TEST 3: Testing The MAF Signal Circuit
So far, in your troubleshooting tests, you've confirmed that:
- The MAF sensor has power.
- The MAF sensor has Ground.
With power, ground and air flowing thru' it, the mass air flow (MAF) sensor has to produce a MAF signal. This MAF signal is then sent to the PCM for fuel injection calculations and the like.
In this last test, I'll show you how to test this MAF signal with your multimeter.
NOTE: To test the MAF sensor output signal, it's important that engine be at normal operating temperature. So, if it isn't already warmed up, go ahead and start'er up and let run for a few minutes. When ready, this is what you'll do:
IMPORTANT: The MAF sensor must be connected to its connector. You'll need to use a back probe on the connector or a wire piercing probe on the wire. You can see an example of this tool here: Wire Piercing Probe.
Let's get started:
Probe the wire identified with the letter A with the red multimeter test lead and an appropriate tool.
This is the yellow (YEL) wire of the MAF Connector.
Put the multimeter in Hertz Frequency (Hz) mode
Don't have a digital multimeter that can read Hertz frequency? Check my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.
Connect the black multimeter test lead to the battery negative (-) terminal.
Start the already warmed up engine.
Once the engine is idling, note the Hertz 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.
Manually accelerate the engine from the engine compartment as you watch the multimeter's frequency readings. The Hertz Frequency readings should increase as you rev up the engine.
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 6 of this test.
At an idle of about 680 RPM's, the MAF sensor outputs about 3.2 K Hertz.
At about at 1500 RPM;s the MAF signal output is about 4.2 K Hz.
At 2500 RPM's it hovers around 5.2 K Hz.
As you rev up/down the engine, the Hertz reading should increase/decrease
If the MAF sensor is defective, the Hertz value will stay stuck at one number as rev up the engine.
Let's take a look at what your test results mean:
CASE 1: If the Hertz (Hz) signal rose smoothly and decreased smoothly as the engine was accelerated and decelerated respectively, then this indicates that the mass air flow (MAF) sensor is working correctly.
CASE 2: If the Hertz (Hz) signal DID NOT rise smoothly nor decreased smoothly as the engine was accelerated and decelerated respectively, then this indicates that the mass air flow (MAF) sensor is BAD. Replace it.
Oscilloscope Waveform Of The GM MAF Sensor
If you have access to an oscilloscope, this is what the mass air flow (MAF) sensor waveform looks like at idle.
If the MAF sensor is good then at idle and at any RPM, the waveform will stay perfectly formed. Also, as you accelerate the engine, the wave-length will become shorter while the wave amplitude stays the same.
Now, if the MAF sensor is bad, the waveform will have missing pieces or no waveform will be formed at all.
Where To Buy the MAF Sensor And Save
The following links will help you comparison shop for the MAF sensor:
Not sure if the MAF sensor will fit your particular GM vehicle? Don't worry, once you get to the site, they'll make sure it fits. If it doesn't, they'll find you the right one.
If this info really saved the day, buy me a beer!