TEST 3: Verifying The Ground Circuit
The next step (after verifying the MAP signal and power circuit) is to make sure that the MAP sensor on your GM van (mini-van, car, pick up, SUV) 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't short this wire to battery power (12 Volts), or you WILL FRY 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 number 3 in the image viewer, 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).
Now connect the red multimeter test lead on the battery's positive (+) post.
Your multimeter should show you either:
1.) 12 Volts DC
2.) 0 Volts.
OK, let's take a look at what your results mean:
CASE 1: The multimeter registered 12 Volts. This is the normal result and it means that the MAP sensor your GM van (mini-van, car, pick up, SUV) is fried and needs to be replaced.
Here's why: In MAP TEST 1 and 2, you verified that the MAP sensor is not producing the correct values (when you applied vacuum) and that it does have power. Since in this test step you have confirmed that the MAP sensor does have a solid path to Ground, these results, interpreted together, indicate that the MAP sensor is bad.
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:
- The MAP sensor's O-rings, the ones located on the vacuum inlet nipple are missing or are too old and they're not sealing properly. Visually check the O-rings and replace if necessary.
- 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.
- The MAP sensor is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:
- 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.
- 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.
- 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.
- There's a good chance that the PCM is fried and that it's setting a false MAP sensor code.
Related Test Articles
I've written several more tutorials that may be of interest to you. These are specific 4.3L, 5.0L, 5.7L GM van (mini-van, car, pick up, SUV) and you can find them in this index:
Here's a sample of the tutorials you'll find in the index:
- Coolant Leaking From Intake Gaskets (GM 4.3L, 5.0L, 5.7L).
- How To Diagnose A Hard To Find Misfire Case Study (GM 4.3L, 5.0L, 5.7L).
- Troubleshooting the fuel pump (GM 4.3L, 5.0L, 5.7L).
- How To Test The Engine Compression (GM 4.3L, 5.0L, 5.7L).
- How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).
- Shift Solenoid A And B Tests (GM 4.3L, 5.0L, 5.7L).
- How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04) (at: easyautodiagnostics.com).