Air Mass Meter Question

Howdy, I have a 1991 Saab 900S and I think something is a little funny with the air mass meter. I am hoping to get some advice.
The check engine light comes on for about a second or two
periodically. Sometimes this is accompanied by the engine stumbling for a split second, sometimes not. I checked the check engine codes and the first one is 12231 (no rpm signal) which is normal, since the engine is off. The next code is 12221 (no signal from air mass meter), which I why I think that's where the problem is. The third code is not listed in the Bentley manual (as far as I can tell), it just repeatedly makes long flashes. I'm not sure what this means. If I cycle it again it starts again with the no rpm signal code and repeats.
So I followed Bentley's instructions for checking the air mass meter. I unhooked it from the air filter, ran the engine above 2500 rpm and then shut it off and watched the wire glow red for a second (burn off). According to the manual this means that the air mass system is probably working fine.
Does anyone have any suggestions or experience with this problem? I hate to put a new part in there if it isn't the problem (the meter is very expensive). I should say that I got a new air mass meter at 118k. It only lasted until 132k and was replaced under warranty. I'm now at 180k. Is 50k the expected life of this part? Is it sensible for the failure to be so intermitent and short lived?
Thanks, mark
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Mark, you've written perhaps the most descriptive and comprehensive description of a problem that I've seen in a long time. I'm guessing you do something heavy-duty technical for a living, for some reason.

Could it also mean "signal out of expected range", like it's out of tolerance, or is it only "no signal" which would indicate a disconnect?

Well, that means that the filament is intact. There's more to it than that, though; the temperature sensor, the feedback circuitry needed to keep the temp of the wire constant as the mass of the air cooling it changes the temp, ... that tells you the filament is OK but not the rest of it.

I'm wondering if you have some sort of cabling issue that is causing this to fail, or causing it to _appear to fail_. Have you polished the contacts on the AMM and on the harness? I remember somewhere seeing a resistance figure between two pins on the harness to see if it's in range, but (a) that was for an '88, and (b) of course, I can't google it up right now for some reason.
Dave Hinz
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Hi Dave, thanks for the response. I'm a mechanical engineer, maybe the style of my post reflects that. :)
The Bentley manual says that the 12221 code means "no signal from air mass meter." I don't know how literally they mean that. It seems to be that a loose connection could cause my problem, but I'm not quite sure how I'd find that. The connector on the AMM seems to be in good shape. I thought the contacts looked clean but I will take another look at them and clean them as best I can.
I see your point that more needs to work than just the wire burn off. Is the circuitry that performs the control loop in the AMM or is it located in the LH unit?
I do think something is failing (not just appearing to fail) because the engine definitely stumbles for a split second, periodically, and it is often accompanied by the check engine light illuminating for a second.
Thanks again, mark
wrote:

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I'm software, but I play electrical and mechanical in my spare time.

I believe the bridge circuit is right in the AMM, since they're pretty balance-sensitive and are measuring very low differences in resistance. Probably the amplifiers which drive the filament heat as well. There's only like 6 pins on the AMM's connector, right? (going from memory of my '88)

But where in the feedback loop is the problem? Is it stumbling because the AMM is sensing a problem, or is the AMM providing wrong input which causes the fuel injection to make it stumble? I'm not sure what the answer is.
That having been said, a few dozen thousand miles of life on an AMM is unusual. I suspect the problem is elsewhere, unless you're just plain unlucky.
Dave Hinz
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wrote:

The connector is 6 pins but it only use 5 of them. My contacts seem good and clean. But if the cicuitry is inside the meter then I suppose there could be something flaky in there... I wonder if I can open it up and check the circuit board for cracks...

I wish I knew. :) If the manual is to be taken literally then the AMM is at fault. I did unplug the AMM, run the car, and then checked the error codes and it showed the same AMM error. But I don't think that is a conclusive test.

Well, I have been known to be unlucky. :) I'm thinking of buying a used one cheap (either off eBay or from a local junker). It seems like the cheapest way to find out if it's the AMM. Although you never can be sure when you're testing with old parts...
Unless of course you or someone else has a better idea. I'd love a better idea (that doesn't involved spending $400 on _another_ AMM).
Thanks for your help, mark

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Mark wrote:

You can try this idea. Buy another Classic Saab of the same year in working order. You can try the AMM from the known working car in your car to see if that fixes the problem. This is how the IBM Computer Engineers diagnosed problems when I worked in data centers. They would keeping swapping parts from a working system into the "broken" system until it was fixed. At least you could try to use it to justify owning another Saab :-)
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