This problem started when I upgraded to a higher output alternator. I am using Bosch #Al559x which is 105amp. output. I believe it is stock for a mid 80's Cavalier or Corsica. The first one lasted about 1 week, then voltage dropped to 12v. The second has only about 50mi. on it and when I started it up tonight after sitting about 1 week it is back to 12v. I checked for power at all the important connections and it is good. I don't have a multi-meter so I cannot check voltage at different points. I have to rely on my auto meter guage. I have dual yellow top batteries wired into a marine type switch. The alternator output wire is 8ga. running directly to the starter wire post on the switch. I have the sensing wire hooked to the junction block on the firewall. It seems that the requlator is not engaging. Why are these alternators failing? I have very good wiring and good connections but something is obviously wrong. This is getting ridiculous. Any ideas appreciated.