How To Test The TBI Fuel Injectors (1989-1991 3.9L V6 Dodge Ram Pickup)

TEST 4: Fuel Injector Activation Signal Test

NOTE: Only run this test if both injectors failed to spray fuel in TEST 1.

If you've made it here, then you've already confirmed two things:

• TEST 1: Neither injector is spraying any fuel.
• TEST 2: Both injectors are getting 10 to 12 Volts of power.

Now in this section, we're gonna check whether the injectors are getting their activation signal —and we're gonna do it using an LED test light.

From my experience, using an LED is the easiest and safest way to check for those fuel injector pulse signals without risking damage to the fuel injection computer.

If you don't have an LED light, you can see what one looks like and you can also buy it here: Oznium Flush Mount 12V LED Light (Amazon affiliate link).

NOTE: Don't use a standard 12V test light for this check. A regular test light can overload the injector driver inside the fuel injection computer and damage it.

Alright, here's how to do the test:

1. 1

   Unplug the 4-wire connector from the fuel injectors. See the photo above to ID it.

2. 2

   Hook the red lead of your LED test light to terminal 1 on the connector.
   
   NOTE: Double-check that this terminal connects to one of the dark green with black stripe (DK GRN/BLK) wires —that's a power wire.

3. 3

   Now connect the black lead of the LED to terminal 2.
   
   Terminal 2 should correspond to the tan (TAN) wire, which delivers the activation signal for injector #2.

4. 4

   Have your helper crank the engine.
   
   CAUTION: Keep your hands, tools, and wiring clear of anything that moves while the engine cranks.

5. 5

   Watch for the LED to blink ON and OFF as the engine cranks.

6. 6

   Now disconnect the red LED lead from terminal 2.

7. 7

   Move the red lead to terminal 3.
   
   Terminal 3 should connect to the white with dark blue stripe (WHT/DK BLU) wire —this one controls injector #1.

8. 8

   Again, have your helper crank the engine.
   
   CAUTION: Make sure you stay clear of moving parts during this step too.

9. 9

   The LED should blink ON and OFF as the engine cranks.

Let's interpret your results:

CASE 1: The LED blinked ON and OFF on both signal wires. That's exactly what you want —it means the fuel injection computer is trying to fire both injectors.

If you've already confirmed all of the following:

• Both injectors aren't spraying (TEST 1).
• They're both getting proper voltage (TEST 2).
• They're receiving a firing signal from the computer (this test).

Then you've got two bad injectors if the fuel pump pressure is at 14.5 PSI. Time to replace them.

If you haven't checked the fuel pump's pressure, this tutorial will help you:

• How To Test The Fuel Pump (1989-1991 3.9L V6 Dodge Ram Pickup).

CASE 2: The LED did NOT blink on either signal wire. That tells you the injectors aren't being activated.

If you've already confirmed they have power (from TEST 2), the most likely issue is a break or open in the wiring between the injectors and the fuel injection computer.

Next step: test the wiring for continuity between the injector plug and the computer connector.

CASE 3: The LED blinked on only one of the signal wires. That usually means the other one isn't getting its signal because of a wiring issue between the fuel injector and the computer —assuming it's getting power (TEST 2).

Next step —check the wiring for continuity between the fuel injector connector and the fuel injection computer.

TEST 5: Making Sure The Fuel Injector Is Getting Power

NOTE: This test applies only if one injector is spraying fuel and the other isn't.

Since one injector is working, that tells us the ASD relay and the distributor pickup coil are both doing their job.

We also know that the working injector is getting voltage. What we need to figure out now is whether the injector that's not spraying is getting power too.

Here's how to run the test:

1. 1

   Unplug the 4-wire connector from the fuel injectors.

2. 2

   Set the multimeter to DC Volts mode.

3. 3

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

4. 4

   Use the red lead to probe terminal 1 of the connector.
   
   Make sure it's connected to one of the dark green with black stripe (DK GRN/BLK) wires.

5. 5

   Have your helper turn the key to the ON position (don't crank the engine).

6. 6

   You should see a reading between 10 and 12 Volts for about 3 seconds.
   
   This is the key on, engine off (KOEO) part of the test.
   
   If the engine doesn't get cranked, the computer cuts power after a few seconds —that's normal.

7. 7

   Now, have your helper crank the engine.
   
   CAUTION: Keep your hands and tools clear of any moving parts during cranking.

8. 8

   You should see 10 to 12 Volts the whole time the engine is cranking.
   
   This part is the key on, engine running (KOER) test.

9. 9

   Repeat this same voltage check on terminal 4.
   
   NOTE: Make sure terminal 4 connects to the other DK GRN/BLK wire.

Here's how to read what your multimeter tells you:

CASE 1: You've got 10 to 12 Volts on both terminals during KOEO and KOER. That's the result we're hoping for.

