TEST 3: Testing The MAF Signal With A Multimeter
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.
As mentioned at the beginning of this tutorial, the MAF signal reacts to amount of air the engine is breathing. So if you accelerate the engine, the MAF sensor signal voltage will increase.
If the MAF sensor is bad on your Nissan Sentra, the voltage reading will stay stuck in one value no matter how much you accelerate the engine.
IMPORTANT: The MAF sensor must be connected to its connector to perform this test. So you'll need to use a back-probe or a wire piercing probe to access the signal inside the wire. To see an example of a wire piercing probe, check this link out: Wire Piercing Probe.
These are the test steps:
Connect the red multimeter test lead to the wire labeled with the number 1 shown in the photo.
Put your multimeter in VOLTS DC mode.
Connect the black multimeter test lead to the battery negative (-) terminal and start the already warmed up engine.
Note the Volts reading on your multimeter at idle. 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 as you watch the multimeter's voltage readings.
The MAF signal voltage value should increase as you rev up the engine.
Let the engine return to idle.
The MAF signal voltage value should decrease as the engine returns to idle.
The voltage value should rise/fall everytime you rev up/down the engine.
If the MAF sensor is defective, the MAF signal voltage value will stay stuck in one single number as you rev up/down the engine.
Let's interpret your MAF signal test result:
CASE 1: The MAF signal rose smoothly and fell smoothly according to the engine's RPMs. This test result tells you the MAF is OK. In other words, the MAF sensor is creating a MAF signal and IS NOT defective.
CASE 2: The MAF signal DID NOT rise smoothly or fall smoothly according to the engine's RPMs or if there was no MAF signal at all. This test result indicates the MAF sensor is bad.
Here's the reason why:
- You have confirmed that the MAF sensor is getting power (TEST 1).
- You have confirmed that the MAF sensor is getting Ground (TEST 2).
- In this test step, you have confirmed that the MAF sensor is NOT creating a voltage signal that increases with higher engine RPMs and decreases when the engine returns to idle.
Taking the above into account, you can conclude that the MAF sensor is bad and needs to be replaced.
If this info really saved the day, buy me a beer!