MAF TEST 3: 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.
As I mentioned earlier, you'll need a multimeter that can read Hertz Frequency values since the MAF digital signal voltage output oscillates pretty darn fast.
Before you start the test, it's a good idea to start the engine and let it reach its normal operating temperature.
Once the engine has reached its normal operating temperature and the engine's idle has stabilized, you'll take a Hertz reading of the MAF signal which you'll need as a base reading to diagnose the MAF sensor.
NOTE: The MAF sensor must be connected to its connector to perform this test.
These are the test steps:
With the Key in the Off position, place your multimeter in Hertz (Hz) mode.
- If you need to buy a Hertz enabled multimeter or need to upgrade to one, check out my recommendations here: Buying A Hertz Digital Multimeter for Automotive Diagnostic Testing.
With a suitable tool connected to the red multimeter test lead, probe the wire labeled with the letter A in the photo. Connect the BLACK lead to the battery (-) negative terminal.
Start the already warmed up engine.
Once the idle stabilizes, observe the Hertz reading on your multimeter is registering. 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.
Manually accelerate the engine from the engine compartment as you watch the multimeter's frequency readings. The Hertz Frequency readings should increase.
When you let go off of the throttle and the engine returns to idle, the Hertz reading should come down to the base Hertz reading you observed in step f of this test.
Repeat this as often as you need to verify that the Hertz numbers on the multimeter rise and decrease smoothly every single time.
If the MAF sensor is good, the Hertz numbers will increase and decrease smoothly. If the MAF sensor is bad, you WON'T see a Hertz number 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 Hertz numbers rose and fell smoothly as you accelerated and decelerated the engine: This tells you that the MAF sensor is good and not the cause of the MAF issue or diagnostic trouble code.
To be a bit more specific: If the Hertz reading increased as you revved up the engine and then decreased as the engine returned to its idle RPMs, then the MAF sensor is working like it should.
CASE 2: The Hertz numbers jumped crazily or none were registered: Recheck all of your multimeter connections and retest. If you still do not see the indicated Hertz values, then the mass air flow (MAF) sensor is bad. Replace the MAF sensor.
The most common type of failures of the MAF sensor are: no MAF signal Hertz values being registered on your multimeter or the Hertz value is stuck at one value no matter how much you rev up the engine.
If this info really saved the day, buy me a beer!