Now that we know the non-spraying injector is getting power, it's time to find out if it's also receiving a firing signal. Go to: TEST 6: Making Sure The Fuel Injector Is Getting An Activation Signal.

CASE 2: One terminal shows no voltage in both KOEO and KOER. That means the non-working injector isn't getting power —but the other one is.

The most likely reason is a break in the wiring between that injector and the engine harness. Troubleshooting that kind of open-circuit issue is outside the scope of this guide, but you'll need to track it down and fix the break to restore power to that injector.

TEST 6: Making Sure The Fuel Injector Is Getting An Activation Signal

NOTE: This part only applies if one of the injectors didn't spray fuel back in TEST 1.

If you've made it to this step, here's what you've already know:

• TEST 1: One injector is working; the other one isn't spraying.
• TEST 3: Both injectors show resistance within the correct range.
• TEST 5: The injector that isn't spraying is getting voltage.

In this step, we're gonna find out if that non-spraying injector is getting its activation signal —and we'll do it using a basic 12 Volt automotive LED light.

In my experience, an LED light is the safest way to do this. It protects the fuel injection computer's injector driver circuit from overload, and it makes it easy to spot those activation pulses.

If you don't have an LED light, you can see what one looks like and you can also buy it here: Oznium Flush Mount 12V LED Light (Amazon affiliate link).

NOTE: Don't use a regular 12V test light here. It can overload and possibly burn out the injector driver circuit inside the fuel injection computer.

Alright, let's get to it:

1. 1

   Unplug the 4-wire connector from the fuel injectors.

2. 2

   Connect the red lead from your LED test light to terminal 1 on the connector.
   
   NOTE: Double-check that this terminal connects to a dark green with black stripe (DK GRN/BLK) wire.

3. 3

   Connect the black lead of the LED to terminal 2.
   
   Terminal 2 should be wired to the tan (TAN) wire —this one supplies the activation signal to injector #2.

4. 4

   Have your helper crank the engine.
   
   CAUTION: Stay alert and keep your hands and tools away from any moving parts.

5. 5

   The LED should blink ON and OFF steadily as the engine cranks.

6. 6

   Now move the red lead of the LED to terminal 3.

7. 7

   Terminal 3 connects to the white with dark blue stripe (WHT/DK BLU) wire. This wire supplies the activation signal for injector #1.

8. 8

   Again, have your helper crank the engine.
   
   CAUTION: Just like before, be careful around moving parts.

9. 9

   The LED should blink ON and OFF while the engine is cranking.

Here's how to break down the results:

CASE 1: The LED blinked ON and OFF for both terminals. That's the expected result —it shows that the computer is sending the signals to fire both injectors.

If you've already confirmed all of the following:

• The injector isn't spraying fuel (TEST 1).
• Both injectors have correct resistance (TEST 3).
• The non-working injector is getting power (TEST 5).
• And now, both injectors are getting activation signals (this test).

Then that non-spraying injector is the problem —go ahead and replace it.

CASE 2: The LED didn't flash on one of the signal wires. That means that injector isn't getting the pulse it needs to fire.

If you know it's getting power (from TEST 5), then there's probably a break in the activation wire between the injector and the fuel injection computer.

Next, you'll want to check continuity between the injector connector and the computer's connector. If that wire's open, fixing it should get the injector firing again.

Tech Tip: Always Replace Injector O-Rings

If you're planning to pull out one or both fuel injectors —whether you're swapping them for testing or replacing them outright— it's really important that you don't reuse the old O-rings.

Here's why that matters: O-rings are what create the tight, leak-proof seal between the injector and the throttle body pod. But once you remove an injector, those rings can get nicked, stretched, or slightly deformed —even if they still look fine.

Reusing them might seem like no big deal, but it seriously raises your chances of a fuel leak after the engine's shut off.

To stay on the safe side, here's what you need to do:

1. Use brand-new O-rings made specifically for your engine and fuel injectors.
2. Apply a light coat of petroleum jelly to the new O-rings so they slide in easier and don't get torn during install.

It's just a couple of extra minutes, but doing this can save you a major hassle later on.

More 3.9L V6 Dodge Ram Pickup Tutorials

You can find a complete list of diagnostic tutorials for the full-size Dodge Ram pickups in this index:

• 3.9L V6 Dodge Ram Pickup Index Of Articles.

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

• How To Test The Ignition Coil (1989-1991 3.9L V6 Dodge Ram Pickup).
• How To Test The Distributor Pickup Coil (1989-1991 3.9L V6 Dodge Ram Pickup).
• Throttle Body Temp Sensor Tests (1989-1991 3.9L V6 Dodge Ram Pickup).
• How To Test For A Blown Head Gasket (1989-2001 3.9L V6 Dodge Ram Pickup).