How To Test The TPS (1990-1995 4.0L Ford Aerostar, Explorer, Ranger. 1994 4.0L Mazda B4000)

The throttle position sensor on your 1990-1995 4.0L Ford Explorer, Aerostar, or Ranger can be tested very easily and without an expensive diagnostic scan tool. All you need is a multimeter. In this tutorial, I'll show you how to test it in a step-by-step manner. With the results of this test, you'll be able to say that the TPS is bad or not.

The contents of this tutorial at a glance:

  1. Symptoms Of A Bad Throttle Position Sensor.
  2. TEST 1: Testing The TPS Voltage Signal.
  3. TEST 2: Verifying Throttle Position Sensor Has 5 Volts and Ground.
  4. Where To Buy Your Throttle Position Sensor And Save.

En Español You can find this tutorial in Spanish here: Cómo Probar el Sensor TPS (1991-95 Ford 4.0L Explorer, Ranger, Aerostar) (at: autotecnico-online.com).

NOTE: Since the exact same throttle position sensor is used on several years of the 4.0L Ford Explorer, Aerostar, Ranger, (4.0L Mazda: B4000 and Navajo); you should take a look at the Applies To: box on the right hand column of this page to see if this tutorial applies to your specific vehicle.

If your 1995 or newer 4.0 liter Ford vehicle has the newer style of TPS, the following tutorial will help you to test it: How To Test The 4.0L Ford Throttle Position Sensor (TPS).

Symptoms Of A Bad Throttle Position Sensor

As you're probably already aware, the accelerator pedal is connected to the throttle plate via a cable. As you step on the accelerator pedal, the throttle plate opens. With the engine running, this results in more air entering the engine. As you step off the accelerator pedal, the throttle plate closes and the less air enters the engine.

The fuel injection computer on your 4.0 liter Ford needs to know how much the throttle plate opens (as you step on or off the accelerator pedal) to calculate a host of things to keep the engine running smoothly and optimally. Since the TP sensor a critical sensor for the engine management system, when it fails, engine performance suffers.

These are some of the symptoms that you're going to see when the TP sensor fails:

  1. One of the following trouble codes lighting up the check engine light (CEL):
    1. 23: Throttle Position Sensor Out of Self-Test Range.
    2. 43: Throttle Position Sensor Below Idle Spec.
    3. 53: Throttle Position Sensor Above Maximum Voltage.
    4. 63: Throttle Position Sensor Below Minimum Voltage.
    5. 121: Closed Throttle Position Sensor Voltage Higher or Lower Than Expected.
    6. 122: Throttle Position Sensor Below Minimum Voltage.
    7. 123: Throttle Position Sensor Above Maximum Voltage.
    8. 124: Closed Throttle Position Sensor Voltage Higher Than Expected.
    9. 125: Closed Throttle Position Sensor Voltage Lower Than Expected.
  2. Hesitation when accelerating the engine.
  3. Lack of power.
  4. Bad gas mileage.

TEST 1: Testing The TPS Voltage Signal

How To Test The TPS (1990-1995 4.0L Ford Aerostar, Explorer, Ranger. 1994 4.0L Mazda B4000)

You're going to need a multimeter to be able to test the throttle position sensor. If you don't have one, check out my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.

As I mentioned earlier, the TPS produces a voltage signal that depends on the position of the throttle plate. When the throttle plate is closed, the TPS produces a small voltage signal. As the throttle plate starts to open, the TPS produces a higher voltage signal. When the throttle plate reaches its maximum wide open position, the TPS produces a voltage of about 4.5 volts DC.

By tapping into the TPS signal wire with a multimeter, you and I can see if it's functioning correctly or not. This is a pretty simple test, but you will need one more tool. This tool is a wire piercing probe or a back probe.

OK, these are the test steps:

  1. 1

    Turn the key to the ON position but don't start the engine, and place your multimeter in Volts DC mode.

  2. 2

    Probe the wire labeled with the #2 (in the photo above) with the RED multimeter lead. This wire is usually a gray with white stripe (GRY/WHT) wire.

    Ground the BLACK multimeter test lead on the battery negative terminal.

    NOTE: The TP sensor must remain connected to its electrical connector. You'll need to use a back probe or a wiring piercing probe to tap into the signal of the middle wire. To see what a wire piercing probe looks like and where to buy one, go here: Wire Piercing Probe.

  3. 3

    Your multimeter should report a voltage between .2 to .9 Volts DC with the throttle plate closed. If your multimeter doesn't, don't worry about it just yet, continue with the other steps.

  4. 4

    Slowly open the throttle (by hand and from the engine compartment). The voltage numbers should increase as the throttle plate opens.

    This increase in voltage should be smooth and without any gaps or skips. Once the throttle is wide open, your multimeter should read somewhere between 3.5 to 4.5 Volts DC.

  5. 5

    Slowly close the throttle. As the throttle is closing, you should see the voltage decrease smoothly and without any gaps or skips, to the exact same voltage you noticed in step 3.

  6. 6

    Lightly tap on the throttle position sensor with the handle of a screw-driver (or something similar, and I want to emphasize the words ‘lightly tap’) as you slowly open and close the throttle and observe the multimeter.

    If the TPS is bad, the tapping will cause the voltage numbers to skip or go blank. If the TPS is OK, the tapping will have no effect on the voltage numbers.

  7. 7

    Repeat step 6 several times to make sure of your multimeter test results.

Let's take a look at your test results:

CASE 1: The throttle angle voltage increased and decreased as you opened and closed the throttle plate. This is the correct test result. It tells you that the throttle position sensor IS NOT defective.

This test result also confirms that the TPS is getting both power and ground from the fuel injection computer.

CASE 2: The throttle angle voltage DID NOT increase (or decrease) as you opened and closed the throttle plate. This test result usually tells you that the TPS is bad.

Just to tie up any loose ends, I recommend that you make sure that it's getting both power (5 Volts) and ground. For these tests, go to: TEST 2: Verifying Throttle Position Sensor Has 5 Volts and Ground.

CASE 3: Multimeter DID NOT register any voltage, this test result doesn't condemn the TP sensor as BAD just yet. Why? Because...

... the TP sensor may be missing either power or ground. So the next step is to check that the TP sensor is getting power and ground, go to TEST 2: Verifying Throttle Position Sensor Has 5 Volts and Ground.