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Many thanks to all who responded! I did unplug the AMM, started it up and, sure enough, it idles fine. So we know the AMM is the culprit. When the first AMM failed, a couple of months ago, I replaced it with one salvaged from a wrecked 87 240. The airbox thermostat was bad, so I disconnected the warm air hose thereby ensuring, I thought, that any air entering the airbox would be cold. Now that the second (salvage) AMM has also failed, I'm wondering if it is just coincidence, or if there is an underlying common cause between the two failures...
I understand there is a feature that applies high voltage to the wire in the AMM to burn off residuals when you turn off the engine. Perhaps this feature has failed, leaving residuals on the wire that build up and, ultimately, result in AMM failure? Here's yet another interesting point: When I bench tested the first failed AMM by turning the adjusting screw while measuring the resistance, it was within spec and seemed to work just fine. So today I put that AMM back in, just to see if it would work. It did, the car drove fine...for two blocks! Could the residuals have fallen off while I was bench testing it, and then built right back up again after two blocks of driving? It seems unlikely...
Primary concern now is investing in another AMM, only to ultimately lose my investment because I haven't yet discovered the root cause of two AMM failures in close proximity to each other.
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