Good evening,

Yesterday, the car started acting funky and sure enough after checking the diagnostic code reader, I got a 121. So I changed out the 016 AMM and now all is well again. The fact is, this has been going on for the last year, where about every 2 months or so, the car starts stalling or gas mileage drops dramatically, I check the code reader and sure enough 121, so I replace the AMM.

As I recall, I've now used four AMM's. Now all have been used, which I picked up on ebay, so the hit hasn't been too bad. You might say ah-hah, your thermostat is stuck and allowing preheated air to torch your AMM. However, I've attempted to keep the car stone stock, so I've left things the way they are (including the heat shields) but I have replaced the thermostat and have checked it's operation to ensure the flapper works correctly.

I'm thinking that maybe the burn off function may be the problem. I see there is a procedure in Bentley's for checking burnoff but it doesn't say what voltage spike I should be observing four seconds after the car is shut down. Nor does it say where the main relay is or how to check it. So I'm looking for some help as to how to proceed with diagnosing the problem....if a problem does in fact exist.

I suppose that with the used AMM's, maybe I've been especially unlucky with units that had very limited life, but I would think that 2-3 months for all four is a detail that is telling me something.
I guess I could wire the flapper as so many posts have suggested, but I don't think that's the problem.

Lastly, can I bench test the ones that are 121 dead. Maybe a soaking in carb cleaner could clean them up. I'd like to diagnose exactly why they are dead too. Can they be visually inspected?

Can you offer some help. Thanks a lot,

Marty Wolfson
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93 244 175K, 93 244 109K, 93 245 115K, 99 V-70 84K

I wonder if the mating portion of the AMM connector that is fixed to the car could be the issue? Vis, change AMM, connector corrosion/crud/loose crimp/whatever on car portion gets perturbed enough to make a good connection for some period of time, then reverts back to resistive mode?

Affirmative. I had problems with the little metal prongs on the amm connector loosing tension, and also being covered with a slight oxidation. It was just enough to screw up the signal, and make the car run like crap. Look at it closely with a loupe. And, blocking off the hot air as suggested by Ken C is a good idea to boot. Cover both bases.
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Gary Gilliam Sumerduck VA, '94 940 na Regina 160k '86 240 190k

I'd still consider the possiblity of something wrong in your air filter box -- it's a well documented cause.

Just because you replaced the thermostat, are you sure the flapper is sealing well?

In any case, if you don't find the problem, and unless you've got a bottomless wallet and like to buy AMMs every few months, I suggest reconsidering ripping out the mechanism and sealing the passage in the box -- you can still leave the hose (for a visible confirmation that you haven't tampered with it -- with how much trouble it is to change the air filter, you just know that no inspector is ever going to go to the trouble of opening your air filter box to see if you've tampered with it).

And contrary to some comments on this list, the truth is that this pre-heating system has *nothing* to do with driveability of the car when starting off in cold weather. In reality, the reason for the system is just to meet EPA requirements for rapid warmup, to shorten the period when the engine, cold, puts out more polutants -- while air quality has it's merits, in my opinion it's not worth the impact on our wallets for shelling out money for replacement AMMs.
I've ripped out this thermostat, and sealed the openings, on all three of my 240's -- and I've driven all over New England in the coldest weather (winter ski trips, etc.), without any driveability problems.