG RTN).

With these three tests, you'll quickly and easily know if the sensor is OK or if it's toast and needs to be replaced.

APPLIES TO: This tutorial applies to the following vehicles:

• 5.0L V8 Ford Mustang: 1994, 1995.

1990-1993 TPS DIAGNOSTIC TESTS:

• How To Test The Throttle Position Sensor (1990-1993 5.0L V8 Ford Mustang).

Symptoms Of A Bad Throttle Position Sensor (TPS)

The throttle position sensor's job is to let the fuel injection computer know how much you step on or off the accelerator pedal by measuring the throttle plate angle.

And as you know, the gas pedal is connected to the throttle plate (in the throttle body) through a mechanical accelerator cable.

Now, when the TPS fails, the lack of throttle angle information to the fuel injection computer will cause one or more of the following problems:

• Check engine light with TPS codes:
  • Code 121: Closed Throttle Voltage Higher Or Lower Than Expected (TPS voltage inconsistent with the mass airflow sensor).
  • Code 122: Throttle Position Sensor Circuit Below Minimum Voltage.
  • Code 123: Throttle Position Sensor Circuit Above Maximum Voltage.
  • Code 124: Throttle Position Sensor Voltage Higher Than Expected.
  • Code 125: Throttle Position Sensor Voltage Lower Than Expected.
• Hesitation on acceleration: Stumbling or hesitation when you press the gas pedal.
• Loss of power: Intermittent lack of power while accelerating.
• Bad gas mileage: Noticeable drop in gas mileage.
• Hard starting: Engine takes longer than normal to crank and fire up.
• Rough/lopey idle: Idle speed surges up and down instead of staying steady.
• No-start condition: If TPS voltage is stuck high, the engine may not start at all.

Where To Buy The TPS And Save

When you're ready to replace the throttle position sensor on your 5.0L V8 Mustang, I have two recommendations for you from well-known automotive brands I've trusted for a long time and don't hesitate to recommend:

TEST 1: Checking The Throttle Position Sensor (TPS) Signal

To wrap our heads around the TPS signal voltage output check we're going to do in this section, we need to know that:

• The TPS voltage signal increases as the throttle plate opens.
• The TPS signal output decreases as the throttle plate closes.

You and I can easily verify this by tapping into the gray with white stripe (GRY/WHT) wire —which is the TPS signal wire— of the TPS sensor's 3-wire connector.

If during our test the TPS signal voltage stays stuck at one value no matter how much we open or close the throttle plate, we can conclude the sensor is toast.

The one thing to keep in mind when checking the TPS output voltage is that it must remain connected to its connector. So, to check the signal in the GRY/WHT wire, you'll need to use either a back probe or a wire-piercing probe.

If you don't have one of these tools, this is the one I use and recommend (it works as both a back probe and a wire-piercing probe):

• Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

If you don't have a multimeter or need to upgrade yours? Check out my recommendation: Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (Amazon affiliate link).

Here are the test steps:

1. 1

   Hook up the black test lead of your multimeter to the negative (-) battery post.

2. 2

   Switch your multimeter to DC Volts mode.

3. 3

   Turn the key to the ON position (engine OFF). Do not crank or start the engine.
   
   This step powers up the TPS.

4. 4

   Probe the middle wire of the TPS connector with the red multimeter lead and an appropriate tool (like a back probe or a wire-piercing probe).
   
   The middle wire of the TPS connector should be a GRY/WHT wire.
   
   NOTE: The TPS must stay connected to its 3-wire connector during this test.

5. 5

   With the throttle plate completely closed, the voltage should read between 0.3 and 0.9 Volts DC.
   
   This is your base reading.

6. 6

   Open the throttle plate slowly by hand. The voltage should climb smoothly as the plate opens.

7. 7

   At wide open throttle (WOT), the voltage should be between 3.5 and 4.5 Volts DC.

8. 8

   Close the throttle plate slowly. The voltage should fall back down to the same number you recorded in step 5.
   
   Verify that the closed-throttle reading matches your starting value.

9. 9

   Lightly tap the TPS body with the handle of a screwdriver while opening and closing the throttle plate.
   
   NOTE: Tapping on the sensor should not cause the readings to spike, cut out, or glitch. If it does, the TPS is faulty and must be replaced.

Here's what your readings mean:

CASE 1: The TPS voltage signal went up and down smoothly without any glitches. That's the result we want to see, and it tells you the TPS is functioning like it should.

This also verifies the sensor is getting both its power (VREF) and Ground (SIG RTN). At this point, no further testing is needed.

CASE 2: The TPS voltage signal stayed flat and didn't change. Most of the time this points to a bad TPS, but not in every case.

To be sure, you'll need to do two more checks. The first one is here: TEST 2: Making Sure The TPS Is Getting Power (VREF).

CASE 3: The TPS voltage signal jumped or cut out only when the sensor was tapped. If that happens, the TPS is faulty and needs to be replaced.

TEST 2: Making Sure The TPS Is Getting Power (VREF)

If your TEST 1 result shows that your Mustang's TPS voltage signal is stuck at one value, we now need to make sure the brown with white stripe (BRN/WHT) wire is actually delivering 5 Volts from the fuel injection computer.

