How To Test The MAP Sensor (1987-1995 Jeep 4.0L)

The manifold absolute pressure (MAP) sensor on your 2.5L, 4.0L Jeep Grand Cherokee, or Wrangler, or Cherokee, can be tested with a multimeter and a vacuum pump. You don't need a scan tool! So if a diagnostic trouble code P0108 (OBD II equipped) or a DTC 13, 14 (pre-1995 OBD I equipped) is lighting up the check engine light (CEL) on your Jeep SUV, this is the article for you.

This article will show you how to do a very accurate MAP sensor test that's explained in step-by-step detail. The MAP test described here in this article can be done in under 15 minutes and covers two different styles of MAP sensor.

You can find this tutorial in Spanish here: Cómo Probar El Sensor MAP (1987-1995 4.0L Jeep) (at: autotecnico-online.com).

NOTE: For the MAP sensor test covering the 1997-2003 4.0L equipped Grand Cherokee, Cherokee, and Wrangler... go to this tutorial: How To Test The MAP Sensor (1997-2003 Jeep 4.0L).

Symptoms Of A Bad MAP Sensor

Since your Jeep uses a ‘speed density’ type of multi-port fuel injection system, the MAP sensor is one of the most critical components the PCM uses to calculate the amount of air entering the engine and thus the correct amount of fuel to inject.

So, when the MAP sensor fails, your Jeep will definitely resent it and experience one or several of the following symptoms:

1. The check engine light will be on with a MAP diagnostic trouble code (DTC) stored in the fuel injection computer's memory:
2. If your Jeep SUV is OBD II equipped, you'll see:
   • DTC P0108.
3. If your Jeep is 1994 or older (OBD I), you'll see:
   • DTCs 13 or 14.
4. The Jeep won't start or will have a long cranking time before it starts.
5. Black smoke coming out of the tailpipe along with really bad gas mileage.
6. The engine idles rough when running and has a lack of power when accelerated.

Quite a few things can fool the PCM into thinking the MAP sensor is bad when it isn't. Among those things are: a bad fuel pump, low engine compression, and intake manifold vacuum leaks.

It's always a good idea to check fuel pump pressure (with a fuel pressure test gauge), engine compression, and for vacuum leaks if your Jeep has a very high mileage engine.

Where To Buy The MAP Sensor And Save

The best place to comparison shop and get an idea of how much your Jeep's MAP sensor costs (and how much you can save) is here:

Not sure if the above MAP sensor fits your particular Jeep Grand Cherokee? Don't worry, once you get to the site, they'll make sure it fits by asking you the particulars of your vehicle. If it doesn't fit, they'll find you the right one.

MAP SENSOR TEST 1: Checking The MAP Signal

The very first thing that you'll do, is to test the signal that the MAP sensor is generating while applying vacuum with a vacuum pump.

If you don't have a vacuum pump, you can run down to your local auto parts store (like AutoZone or O'Reilly Auto Parts) and you can rent one from them (and when you get the chance, buy one online).

NOTE: If you still can't get a hold of a vacuum pump, you can use your mouth to apply vacuum to the MAP sensor. With this method you won't be able to pull down the voltage to 1 Volt like you would with a vacuum pump but you WILL see the voltage move.

OK, to get this show on the road, this is what you need to do:

1. 1

   Disconnect the vacuum hose from the MAP sensor but leave the MAP sensor connected to its electrical connector.

2. 2

   Now, connect the vacuum pump to the MAP sensor using a small piece of vacuum hose.

3. 3

   Set your multimeter's selector in Volts DC.

4. 4

   Connect the red multimeter test lead to the wire labeled with the number 2 in the photo above.
   
   You'll need to use a back-probe on the connector or a wire piercing probe on the wire. To see what a what wire piercing probe tool looks like, click here: Wire Piercing Probe.

5. 5

   Connect the black multimeter test lead directly to the battery negative (-) terminal.

6. 6

   Turn the key to the ON position but don't start the engine. This will power up the MAP sensor.

7. 7

   Now, as you actuate the vacuum pump to apply vacuum to the MAP sensor, you'll should see:
   
   1.) 0 in. Hg ...... 4.7 Volts.
   
   2.) 5 in. Hg ...... 3.9 Volts.
   
   3.) 10 in. Hg .... 3.0 Volts.
   
   4.) 20 in. Hg .... 1.1 Volts.

OK, let's take a look at what your vacuum pump test results mean:

CASE 1: The multimeter showed a decrease and increase in the voltage as you applied and released vacuum. This result verifies that the manifold absolute pressure (MAP) sensor on your Jeep 4.0L Grand Cherokee (or Wrangler or Cherokee) is good and not bad. You can stop here, since no further testing is required.

CASE 2: The multimeter DID NOT show a decrease and increase in the voltage as you applied and released vacuum. This usually means that the MAP sensor is fried. To be absolutely sure, I suggest confirming that the MAP sensor has power and Ground. If both (power and Ground) are present, the MAP sensor is bad. To test for power, go to: MAP TEST 2: Checking The Power Circuit.