And then I decided to take another approach. Clearly my assumption that the problem was a bad wire (or perhaps the oil pressure switch - my first assumption) was incorrect.
So, I got out my voltmeter and checked the voltage directly at the terminal 1 connector. (I probably should have done this much earlier.) When the ignition switch was on and the engine stopped, I saw 13.1 volts - battery voltage (and I had to ignore the low oil pressure alarm, screaming in my ear). This is as it should be.
Then I started the engine and kept it below 1000 RPM. I still was seeing the 13.1 V at terminal 1. Then I wiggled the connector around a bit and the voltage shot up to 14.1 - the alternator had started running, and was feeding itself.
I was able to repeat this several times.
Treating the connector with WD40 and plugging/unplugging it several times did not help.
- The problem may be a bad connection internal to the connector, where the wires join the terminal.
- The problem may be a bad connector, making a poor connection with the alternator terminals (actually, these two terminals are on the internal voltage regulator).
- The problem may be a loose connection at the voltage regulator, inside the alternator
- The problem may be a bad voltage regulator.
- The exercise was worth it, in understanding how the wiring actually is run, and in removing the last mystery wire from the boat. I will make a note on the schematic reflecting the actual wiring.
But I'm tired. Having gotten this far, I'll leave the next stage for another day.
I hear a beer calling to me (faintly, from inside the refrigerator)...
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