TEST 3: Testing The 2nd Ground Circuit
Now, we'll check the second Ground Circuit of the MAF sensor (this is known as the MAF RTN circuit).
This Ground is provided by the PCM (Powertrain Control Module=Fuel Injection Computer) internally.
Be careful and take extreme care not to short-circuit this wire to Ground or power as you're probing it.
Alright, here are the test steps:
Turn key to the OFF Position.
Place the multimeter in Volts DC mode.
Probe the MAF sensor connector's wire identified with the number 2 as shown in the photo with the black multimeter test lead.
Connect the red multimeter test lead to the battery (+) positive terminal.
Turn the ignition switch to the RUN position but don't crank or start the engine.
You should see a voltage of 10 to 12 Volts.
Let's interpret your test results:
CASE 1: The multimeter registered 10 to 12 Volts. This is the correct test result and it indicates that Ground is present in the wire.
The next step is to test the MAF signal. For this test go to: TEST 4: Testing The MAF Signal.
CASE 2: The multimeter DID NOT register 12 Volts. The MAF sensor is not the problem. Without a clear path to Ground in this circuit, the MAF sensor will not work.
Correcting the cause of this missing Ground will get the MAF sensor working again.
TEST 4: Testing The MAF Signal
Now that the basics have been checked, we'll check the MAF signal coming out of the sensor and going to the ECM.
Start the engine and let it reach it's normal operating temperature. You'll be using the voltage reading you will obtain at idle as a base to diagnose the MAF sensor.
The MAF sensor on your Ford (Mercury or Lincoln) must be connected to its connector to perform this test.
Turn the key in the OFF position.
Probe the wire labeled with the number 1 with the red multimeter test lead and a suitable tool.
Put the multimeter in Volts DC mode.
Connect the black multimeter test lead to the battery (-) negative terminal.
Start the already warmed up engine and let it run.
Note the Volts reading on your multimeter at idle (should be about 0.9 to 1 Volt DC).
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.
Accelerate the engine (by manually opening the throttle plate) as you watch the multimeter's voltage readings. The voltage value should increase as you accelerate the engine.
At about 1500 RPM you'll see about 1.4 Volts DC.
At around 2500 RPM the multimeter should register around 1.8 Volts DC.
The actual voltage reading on your multimeter may/will vary slightly.
When you release the throttle and the engine returns to idle, the voltage reading should decrease and hover around the reading that your multimeter registered in step 6 (about 0.9 to 1 Volt DC).
Accelerate/decelerate the engine as often as you need to verify that the voltage numbers on the multimeter increase/decrease every single time.
Let's take a look at what your test results mean:
CASE 1: The signal voltage increased/decreased as you accelerated/decelerated the engine. This test result lets you know that the MAF sensor is working OK.
If the mass air flow (MAF) sensor was bad, the output voltage number would stay stuck at one value irregardless of how much you rev up or rev down the engine.
CASE 2: The signal voltage stayed stuck in one value as you accelerated/decelerated the engine. Recheck all of your connections and retest.
If still the multimeter does not show the voltage signal increasing/decreasing, you can confidently conclude that the MAF sensor is defective only if:
- If you have confirmed that the MAF sensor is getting power (TEST 1).
- MAF sensor is getting chassis Ground (TEST 2).
- MAF sensor is getting PCM ground (TEST 3).
If all of the above are present, then you can conclude that the MAF is bad and needs to be replaced.
If this info really saved the day, buy me a beer!