TEST 3: Testing The MAF Signal Circuit

Testing The MAF Sensor Signal With A Multimeter (1997, 1998 3.0L Mitsubishi Montero)

The MAF signal the sensor creates is based on the amount of air the engine is breathing. In really simple terms, this means that the MAF signal increases when the engine accelerates and decreases when the engine returns to its idle RPMs.

We can tap into the wire that carries this signal, with a Hertz frequency capable multimeter, and see/confirm that the MAF sensor is actually doing this.

The MAF sensor supplies this MAF signal on the wire that connects to terminal #3 of the MAF sensor connector and this is the wire that we will tap into to check the sensor's performance.

If your Montero's MAF sensor is working correctly, you'll see this MAF Hertz frequency signal increase/decrease as you manually accelerate/decelerate the engine.

If the MAF sensor is bad, you'll see this Hertz frequency signal stay stuck in one value no matter how much you accelerate the engine.

NOTE: The MAF sensor must remain connected to its connector for this test to work. To access the signal inside the wire you'll need to use a back probe on the connector or a wire piercing probe on the wire. You can see an example of this tool here: Wire Piercing Probe.

OK, these are the test steps:

  1. 1

    Put the multimeter in Hertz (Hz) frequency mode.

  2. 2

    Connect the red multimeter test lead to the wire identified with the number 3 with a back probe or wire-piercing probe.

  3. 3

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

  4. 4

    Start the engine and note the Hertz reading on your multimeter at idle.

    Now, to give you a reference point, this Hertz value usually hovers around 10 to 14 Hertz at idle. This reading may be stable (with only small fluctuations) or unstable with very extreme fluctuations. No matter what the instability in the reading, this will be your base reading.

  5. 5

    Manually accelerate the engine from the engine compartment as you watch the multimeter's frequency readings. The Hertz frequency readings should increase.

    At around 2,500 RPM's this Hertz reading will oscillate around 70 Hertz.

  6. 6

    Let go of the throttle and let the engine return to idle. The Hertz reading should come down to the base Hertz reading you observed in step 4 of this test.

  7. 7

    Acelerate/decelerate the engine several times. The Hertz numbers on the multimeter should increase/decrease every time you accelerate/decelerate the engine.

Let's take a look at what your test results mean:

CASE 1: The Hertz (Hz) signal increased/decreased as the engine was accelerated/decelerated. This is the correct test result and it indicates that the mass air flow (MAF) sensor is producing a good MAF signal.

You still need to do one more test and this is to check that the MAF Reset Signal is being created. For this test, go to: TEST 4: Testing The MAF Reset Signal.

CASE 2: The Hertz (Hz) signal DID NOT increase/decrease as the engine was accelerated/decelerated. Then this indicates that the mass air flow (MAF) sensor is bad.

I'll explain why: You have confirmed the basics, which was checking/confirming the sensor is being fed power and Ground. Since the MAF sensor is getting both, then it should create a Hertz frequency signal that should increase/decrease as you accelerate/decelerate the engine. Since it did not, the MAF sensor is defective.

TEST 4: Testing The MAF Reset Signal

Testing The MAF Sensor Reset Signal With A Multimeter (1997, 1998 3.0L Mitsubishi Montero)

The MAF Reset Signal is just an On/Off type DC voltage signal that turns ‘On’ when the throttle plate opens and turns ‘Off’ when the throttle plate closes.

This is a pretty simple test that we can accomplish with the multimeter in Volts DC mode.

NOTE: This test is performed with the MAf sensor connected to its harness connector. To access the signal inside the wire you'll need to use a back probe on the connector or a wire piercing probe on the wire. You can see an example of this tool here: Wire Piercing Probe.

Here are the test steps:

  1. 1

    Place the multimeter in Volts DC mode.

  2. 2

    Connect the red multimeter test lead to the wire that connects to the connector terminal identified with the number 7 (see the illustration above) using an appropriate tool.

  3. 3

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

  4. 4

    Crank and start the engine.

  5. 5

    Manually open and close the throttle plate from the engine compartment as you observe the multimeter.

    NOTE: Since the engine will be running, take all necessary safety precautions.

  6. 6

    With the engine at idle and the throttle plate closed, your multimeter should register around 1 Volt DC or less. This is the ‘Off’ voltage reading.

  7. 7

    Open the throttle plate about 1/3 or more. Your multimeter should register 6 to 9 Volts. This is the ‘On’ voltage reading.

  8. 8

    Let go of the throttle plate and let it close (causing the engine to return to idle). The multimeter should register the voltage you observed in step 6 of this test.

Let's take a look at what your test results mean:

CASE 1: The multimeter registered the ‘On’ and ‘Off’ voltage readings. This is the correct test result and it indicates that the mass air flow sensor is creating the MAF Reset Signal. Your MAF sensor is working properly.

CASE 2: The multimeter DID NOT register the ‘On’ and ‘Off’ voltage readings. Recheck all of your multimeter connections and retest. If the indicated voltages are still not present then your 3.0L Mitsubishi Montero's MAF sensor is not functioning correctly. Replace the MAF sensor.