Disconnect the battery.
Remove the large wire from the back of the alternator and wrap it electrical tape and reconnect the battery.
If the current draw is gone, the alternator fried a diode and was drawing current from the battery.
Tester
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If it were me, at this point I would spring for a manual with a factory wiring schematic. Wandering the desert for 40 years like Moses without any tangible results would get awful tiring.
I’ve purchased manuals and tools for one time use. It can be a bit painful but compared to the Moses scenario it’s a pretty good option. Did this once with an electrical manual. 90 bucks hurt a bit but it quickly led to solving the problem.
I bought a pricey Snap-On wrench, walked off the tool truck, and went straight to the back room where I took a torch and modified it with a couple of bends.
Just so I’m not one of those guys that comes to a forum, asks questions, and leaves without follow-up, here’s the outcome:
I took it into a shop locally and left it there a few days. They tested it and found nothing. It was left hooked to the battery for a weekend before I got back to pick it up, and when I got there it turned over no problem. The car was returned to the owner, and they’ve reported no problems since. I keep feeling like it’ll come back, but so far so good. The only plausible thing I can come up with is that in all my testing, I managed to tighten something that was previously a poor connection that was ‘waking’ the car up and draining it.
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