How To Test The TPS (1989-1990 3.9L V6 Dodge Ram Pickup)

The throttle position sensor (TPS) is one of the easiest fuel injection system components to check, and you don't need any expensive diagnostic tools to test it.

In this tutorial, I'm gonna show you how to check the TPS using three quick tests. You'll just need a multimeter for all of them.

Once you've got your results, it'll be simple to figure out if the sensor is fried or if it's good.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.9L V6 Dodge Ram 100 Pickup: 1989.
• 3.9L V6 Dodge Ram 150 Pickup: 1989, 1990.
• 3.9L V6 Dodge Ram 250 Pickup: 1989, 1990.

Symptoms Of A Bad Throttle Position Sensor

Your Dodge Ram's fuel injection computer keeps an eye on the throttle position all the time as soon as you turn the key to the ON position and especially when the engine is running.

If the throttle position sensor (TPS) goes bad, the computer throws a diagnostic trouble code (DTC), and the check engine light pops on:

• Code 24: TPS Voltage Is Too Low.
• Code 24: TPS Voltage Is Too High.

Besides the code, you'll see one or more of the following:

• Engine hesitation: It stutters or hesitates when you hit the gas pedal.
• Loss of power: Acceleration sucks, especially when the truck's under load.
• Bad gas mileage: The fuel injection computer kicks into "limp-in" mode and dumps extra fuel to protect the engine —your gas mileage takes a big hit.

How The TPS Works

To wrap our heads around the TPS sensor tests we're gonna do in this tutorial, it's helps to understand what the sensor actually does and how it generates its signal output.

In a nutshell, the TPS keeps track of how far the throttle plate opens or closes when you press or let off the gas pedal. It sends that info to the fuel injection computer as a variable voltage signal.

Here's what a good TPS should show:

• Throttle closed (idle): Voltage should be somewhere between 0.4 and 0.9 Volts DC.
• Throttle opening (as you hit the pedal): Voltage needs to rise smoothly without jumping.
• Wide Open Throttle (WOT): Voltage should climb up to around 4.5 to 4.7 Volts DC.
• Throttle closing (as you let off the pedal): Voltage needs to decrease smoothly without gaps.

Put simply, the voltage should go up (or down) smoothly as the throttle moves. If it's doing this, it's doing its job.

If the voltage stays stuck and doesn't match pedal movement, there's a good chance the sensor's bad (we still need to make sure it's getting power and Ground).

We're gonna use a multimeter in this tutorial to check that voltage as you go from idle to full throttle and see if the TPS is working correctly.

Throttle Position Sensor Circuit Descriptions

In the table below, you'll find a short description of what each of the 3 wires coming out of the 3-wire does:

Pin	Wire Color	Description
1	Violet with white stripe (VIO/WHT) or Violet (VIO)	5 Volts
2	Orange with dark blue stripe (ORG/DK BLU)	Throttle Position Signal
3	Black with light blue stripe (BLK/LT BLU)	Sensor Ground

Where To Buy The TPS And Save

The throttle position sensor, for your Dodge Ram pickup, isn't an expensive component. The following links will help you comparison shop and hopefully save you a few bucks on its purchase:

The following links:

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

Most of the time, when the throttle position sensor (TPS) fails, it just stops creating its variable voltage signal that shows the throttle angle.

Basically, it quits tracking and reporting how far the throttle plate's opening/closing. You and I can check this easily with a multimeter and that's exactly what we're gonna do in this test section.

IMPORTANT: You've gotta leave the TPS plugged into its 3-wire connector while testing it. So either back-probe the connector or use a wire-piercing probe to reach the signal wire. You can check out what that tool looks like and where to grab one here: Wire Piercing Probe Tool.

Here's how to run the test:

1. 1

   Set your multimeter to Volts DC mode.

2. 2

   Connect the black test lead straight to the battery's negative (-) terminal.

3. 3

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

4. 4

   Touch the red test lead to the ORG/DK BLU wire on the TPS connector.
   
   The ORG/DK BLU wire is the one that connects to the male terminal 2 of the 3-wire TPS connector.
   
   NOTE: Keep the TPS plugged into its connector while you test the signal.

5. 5

   You should see a voltage between 0.3 and 1.0 Volts DC at this point with the throttle plate fully closed.

6. 6

   Open the throttle plate by hand.
   
   You'll get the best results doing this right at the throttle body instead of pressing the gas pedal.

7. 7

   Voltage should rise smoothly as the throttle plate opens.
   
   With the throttle wide open, your multimeter should read somewhere between 3.2 and 4.9 Volts DC.
   
   Again, do this by hand at the throttle body for better accuracy.

8. 8

   Slowly close the throttle plate. The voltage should drop smoothly.

9. 9

   Tap the TPS gently with a screwdriver handle while opening and closing the throttle. Watch the multimeter.
   
   This helps you check for glitches in the voltage signal. A good TPS keeps a smooth signal even when tapped like this.

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

CASE 1: Voltage went up and down smoothly with no gaps. That's exactly what we wanna see. It means the TPS on your Dodge Ram's working fine.

Since the ORG/DK BLU wire shows a clean variable voltage signal, this also tells you:

1. The TPS is getting 5V power.
2. It's also getting Ground.

CASE 2: Voltage didn't change at all. That usually means the TPS is probably shot.

To make sure, we need to make sure it's actually getting power and Ground. For that, head over to: TEST 2: Making Sure The TPS Is Getting 5 Volts

CASE 3: Voltage jumped or cut out when you tapped the sensor. If the signal only glitched while tapping the TPS with the screwdriver, it's a clear sign it's bad and needs to be replaced.