1991 Grand prix 3.1L turbo frying up ECM

I've got a '91 grand prix 3.1L turbo that has mechanics stumped!! It sat for a few months while I was away, and when i came back i put a new alternator in that is needed along with the belt. Ran beautifully all day!! Later that night I was driving it around . stopped somewhere for an hour . . went to start it and nothing. Seems now that it will only start from dead cold and it idles all out of wack. Mechanic changed the map sensor, crank sensor, ignition module, and ECM. Started up and something shorted and fried the ECM. Mechanic looked around . .found nothing wrong, put a 2nd ECM in it and fried that too. Anybody have a clue on where to start looking?!?!? Please help?!?

Since the ECM controls the alternator signal, the alternator may have been a symptom, not a true problem. The ECM on your car is mounted under the hood, so if you live in a state where salt is used on the roads in the winter, there is a high likelyhood of a corroded ECM harness. That is where I would start.

Then i would look at other connections to the alternater and make sure all connection are grounded. This is AFTER you check the ecm harnes as stated above.richtazz is good, i would start with what he said about the ecm harnes.