TEST 2: Checking The 5 Volt Power Signal

Checking The 5 Volt Power Signal. Manifold Absolute Pressure MAP Sensor Test (Ford 4.9L, 5.0L, 5.8L)

So far, in MAP TEST 1, you have verified that the MAP sensor on your Ford car (pick up, van) is not creating a signal or the signal is erratic

And so, in this MAP sensor test step, you're gonna' verify that the MAP sensor is getting power, since without power it won't work. This power comes in the form of 5 Volts and the PCM (Powertrain Control Module = Fuel Injection Computer) is the one that provides this juice.

Since this circuit is directly connected to the PCM, be careful and don't short this wire to battery power (12 Volts), or you run the risk of frying the PCM.

NOTE: Power to the MAP sensor is 5 Volts DC but depending on the age of your vehicle, this voltage may be as low as 4.5 Volts DC. If you see 4.5 Volts on your specific vehicle, you can consider this as 5 Volts.

Alright, this is what you'll need to do:

  1. 1

    With your multimeter still in Volts DC mode from the previous test and the Key On (but engine Off).

  2. 2

    Probe the wire labeled with the number 1, in the image above, with the red multimeter test lead.

    You can test for these 5 Volts with the MAP sensor's electrical connector connected to the MAP sensor or not, just avoid probing the front of the connector.

  3. 3

    Now Ground the black multimeter test lead on the battery's negative post.

  4. 4

    Your multimeter should show you 4.5 to 5 Volts DC. Anything under 4.5 Volts DC and you've got a problem.

OK, now that the testing part is done, let's take a look at what your results mean:

CASE 1: The multimeter registered 4.5 to 5 Volts. This is the correct result and it's starting to look like the MAP sensor is bad but you still need to check that the MAP sensor is getting Ground. For the Ground test, go to: TEST 3: Checking The MAP Sensor Ground.

CASE 2: The multimeter DID NOT register 4.5 to 5 Volts. This results lets you know that the MAP sensor is not bad, since without these 4.5 to 5 Volts DC, the MAP sensor can not function.

Although it's beyond the scope of this article to troubleshoot the cause of these missing voltage, you have now eliminated the MAP sensor as bad. Resolving the issue that is keeping these 5 Volts from being supplied will solve the MAP sensor issue on your 4.9L, 5.0L, 5.8L Ford car or pick up or van.

TEST 3: Checking The MAP Sensor Ground

Checking The MAP Sensor Ground. Manifold Absolute Pressure MAP Sensor Test (Ford 4.9L, 5.0L, 5.8L)

The next step (after verifying the MAP signal and power circuit) is to make sure that the MAP sensor on your Ford car (pick up, van) has a good path to Ground. So, in this test step, you're gonna' verify that the MAP sensor is getting Ground using your multimeter once again.

A word of caution: since this circuit is directly connected to the PCM, be very careful and don' short this wire to battery power (12 Volts), or you WILL FRY the PCM.

Alright, this is what you'll need to do:

  1. 1

    With your multimeter still in Volts DC mode from the previous test and the Key On (but Engine Off).

  2. 2

    Probe the wire labeled with the number 3 in the image above, with the black multimeter test lead.

    It doesn't matter if you probe this circuit (wire) with the connector connected to the MAP sensor or not, but do not probe the front of the connector (if you decide to unplug the connector to test for this path to Ground).

  3. 3

    Now connect the red multimeter test lead on the battery's positive (+) post.

  4. 4

    Your multimeter should show you 10 to 12 Volts DC (if Ground is present).

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

CASE 1: The multimeter registered 12 Volts. This is the normal and correct result and let's you know that the MAP sensor is being fed Ground by the PCM. Now, you can consider the MAP sensor bad ONLY if:

  1. Your MAP sensor did not pass TEST 1.
  2. Your MAP sensor is getting fed power (TEST 2).
  3. Your MAP sensor is being fed ground.

Just to paraphrase: If you have confirmed that the MAP sensor is not producing the indicated signal in TEST 1, and it's getting both power and Ground, then you can conclude the MAP sensor is bad and needs to be replaced.

CASE 2: The multimeter DID NOT register 12 Volts. Double check your multimeter connections and repeat the test. If your multimeter results still do not indicate 12 Volts, then the MAP is not fried and not the cause of the MAP diagnostic trouble code (DTC) issue.

Here's why: Without a good path to Ground, that the PCM provides internally, the MAP sensor will not work. With this test result, you have eliminated the MAP sensor as bad.

MAP Sensor Code Won't Go Away

So you've tested the MAP sensor and according to the test results, it's good. But the check engine light keeps coming back on even after you erased the diagnostic trouble code (DTC) from the computer's (PCM) memory. Well, here are a couple of suggestions that might inspire your next diagnostic move:

  1. The MAP sensor's vacuum hose or line is torn or cut or clogged.
  2. The engine has several cylinders with very low engine compression causing it to idle rough and thus producing low or erratic vacuum. For this I suggest a compression test:
    1. How To Test Engine Compression (4.9L, 5.0L, 5.8L).
  3. The MAP sensor is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:
    1. I have found that the best way to test these intermittents is to slightly tap the MAP sensor with the handle of a screw-driver and see if this tapping screws up the voltage readings as I apply vacuum.
  4. The MAP sensor's connector is bad, usually the locking tab is broken and the connector has worked itself loose, causing an intermittent false connection.
  5. Your fuel pump is starting to go bad and is not sending enough fuel and/or fuel pressure up to the fuel injectors. I suggest a fuel pump test:
    1. How To Test The Fuel Pump (Ford 4.9L, 5.0L, 5.8L).

More Ford 4.9L, 5.0L, 5.8L Tutorials

You can find a complete list of tutorials for 4.9L, 5.0L or 5.8L equipped Fords here: Ford 4.9L, 5.0L, 5.9L Index Of Articles.

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

  1. Manifold Absolute Pressure MAP Sensor Test (Ford 4.9L, 5.0L, 5.8L).
  2. Ignition Coil Test -No Spark No Start Tests (Ford 4.9L, 5.0L, 5.8L).
  3. How To Troubleshoot A No Start (Ford 4.9L, 5.0L, 5.8L).
  4. Testing A Blown Head Gasket (Ford 4.9L, 5.0L, 5.8L).
  5. How To Test Engine Compression (4.9L, 5.0L, 5.8L).
  6. How To Test The Ford Mass Air Flow (MAF) Sensor (at: easyautodiagnostics.com).
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Ford Vehicles:

  • Bronco 4.9L, 5.0L, 5.8L
    • 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996
  • Crown Victoria 5.0L
    • 1989, 1990, 1991
  • E150, E250, E350 4.9L, 5.0L, 5.8L
    • 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996

Ford Vehicles:

  • F150, F250, F350 4.9L, 5.0L, 5.8L
    • 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997
  • Mustang 5.0L
    • 1988, 1989, 1990, 1991, 1992, 1993
  • Thunderbird 5.0L
    • 1988, 1989, 1990

Lincoln Vehicles:

  • Continental
    • 1988, 1989, 1990
  • Mark VII
    • 1988, 1989, 1990, 1991, 1992
  • Town Car
    • 1988, 1989, 1990

Mercury Vehicles:

  • Cougar 5.0L
    • 1988, 1989, 1990

Mercury Vehicles:

  • Grand Marquis 5.0L
    • 1988, 1989, 1990, 1991