Mass Air Flow sensor voltage and testing


vettepin
10-19-2010, 04:46 PM
i have a 99 astro 149k miles and the MAF code came on. i tested the 3 wires at idle and reving it . i sprayed the maf to clean it before starting the test. The van had a bad spider that cause a high idle and was changed and now i get this code.

yellow wire = 4.40 volts at idle
yellow wire = 3.20 volts at 3500rpms
black = ohms to ground fine
red wire = reads 13.6 volts at idle

Are these numbers in spec ???

Blue Bowtie
10-19-2010, 10:40 PM
The MAF on your vehicle is not actually a variable voltage device. Some mid-1980s GM V-8s used analog MAF sensors (variable output voltage) but most everything else uses variable frequency output MAF sensors for better signal integrity regardless of connection integrity. The reason you are measuring a variable D.C. voltage is because of the meter's sampling rate and how it averages (similar to RMS) the input over time. The MAF produces a variable rate square wave output at about a 50% duty cycle at a frequency proportional to the measured intake air rate (difference in cooling rates of the sampling junction compared to the reference junction). The output waveform will look like this:

http://www.wwdsltd.com/files/1226869A.jpg


Since the output P-P voltage is fixed and duty cycle is fairly consistent, the measured voltage indication can vary if you are not using a true RMS meter:

http://www.wwdsltd.com/files/Fluke8026B.jpg

The best way to measure this is with an oscilloscope or meter with a frequency counter function. Even a GM Tech II or SnapOn scanner will not diagnose a MAF properly every time, since they only display what the PCM thinks it is reading, which isn't always what the sensor is providing the PCM.

There is no specification for voltage output at a given air flow. It is good that you have verified a good supply voltage and good ground at the sensor, but now it is time to complete the remaining diagnosis.

vettepin
10-20-2010, 01:03 AM
well that meter looks like it costs more then a new maf . i guess i will get a junk yard part for more testing.....

old_master
10-20-2010, 09:36 PM
What problems are you having with the vehicle?
What was the DTC?
Was that the only DTC?
Have you checked fuel pressure and leakdown?
I've seen low fuel pressure trip a MAF DTC more times than I can count ;)

vettepin
10-20-2010, 09:59 PM
What problems are you having with the vehicle?
service light on dash and poor fuel mileage. poor idle

What was the DTC?
low volt maf, 2x low volt map, low volt coolant sensor code.

Have you checked fuel pressure and leakdown?
no .

well i changed the maf and still had codes, i looked in the engine and saw the map sensor plug not in all the way ,dam it. i also had to clean the coolant temp sensor plug because it was corroded and that fixed its code. mostly my fault , the van now runs great since i have replaced or fixed everything on it or in it. :banghead: thanks

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