AFM testing

[rob101]

I tested my AFM on my e28 (which is L-jet but don't worry about that) and it gave me inconsistent readings testing it with the multimeter. eg along its door travel would go along normally until bam it went to infinite resistance then normal then bang infinite resistance repeat until the end of the travel. so every so often it'd spike but all throughout the travel of the door there were these "spikes".

From the looks of the carbon track the substrate had worn right through to the layer of the board. So anyway we sent it somewhere to be fixed and low and behold when we got it back it was still the same. So we said well, it ain't fixed.

The guy then told use some **** about the reference voltage that the multimeter uses not being large enough to get a decent resistance and that we should test it using the voltage supplied through the pins. personally i think he's full of it and also my car ISN'T FIXED i still have the same BS surging crap that is mostly likely due to the AFM being worn out.

Now my question is: is this a common sign that its screwed and does this guy have a point? I have a warranty and if its still rooted I want to get my money back (because these people were a bunch of d*ckheads who didn't even consult us before starting when we wanted a quote.)

If anyone who has experience diagnosing a worn out AFM can chime in with their experiences.

[TC535i]

So hook it up and bench test it?

[TC535i]

Can you just pick up another used one and try it?

[rob101]

I already paid $350 and i don't want to spend another $150 on a used AFM that may or may not work!
just a side note, I live in australia where good expertise is hard and time consuming to find. I don't want to have to send it anywhere I just want to know whether i should be bothered hooking it up or that the multimeter should give me an indication. considering a multimeter is 9V and the L-jet uses 7.5V i don't really understand what the electronics dude is getting at. of course he assuming i don't understand electronics at all or understand resistance. I am just wondering whether i missed something here?

[winfred]

resistance is not the whole story on testing it but there should be a reasonably smooth track as you open and close the door, if it's worn through you can sometimes move the chip a little so it tracks on a fresh part of the trace, with the top off and key on you can probably look at the board and see the point you need to be hooked to to read the voltage and ground the other lead, should be a smooth transition from around 0 to around 5 volts, any spikes/drops ****s with the computer, probably will idle around 1-1.5 volts, that will vary with the condition of the engine and vacuum leaks, closing the little plug richens up the idle mixture and can smooth out a slightly lumpy idle if everything else is in good shape

[rob101]

I can't really open the top bit because that will void the warranty, but I think i'll work something out with alligator clips, I don't have to have the engine running to have the L-jet running voltage through the AFM do I? Just have the ignition on.

[digger]

read this and retest

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

[genphreak]

I can't really open the top bit because that will void the warranty, but I think i'll work something out with alligator clips, I don't have to have the engine running to have the L-jet running voltage through the AFM do I? Just have the ignition on.

Test it with a regulated benchtop power supply or a 6V battery. Dude if its not good, tell them it is not 'fit for service' and get your money back. Talk to me about a MAF conversion. I also have a couple of AFMs here, I have one from the e34 I know is not that bad... I'll send it to you if you'd like it. Nick

[bill g]

My engine (89 E34 535) runs perfectly well yet when I tested the AFM resistance readings they were a bit erratic. In fact that means of testing is a waste of time in my opinion and the Bentley manual is just confusing people - how many perfectly good AFMs have been tossed away after resistance readings looked a bit dodgy ?
I have since tested the voltage output from the AFM on the bench using a 6 volt battery and then later using the AFM supply voltage (5 volts) on the car, with some wiring adapters I made up - these tests showed AFM to be OK - smooth continuous voltage increase from closed to fully open.
If your L-jet AFM is anything like the Jag XJ6 system of early 80s then the voltage output will be different to the E34 AFM and will decrease instead of increase as you open the air flap.

Bill G