Volvo: bad ECM?, air pumps, c unit

Question
QUESTION: I have a 97 V90.  The idle speed motor stays powered on after the ignition is off, draining the battery.  After considerable fault tracing, my tech believes that this is caused by a fault in the ECM (unplugging the ECM stops the idle speed motor, which doesn't start again even if the ECM is reconnected, at least until the car is started again).  The part no. is 0-261-204-239, which seems to be rare.  I managed to find a used ECM with the same number, but it didn't fix things, so I don't know if this second unit had the same fault as mine or if we've mis-diagnosed things.  Any thoughts?  Can I substitute any other ECM for my rare one or must I only use one with the identical part no.?
ANSWER: Peter,
Maybe a faulty wire harness, poor grounds. If you have a c/unit from a 96 to 98 960/S-V90 they all use the motronic 4.4 system and should work for trating.

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QUESTION: Thanks, Ray. When you say that "a c/unit from a 96 to 98 960/S-V90 ... should work for testing," I assume that means I won't fry the test unit or some part of the car by swapping in one of these to test the hypothesis that the ECM is causing the problem.  But do you also mean that a non-identical unit might have other problems and so shouldn't be used as a replacement?

Answer
Peter,
I do not know what is rare about your control unit, since the S/V90 or 960 96 and up use the Motronic 4.4 system. There may however be some C/units that are programmed for applications that have air pump, EGR. Those systems will not keep the car from running, but will turn on the Check Engine light when the C/unit looks for but will not receive the signals from air pumps or EGR. I can not see why anything would dhort out. I have done it many times on the 240's and 850's.