How To Test The TPS (1991 3.9L V6 Dodge Ram Pickup)

The throttle position sensor (TPS), on your 1991 V6 Dodge Ram pickup, is one of the easier components to check using a regular multimeter. You won't need a scan tool for any part of this test.

In this tutorial, I'll walk you through three quick tests to help you figure out if your TPS is working right —or if it’s time to replace it. Let’s get into it.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.9L V6 Dodge Ram 150 Pickup: 1991.
• 3.9L V6 Dodge Ram 250 Pickup: 1991.

Symptoms Of A Bad Throttle Position Sensor

As you probably know, the accelerator pedal connects to the throttle plate with a physical cable. Once the engine's running, here's what goes down:

• Step on the gas pedal, and the throttle plate opens up, letting more air flow into the engine.
• Let off the pedal, and the throttle plate shuts, cutting back the airflow.

Your Dodge Ram's fuel injection system tracks the position of the throttle plate (while you step on or off the pedal) using the throttle position sensor (TPS).

If the TPS stops working right, engine performance takes a hit —big time. Here's what you might run into when the sensor fails:

• Trouble codes: These codes will trigger the check engine light (CEL):
  • Code 24: TPS voltage is reading too low.
  • Code 24: TPS voltage is reading too high.
• Won't start: No matter how long you crank it, the engine just won't fire up.
• Hard to start: You'll have to crank it several times before it catches.
• Stumbling or hesitation: It hesitates when you accelerate, either from a stop or while you're already moving.
• Feels sluggish: You press the pedal and it feels like the engine's holding back.
• Terrible gas mileage: The computer kicks into 'limp-in' mode to protect the engine, but your fuel economy tanks.

Where To Buy The TPS And Save

The following links will help you to comparison shop for the TPS for your 3.9L V6 Dodge Ram pickup. I think they'll save you a few bucks:

If you're unsure if this TPS is compatible with your specific 3.9L V6 Dodge Ram pickup, no worries. Once you visit the site, they'll check and ensure it's the right one by asking for more details about your vehicle. If it doesn't fit, they'll help find the correct one.

TEST 1: Testing The TPS Voltage Signal

Like I mentioned earlier, the throttle position sensor (TPS) creates a voltage signal that matches the throttle plate's position.

Here's what normal TPS behavior looks like:

• When the throttle is closed: The TPS sends out a low voltage signal.
• As you press the gas: The throttle starts to open, and the TPS voltage gradually climbs.
• At Wide Open Throttle (WOT): When the throttle is fully open, the TPS usually reads close to 4.5 Volts DC.
• When letting off the pedal: The throttle closes again, and the voltage smoothly drops back down to its original low reading.

When the TPS goes bad, this voltage won't rise or fall like it's supposed to —it'll stay stuck or jump around instead of following the throttle plate's movement.

NOTE: You'll need a multimeter to run these tests and check if the TPS is doing its job. If you don't have one, this is the one I use and recommend: Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (at: amazon.com).

IMPORTANT: The TP sensor must remain connected to its electrical connector. You'll need to use a back probe on the connector or wiring piercing probe to access the signal in the wire. You can see an example of this tool here and where to buy it: Wire Piercing Probe.

Alright, let's jump into the test:

1. 1

   Hook the black lead of your multimeter to the battery's negative (-) terminal.

2. 2

   Switch your multimeter to Volts DC mode.

3. 3

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

4. 4

   Connect the red test lead to the middle wire on the TPS connector. Use a back probe on the connector or a wire-piercing probe on the wire to do this.
   
   NOTE: The throttle position sensor needs to stay plugged into its connector for this test.

5. 5

   You should see a voltage between 0.2 and 0.9 Volts DC if the throttle plate is closed.
   
   If that's not what you see, no worries —go ahead and continue with the rest of the test.

6. 6

   From the engine bay, slowly move the throttle open by hand.

7. 7

   The voltage should steadily rise.
   
   With the throttle fully open, your multimeter should read somewhere between 3.5 and 4.5 Volts DC.

8. 8

   Now slowly let the throttle close all the way.

9. 9

   The voltage should drop smoothly back down to where it started.

10. 10

    Lightly tap the TPS with the handle of a screwdriver while opening and closing the throttle.
    
    A faulty TPS will glitch and send jumpy signals when tapped. A good one —the voltage stays smooth no matter what.

Now, let's break down what your test results mean:

CASE 1: The TPS voltage rose and dropped correctly as you opened and closed the throttle. That's exactly what we want. This confirms your TPS is working as it should.

It also proves the sensor is getting the right power and Ground signals from the fuel injection system.

CASE 2: The voltage reading never changed —it stayed stuck at one number. That usually means the TPS is shot.

But before you swap it out, you've gotta double-check that it's actually getting power and Ground. Go here next: TEST 2: Making Sure The TPS Is Receiving 5 Volts.

CASE 3: You got no voltage reading at all. This result often points to a dead sensor.

Still, don't rush to replace it just yet. Make sure the sensor's getting power and Ground first. Head to: TEST 2: Making Sure The TPS Is Receiving 5 Volts.