Test for faulty MAF sensor?

[ocampg]

Does anybody know of a way to test for a faulty MAF sensor? Engine has been stumbling, I have replaced the plugs, wires, cap, rotor, O2 sensor, Throttle Pos. Sensor, and fuel filter to no avail...Brought the car to an indy who scanned it and says I have a bad MAF sensor. Before I go replacing yet another part, I would like to know if there are any ways I can verify that it is indeed broken. I have unplugged it while the engine was running rough/stumbling before, and it stalls out completely which means that it is quasi-working, but is there a defnitive test? voltage/resistance test? stomp test? Thanks....
Geoff

[Phil in Vancouver]

He ran a diagnostic on it and came up with the MAF fault code. Replaced the MAF sensor, and my stalling problem was gone.
Sorry I can't be of any more DIY help.
Phil
94 530i man.

[George M]

If your DME kicked a MAF code, try first cleaning the contacts inside and/or the serial connector and check to see if the vane is sticking off idle. As to measuring output or voltage relative to vane position, you can get a crude assessment...believe Bentley covers these values with a multimeter. It has been stated before however this is a crude measurement and the best way to deduce AFM transient output to the DME is with an oscilloscope. If you know of someone with the same car, also can try swapping AFM's as well. These cars won't run a lick without the AFM plugged in.
HTH,
George

[ocampg]

I have the whole thing apart, looks like there is a printed circuit board with various resistor patches tied to a large contact patch...There is a wiper which moves across the contact patch (wiper moves on same axis as the barn door), if u follow the various patches and wire busses around the board, you can trace them back to 3 of the 4 pins on the connector (2 ends, center wiper, i.e. a potentiometer). In addition, There seems to be another element, sorta looks like a diode or capacitor, which sits directly in the air stream. One end is tied to the same pin as one end of the potientiometer circuit, the other end gets its own pin. As for testing, there is no continuity along the contact patch which the wiper runs over, which is very strange, since I can get continuity from the wiper to the pin, and from the outer pins to the outer ends of the contact patch, but thats it. Also, measuring resistance across the other element which sits in the airstream, I get an "open circuit". After all this, I'm still not sure if something is wrong with it, though I assume I should get a variable resistance between the wiper and the two end pins, which I do not have. Regardless, I had already placed an order for a used one this morning through someone on this message board so I wasn't too reluctant in tearing this one apart.

[George M]

well a good description of what's inside but aside from cleaning the contacts, I would button it up and leave it alone and not try to assess continuity within the MAF. Your best effort to determine its function is by measuring output across the male connector pins with the vehicle harness connector off per Bentley's desciption to see if there is an approx. linear relationship between flap door postion and output.
George

[535ise]

Here's a link you might find interesting,all though it's a porsch 944 afm he's desribing it is a motronic set up from around the same era as our e34's so they should be very similar our bm's.

http://frwilk.com/944dme/afm.htm

HTH

Andrew

[ocampg]

great info! I will bench test my MAF later... thanks again...

[ocampg]

So I bench tested my MAF as described by the previous post. It turns out that its working! What else could cause my indy's scanner to report a broken MAF? thanks...

[Anton CH.]

Would a failing MAF sensor cause the engine to be running rougher/worse idle on hot days?

[ocampg]

maybe, there is a component which sits in the airstream and changes in resistance as a function of temperature. If you measure across it with an ohmeter at room temperature, you should have about 2.2k. On my particular MAF, you want to read across the the two outer pins of the connector. Check out the link from 535ise, its pretty much the same thing, there is a table which shows the temperature and their respective resistance.

[ryan roopnarine]

the afm in the m20 525i, like the gentleman's, works in a significantly different way than the maf on your 92 525i....methinks that a maf sensor doesn't really have "degrees" of operation, it would usually throw off the check engine if something were wrong with it, and even then, you'd likely just replace it.

[535ise]

maybe, there is a component which sits in the airstream and changes in resistance as a function of temperature. If you measure across it with an ohmeter at room temperature, you should have about 2.2k. On my particular MAF, you want to read across the the two outer pins of the connector. Check out the link from 535ise, its pretty much the same thing, there is a table which shows the temperature and their respective resistance.

The componant your refering to is the air temp sensor,to help you understand what it does i've copied a page out of the instruction manual to my ECU.

HTH

Andrew

http://homepage.ntlworld.com/a.law47/air%20temp.JPG

[Anton CH.]

Would you say there still is a point to test the air temp sensor? I don't think those things ever fail. Which table would I use for my car?