Without these 5 Volts (VREF), the TPS won't be able to produce its variable voltage signal as the throttle plate opens and closes on the throttle body.

We can confirm the presence of these 5 Volts in the BRN/WHT wire with a simple multimeter voltage test at terminal 1 of the TPS sensor's 3-wire connector.

NOTE: The VREF and SIG RTN tests are done on the TP sensor's 3-wire pigtail connector, which has female terminals. Be careful when probing the terminals of the connector with your multimeter's probe tip to avoid damaging the connector.

Here's what we need to do:

1. 1

   Unplug the TPS from its 3-wire connector.

2. 2

   Switch the key to the ON position (engine off). Don't crank or start the engine —this step only powers up the TPS circuit.

3. 3

   Put your multimeter in Volts DC mode.

4. 4

   Attach the black multimeter lead to the battery's negative (-) post.

5. 5

   Probe female terminal 1 of the TPS connector with the red multimeter lead.
   
   NOTE: Terminal 1 is the 5-Volt supply and should connect to the BRN/WHT wire of the connector.

6. 6

   The multimeter should show between 4.5 and 5.0 Volts DC.

Here's what it all means:

CASE 1: The multimeter shows 4.5 to 5 Volts. That's the reading we're looking for, and it confirms the TPS is getting its power (VREF) feed.

The next thing to check is whether the sensor is getting its SIG RTN Ground. You can do that here: TEST 3: Making Sure The TPS Is Getting Ground (SIG RTN).

CASE 2: No voltage is present. Double-check that your test lead is on the correct terminal and run the test again.

If you still don't see 4.5 to 5 Volts, then the TPS isn't at fault. Without that 5-Volt supply, it simply can't operate.

At this point, the next step is to track down why the 5 Volts are missing and fix the problem so the TPS can work again.

In most cases, the missing VREF voltage comes from an open in the BRN/WHT wire somewhere between the TPS connector and the computer.

TEST 3: Making Sure The TPS Is Getting Ground (SIG RTN)

Up to this point, you've confirmed the following about your Mustang's TP sensor:

• It's not producing a variable voltage TPS signal (TEST 1).
• The BRN/WHT wire is delivering 5 Volts (VREF) (TEST 2).

For our next and final test, we'll make sure the gray with red stripe (GRY/RED) wire is delivering Ground from the fuel injection computer.

This Ground, in Ford-speak, is called the SIG RTN Ground. To check for its presence in the GRY/RED wire, we'll do a simple multimeter voltage test at terminal 3 of the connector.

IMPORTANT: Be careful with this SIG RTN circuit because the Ground is provided internally by the computer. Do not connect this wire to battery 12 Volts or you'll fry the fuel injection computer. The multimeter voltage test we're about to do is a safe way to verify the presence of this Ground.

OK, let's begin:

1. 1

   Unplug the TPS from its 3-wire connector.

2. 2

   Turn the ignition key to the ON position (engine off). Don't crank or start the engine —this step only powers up the TPS circuit.

3. 3

   Put your multimeter in Volts DC mode.

4. 4

   Attach the red multimeter lead to the battery positive (+) post.

5. 5

   Probe female terminal 3 of the TPS connector with the black multimeter lead.
   
   NOTE: Terminal 3 is the Ground circuit and should be the GRY/RED wire in the TPS connector.

6. 6

   Your multimeter should show between 10 and 12 Volts DC.

Here's how to interpret your test results:

CASE 1: Ground is present. That's the result you want to see.

You can conclude the TPS bad and needs to be replaced it if all of the following are true:

• In TEST 1, the TPS signal voltage stayed flat and did not rise or fall with the throttle plate movements.
• In TEST 2, the TPS showed it was receiving its 5-Volt reference signal.
• In this test, you've confirmed the TPS has a Ground feed.

When it's time to swap it out, here are the replacement TPS sensors I recommend (from trusted and known automotive brands):

• Standard Motor Products TH128 Throttle Position Sensor (Amazon affiliate link).
• Walker Products 200-1029 Throttle Position Sensor (Amazon affiliate link).

CASE 2: Ground is missing. First, make sure your test lead is on the right terminal and run the test again to be certain.

If the multimeter still doesn't show 10 to 12 Volts DC, then the TPS itself isn't the issue. Without a SIG RTN Ground, it simply can't function.

Although the fix is outside the scope of this tutorial, your next move is to track down the reason for the missing Ground and restore it so the TPS can go back to doing its job.

Most of the time, the problem comes down to an open in the GRY/RED wire somewhere between the TPS connector and the computer's connector.

More 5.0L V8 Ford Mustang Diagnostic Tutorials

You can find a complete list of wiring diagrams and diagnostic tutorials for the 5.0L V8 Ford Mustang in this index:

• 5.0L V8 Ford Mustang Index Of Articles.

Here's a sample of the diagnostic tutorials you'll find in the index:

• How To Test The Fuel Pump (1990-1995 5.0L V8 Ford Mustang).
• How To Test The MAF Sensor (1991-1995 5.0L V8 Ford Mustang).
• How To Test For A Blown Head Gasket (1990-1995 5.0L V8 Ford Mustang).
• How To Do And Interpret An Engine Compression Test (1990-1995 5.0L V8 Ford Mustang).