I have a 78 amp 12SI alternator, out of a mid-80s Riviera, in my '78 Trans Am.
My new battery went completely dead in a week. A meter showed a current draw of about 300mA with the key off. Removing the two-pin connection from the alternator dropped the current draw to about 9mA. This is a completely repeatable result.
The alternator appears to work fine, except maybe for being a bit weak at idle with the backglass defrost on. However, something is obviously wrong inside it.
The '78 Pontiac service manual refers to diagnosis using the "ALT" light, but my T/A has rally gauges and no warning lights, so I can't use the manual's procedure. Nevertheless, the manual says that "if the ALT light is on with key off, remove the two-pin connector; if the light goes out, replace the rectifier bridge." So I'm guessing that my ALT light would be on with key off, assuming I had an ALT light?
The rest of the troubleshooting procedures involve disassembling the alternator, which is more work in winter than leaving the two-pin connector off when the car is parked!
So my four questions are:
1) Do the symptoms as described point to a bad rectifier bridge in my alterntor?2) For the short term, can I simply disconnect the two-pin connector while not driving, and connect it when I am driving? (It's winter in Ontoario!)
3) I have an overhauled/rebuilt 10SI with unknown components in it. Can the diode trio and rectifier bridge be swapped between a fairly high amperage 10SI and my 12SI?4) Okay, I've studied the factory manual charging system pages and the wiring diagrams. How on earth can a "ALT" light be on with the key off? The light circuit is interrupted! Yet apparently it can happen?
Thanks Ed