MAF TEST 3: 2nd Ground Circuit
Now, we'll check the second Ground Circuit of the MAF Sensor. This ground is provided inside the computer on your Passat.
When testing this circuit, you've got to be careful and take extreme care not to short-circuit this wire to power or you'll fry the computer on your Passat.
OK, this is are the steps:
Put the multimeter in VOLTS DC mode.
With the red multimeter test lead and an appropriate tool, probe the wire identified with the number 2 in the photo. Connector can be plugged or unplugged to the MAF sensor.
With the red multimeter test lead probe Battery Positive terminal.
You should see 12 Volts on the multimeter. Do you have 12 Volts?
Let's interpret your multimeter test results:
CASE 1: Your multimeter registered 12 Volts: This is the correct result, the next step is to make sure the MAF sensor has Ground, for this, go to MAF TEST 4.
CASE 2: Your multimeter DID NOT register 12 Volts: Recheck all of your multimeter connections and retest. If you still do not see these 12 Volts, then this exonarates the MAF Sensor as bad since without this ground it's not gonna' work.
MAF TEST 4: MAF Signal
Now that all of the basic circuits have been tested and their respective signals are present, the last test is to see if the MAF sensor is actually producing a MAF Signal.
It's important that the Engine be at its normal operating temperature. So, start the engine and let it reach it's normal operating temperature.
I know I'm stating the obvious, but the MAF Sensor must be plugged to its connector. As before, the Voltmeter has to be in Volts DC Mode:
Probe the wire labeled with the number 4 in the photo. Connect the BLACK lead to the battery (-) negative terminal.
Crank up the engine and get her to idle.
Once the idle stabilizes, your multimeter should register a voltage that will jump up and down a bit. This is OK. Whatever this voltage is, this will be your base Voltage (of the MAF Signal) that you'll need for the rest of the test steps.
Now, open the throttle (manually) to rev up the engine. As the engine revs up... keep your eyes on the multimeter's voltage numbers. The Voltage should increase.
When you let go off of the throttle and the idle comes back down, the voltage reading should also come down and should hover around the numbers you noticed at the beginning of this test.
Rev up and let the Engine come back down several times. Each time, the voltage numbers on your multimeter should rise and fall smoothly.
If the MAF sensor is good, the voltage numbers on the multimeter should rise and fall smoothly and without gaps. If the MAF Senosr is BAD, you WON'T see a Voltage registered on the multimeter at all or these numbers will spike up and down crazily.
Did the signal rise smoothly with each increase in engine acceleration and stay steady at idle?
CASE 1: The voltage numbers rose and fell smoothly and without gaps: This tells you that the MAF sensor is good and not the cause of the MAF issue or Diagnostic Trouble Code.
CASE 2: The multimeter registered 0 Volts or a voltage that was erratic: Recheck all of your multimeter connections and retest. If you still do not see the indicated voltage rising and falling smoothly, then the Mass Air Flow (MAF) sensor is BAD. Replace the MAF sensor.
If this info really saved the day, buy me a beer!