TEST 1: Verifying The MAP Signal
To get this show on the road, the very first thing that you'll do, is to see (with a multimeter) if the MAP sensor is able to create a good MAP signal. This is done using a vacuum pump while the MAP sensor is still connected electrically to the vehicle.
There's a good chance you may not have a vacuum pump, in this case you have two options: 1.) Rent one from AutoZone or O'reilly Auto Parts or 2.) Use your mouth to suck on the MAP sensor via a vacuum hose (using the good ole’ lungs is not the most accurate way, but it does work).
At the bottom of the article I have included the three different possible results you'll obtain from this test and how to interpret them.
NOTE: If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com).
OK, to get this show on the road, this is what you need to do:
If necessary, remove the MAP sensor from the engine to disconnect it from the vacuum hose or line that supplies it intake manifold vacuum
If you needed to disconnect the MAP sensor's electrical connector to remove it or its vacuum hose, then reconnect it now since this test requires that the MAP sensor be connected to its 3-wire electrical connector.
With a piece of vacuum hose, connect the MAP sensor to the vacuum pump. The vacuum hose that you need to use has to fit tightly around both the vacuum pump and MAP sensor's inlet vacuum nipple.
OK, grab your trusty multimeter, select Volts DC mode on it and with the red multimeter test lead, pierce the wire labeled with the number 2 in the image viewer.
NOTE: The MAP sensor harness connector needs to be connected to the MAP sensor, so you'll need to either back-probe the connector or use a wire piercing probe to get to the signal inside the wire (to see what a wire piercing probe looks like: Wire Piercing Probe Tool).
Ground the black multimeter test lead directly on the battery negative terminal.
Turn the key on but don't start the engine. This will enable the manifold absolute pressure (MAP) sensor to get power in the form of 5 Volts and Ground from the PCM (Powertrain Control Module= Fuel Injection Computer).
With the Key On and the Engine Off and without any vacuum applied to the MAP sensor, your multimeter should register 4.7 Volts DC.
Now, as you pump up the vacuum pump, the MAP sensor should make your multimeter register the following voltages at the following vacuum values (they may differ a little on your specific GM car or pick up or SUV or van or mini-van):
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.
Repeat this test step several times and each time, you should see the same values on your multimeter.
If you're using your mouth to suck on the MAP sensor's vacuum hose, then what you should see is the voltage going down. You won't be able to get it to 1 Volt DC on lung power. The important thing is just to see the voltage go down. When you stop and release whatever vacuum you created with the good ole’ lungs, the MAP voltage should go up to about 4.7 Volts.
OK, let's take a look at what your vacuum pump test results mean:
CASE 1: Your multimeter displayed a decreasing voltage signal as you pumped up the vacuum gauge. Your multimeter test results confirm that the manifold absolute pressure (MAP) sensor on your GM van (mini-van, car, pick up, SUV) is OK and not the cause of the problem or issue. No further testing is required.
Now, if your vehicle still has the MAP sensor code lighting up the check engine light (CEL) on your instrument cluster, take a look at the section: MAP Code Won't Go Away for more info.
CASE 2: Your multimeter DID NOT display a decreasing voltage signal as you pumped up the vacuum gauge. This confirms that the manifold absolute pressure (MAP) sensor on your GM van (mini-van, car, pick up, SUV) is bad and needs to be replaced. Replacing the MAP sensor will solve the issue.
CASE 3: Your multimeter DID NOT register any voltage. 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: TEST 2: Verifying The MAP Sensor Has Power.
TEST 2: Verifying The MAP Sensor Has Power
So far, in MAP TEST 1, you have verified that the MAP sensor on your GM van (mini-van, car, pick up, SUV) is not creating a signal or the signal is erratic
An 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 come 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.
Alright, this is what you'll need to do:
With your multimeter still in Volts DC mode from the previous test and the Key On (but engine Off).
Probe the wire labeled with the naumber 1, in the image viewer, 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.
Now Ground the black multimeter test lead on the battery's negative post.
Your multimeter should show you either:
1.) 5 Volts DC
2.) 0 Volts.
OK, now that the testing part is done, let's take a look at what your results mean:
CASE 1: The multimeter registered 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: Verifying The Ground Circuit.
CASE 2: The multimeter DID NOT register 5 Volts. This results lets you know that the MAP sensor is not bad, since without these 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 5 Volts, 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 4.3L, 5.0L, 5.7L GM SUV, pick up, van, or mini-van.