I had already reassembled the 12SI when most of the discussion here took place. Saturday morning I decided to reinstall it into the car.
First, I checked and the tan/brown wire had zippo voltage on it with the key off. Then I installed the alternator and the same problem: increased current draw with the two-pin connector.
THEN I had the horrible realization.
In mid-December, my battery went bad. It was old, and one of the side terminals had leaked and corroded the connection. I also made sure that the aged wiring to the alternator was corrosion-free.
To clean the regulator connections, I popped the wires and connectors from the plastic body, doused them with contact cleaner, and replaced them.
You can probably see this coming: when reinserting the wires into the plastic connector, I swapped 1 and 2. This results with relatively normal alternator operation, except for a higher key-off drain. I switched the wires to the way they're supposed to be, and now my key- off draw is 10mA with the alternator all connected, and 9mA with the two-pin pulled. A great improvement on the 300mA draw before. And the alternator charged fine; no dead shorts to ground either.
So tomorrow I'll go to the auto electric places and get new bearings pressed in, and also collect a proper 12SI rectifier bridge. Then hopefully I can ignore this alternator for the likely remaining life of the car.
Question though: how do you determine if there's excessive voltage drop in the wiring? I turned on the blower on high and the back defrost on. Trying to read the voltage between the battery and alternator B+ terminal was frustrating, with the readins flipping all around the 0.03V range, sometimes going up to 0.06 but sometimes going a bit negative.
What I'm trying to figure out is why the alternator keeps the voltage up when the car has recently been started, no matter if all the accessories are on. After a while, the voltage will drop at idle with a full electrical load. Is this just increased resistance in the alternator when it gets hot?
....Ed