I am about to order an AMM but thought I would try a few more things and ge some 'final' opinions from you folks. The cheapest rebuilt is about $160, and for that much I can do a lot of typing! ;-)
I wanted to isolate the effects of the IAC valve and related parts to be sure that wasn't the problem, so here's what I did this afternooon and the results.
AS IS - In stock configuration the car starts well, revs to 2000rpm and then drops and stalls when cold. After a few attempts it warms enough that when started will rev to 2000 RPM, drop to well below 500 (varying from 150 to 300 RPM or so) and then will return to around normal idle, but idling very rough. With a medium-rapid pressing and releasing the accelerator I raise the RPMs to 3000 and I can hold it there by continuing to oscilate the accelerator (indicates no exhaust blockage?), but it will not hold the higher RPMs on its own (a symptom of a lean condition IMO).
TEST1 - I removed the IAC valve, and reconnected the wire and the hoses. I did the OBD socket 2, "3 presses test" to cycle the injectors and the IAC, and while blowing into the IAC in hose I could verify that the IAC is functioning, although this is not at all an accurate test as there is no way to judge if the correct volume of air can pass due to the bends in the hoses and their cumulative resistance to air.
TEST1b- Previously I had run the motor and pinched off the idle air hose and the motor immediately stopped showing that it is indeed running the motor at idle.
TEST1c- Disconnecting the IAC electrical plug causes the motor to immediately stall.
TEST2 - I removed the IAC hoses and plugged the idle-air intake metal tube on the intake manifold, in essence making the car in a state as if the IAC didn't exist, although it was connected. The car cranked but would not run or start unless the accelerator was depressed, but once I did that it kept running and it ran MUCH better and noticeably smoother (It felt like it was running richer IMO). Same fuel, less air I guess. (AMM telling the computer that less air is passing its way than is actually the case, so the computer injects less fuel. thus a lean condition?- I am guessing.)
a 1-2-1 "bad/missing signal to/from AMM" code was set. This seems to indicate that there is more going on with the AMM and the 1-2-1 code than would meet the eye. There is some relationship between what the AMM 'sees' and what the computer thinks it should see, and if it doesn't agree within a pre-programmed range it sets the code.- another guess on my part.
TEST3 - I did the above test but did not plug the idle air intake pipe on the manifold. It idled rougher and fast, but kept running on its own. Same 1-2-1 code set. I am thinking that it made the motor so lean with the extra air that the O2 sensor knew something was wrong and the computer tried to adjust. A wild guess.
I did clear the codes if set between tests and verified a 1-1-1 before continuing.
So if you have anything to add, or have any other tests I can try, PLEASE fell free to chime in. If nothing else we are gathering data points for the next idiot.. errr... Volvo home mechanic to use if they have the same problem... ;-)
I can see where the AMM is probalby the most often part replaced unnecessarily- it's a good guess with a high profit margen and it takes very little effort to change it.
__ __ Randy & \ \/ /alerie's \__/olvos '90 245 Estate - '93 965 Estate "Shelby" & "Kate"