How To Test The MAP Sensor (1989-1990 5.2L, 5.9L V8 Dodge Ram Pickup)

TEST 3: Verifying The MAP Sensor Ground Circuit

The last two tests confirmed two important things:

• The MAP sensor isn't sending out a changing voltage signal when you apply or release vacuum (TEST 1).
• The MAP sensor is getting power —4.5 to 5 Volts DC (TEST 2).

Now for the final check: let's make sure the MAP sensor is getting Ground through the black with light blue stripe (BLK/LT BLU) wire of its 3-wire connector.

For this easy test, you'll use your multimeter again —this time for a voltage test to confirm Ground.

IMPORTANT: That BLK/LT BLU wire runs straight to the fuel injection computer. So be very careful NOT to connect it to battery power (12 Volts), or you'll fry the computer. The voltage test I'm showing here is safe and gets the job done.

Here's how to test it:

1. 1

   Unplug the MAP sensor from its electrical connector.

2. 2

   Connect the red multimeter lead to the battery's positive (+) terminal.

3. 3

   Set your multimeter to DC Volts mode.

4. 4

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

5. 5

   Touch the black multimeter lead to terminal number 1 on the connector.
   
   NOTE: Make sure the terminal connects to the BLK/LT BLU wire.

6. 6

   Your meter should read somewhere between 10 and 12 Volts DC.

Here's what the reading means:

CASE 1: You saw 10 to 12 Volts. That means the sensor is getting proper Ground from the computer.

At this point, you've confirmed all three things:

• The MAP sensor doesn't send a changing signal when vacuum is applied or released (TEST 1).
• It's getting power (TEST 2).
• It's getting Ground (this test).

That's enough to say the sensor itself is bad and needs to be replaced. To save a few bucks, check out: Where To Buy The MAP Sensor And Save.

CASE 2: You didn't get 10 to 12 Volts. Go back, double-check your setup, and test again.

If the reading's still missing, then that's your issue —the MAP sensor isn't getting Ground, and that's why it won't send out a signal under vacuum.

In this tutorial, I don't go into finding the cause of that missing Ground, but you've already narrowed things down. You've ruled out the sensor itself.

Next step: get that Ground supply back. Most of the time, it comes down to one of these:

• The Ground wire is damaged somewhere between the sensor connector and the fuel injection computer.
• Much less common, but possible —the computer itself has an internal fault.

MAP Sensor Code Won't Go Away

Alright, so you've tested the MAP sensor and everything checks out. It's doing what it's supposed to. But that check engine light keeps coming back —even after you cleared the trouble code from the computer's memory. Let's dig a little deeper. Here are a few things that could still be causing the issue:

1. Big vacuum leak: A major vacuum leak is throwing off the air/fuel ratio, making the mix too lean. This causes a rough idle, and the computer ends up blaming the MAP sensor when it's actually not at fault.
2. Damaged/cut vacuum hose: The rubber hose that links the MAP sensor to the vacuum port on the intake manifold might be dry-rotted, cracked, or split. That'll definitely mess with sensor readings.
3. Low compression in the engine: If several cylinders have weak compression, it'll cause a rough idle and throw off the vacuum readings. You'll want to run a compression test to check:
   • How To Test Engine Compression (1989-2003 5.2L, 5.9L V8 Dodge Ram Pickup).
4. Intermittent MAP sensor failure: The sensor works fine most of the time but cuts out now and then. Try tapping it lightly with the handle of a screwdriver while applying vacuum. If the voltage reading jumps or drops out, you've found your problem.
5. Loose or damaged MAP sensor connector: A broken/damaged connector could be causing trouble. Most of the time, the locking tab breaks off, and the connector comes loose —leading to an intermittent false connection that makes the computer think the MAP sensor is toast.
6. Weak fuel pump: If the fuel pump is on its way out, it might not be pushing enough fuel or maintaining pressure. That'll definitely throw things off. You'll need to check fuel pressure and compare it to spec:
   • 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 that may be of interest to you that you can find 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 Intake Air Temp (IAT) Sensor (1992-1997 5.2L, 5.9L V8 Dodge Ram Pickup).
• Troubleshooting A Blown Head Gasket (1989-2003 5.2L, 5.9L V8 Dodge Ram Pickup).
• How To Test Engine Compression (1989-2003 5.2L, 5.9L V8 Dodge Ram Pickup).