2 Caddys both electrical probs. Wont start

Hello thanks for reading this message, I have 2, 91 Caddy El Dorado's with about 150k on both One just stopped working all togeather it gives me a ALT Caution error on the diagnostic which I asked many mechanics what it could be and they all shrugged thier shoulders I also tryed changing the BCM and ECM but still no go tried changin the alternator but no go still same error and no codes to tell me if something is wrong. Could it be the chips you put in when replacing computer moduels?

The second car just had a diagnostic and they said the car was fine but injection computer CCV (i think ) was going one day it ran a little rough then was fine the next day tried to start it and it sounded like a rough dragster car then shut off including the whole dash board. Now you try to even just turn on the ignition and it turns on momentarly then shuts right back off but door locks work you can still pop the trunk and nothing seem to be draining the battery. I have also tried swaping computers out of both cars BCM & ECM also keeping EPROM chips with thier original cars. Only thing I havnt tried was the PCM unit. Please a little guidance would be great this is deffinately a wierd problem for me Thanks, Justin