How To Test The MAP Sensor (1991 5.2L, 5.9L V8 Dodge Ram Pickup)

TEST 3: Verifying The MAP Sensor Ground Circuit

Alright, based on the last two checks, here's what you've confirmed so far:

• First, your MAP sensor isn't outputting a variable voltage when you apply or release vacuum (TEST 1).
• Second, it's definitely getting power —between 4.5 and 5 Volts DC (TEST 2).

Now we're gonna do one last test —we need to make sure the MAP sensor's getting a good Ground. You'll use your multimeter again for this step.

This part's pretty simple. You'll be testing the black with light blue stripe (BLK/LT BLU) wire of the sensor's 3-wire connector. That wire is in charge of delivering Ground to the MAP sensor.

IMPORTANT: This Ground wire runs straight to the fuel injection computer. Be very careful not to connect it to battery power —or you'll fry the computer. The test below is a safe and effective way to check for Ground in the circuit.

Here's how to do it:

1. 1

   Unplug the MAP sensor from its 3-wire connector.

2. 2

   Connect the red lead of your multimeter to the battery's positive (+) post.

3. 3

   Set the multimeter to measure DC Volts.

4. 4

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

5. 5

   Touch the black multimeter lead to terminal 1 —the one that connects to the BLK/LT BLU wire.

6. 6

   You should see a reading between 10 and 12 Volts DC.

Now let's go over what those results mean:

CASE 1: You got a reading of 10 to 12 Volts. That confirms the MAP sensor's getting a solid Ground from the computer.

At this point, if you've already confirmed that:

• The MAP sensor isn't producing a variable voltage (TEST 1).
• It's getting proper power (TEST 2).
• It's getting Ground (this test section).

...then the MAP sensor is done. It needs to be replaced.

Looking to save some cash on the replacement? Check out my recommendations: Where to Buy the MAP Sensor and Save.

CASE 2: You didn't get a 10 to 12 Volt reading. Double-check your setup and test again just to be sure.

If you're still not seeing voltage, you've found the real issue —the MAP sensor isn't getting the Ground it needs, so it can't send out a signal.

That means the sensor isn't the problem. What you'll need to do now is fix the missing Ground. The usual suspects are:

• A break or fault in the Ground wire between the sensor's connector and the computer.
• A problem inside the fuel injection computer itself —not common, but it happens.

MAP Sensor Code Won't Go Away

Alright, so you've already tested the MAP sensor and it checks out. It's doing what it's supposed to do. But that check engine light? Still hanging around —even after you cleared the diagnostic trouble code (DTC) from the system?

Let's go a little further. Here are a few other things that could be triggering that stubborn code:

1. Big vacuum leak: A major vacuum leak (like bad intake manifold gaskets) could be messing things up. It throws off the air/fuel mix, makes the engine idle rough, and can trick the computer into blaming the MAP sensor.
2. Cracked or broken vacuum hose: Look at the rubber hose going to the MAP sensor. If it's damaged, split, or loose —that could be the problem right there.
3. Low engine compression: If you've got low compression in a few cylinders, it'll cause a shaky idle and unstable vacuum levels. You'll want to run a compression test to be sure:
   • How To Test Engine Compression (1989–2003 5.2L, 5.9L V8 Dodge Ram Pickup)
4. Intermittent MAP sensor failure: Even if the sensor passed earlier, it might be failing now and then. Try tapping it lightly with a screwdriver handle while applying vacuum. If the voltage reading jumps or glitches, you've found the issue.
5. Loose or damaged MAP connector: Check out the connector. If the locking clip is busted, the connector may not be making a solid connection —causing signal dropouts and weird sensor behavior.
6. Weak or dying fuel pump: If the pump isn't sending enough fuel or pressure to the injectors, you'll get rough running and possible codes. You should check fuel pressure and make sure it's where it should be:
   • How To Test The Fuel Pump (1989–1991 5.2L, 5.9L V8 Dodge Ram Pickup)

More 5.2L, 5.9L V8 Dodge Ram Pickup Tutorials

I've written several more tutorials for the 5.2L, 5.9L V8 Dodge Ram pickups and you can find them in this index:

• 5.2L, 5.9L V8 Dodge Ram Pickup Index Of Tutorials.

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

• How To Test The TPS (1989-1990 5.2L, 5.9L V8 Dodge Ram Pickup).
• How To Test The TPS (1991 5.2L, 5.9L V8 Dodge Ram Pickup).
• How To Test Engine Compression (1989-2003 5.2L, 5.9L V8 Dodge Ram Pickup).
• Troubleshooting A Blown Head Gasket (1989-2003 5.2L, 5.9L V8 Dodge Ram Pickup).