October 23, 2008
Here's my old non-functioning mystery alternator next to my shiny new (re-manufactured) 60amp 280z/zx internally regulated alternator (PN#:14185). It's a direct swap into the Datsun 510, both in terms of bolting it up and wiring it. Excellent.
Here's a close up of the new alternator. It apparently has a "P" terminal that is never used. As in, the original wiring harness from the car that went with this alternator didn't actually have a wire that went to that terminal.
I wonder what it does?
Edit: According to the folks on the Datsun 510 mailing list, here is the answer:
The alternator is "generic" and used for a lot of applications. The ZX Turbo happened to use the same alternator sometimes, but did not use that terminal as it was also "generic" in that car. P stands for "Phase" and the terminal is an auxiliary pickup point for partial alternator output, single phase - approx. 6V, to drive some tachs and other accessories. It's basically the same as a bridge tap on transformers. If you want to go hogwild with 6V, I am sure you can figure out some use for it, otherwise just leave it be.
It's the stator output - the juice actually being made by the alternator before it gets turned from AC to DC. It is "half wave" rectified so that only the positive (top) portion of the current comes through, in pulses. Older vehicles, especially Fords and Toyotas, would sometimes use this current to run a choke heater because it is only present when the alternator is charging, so the heater wouldn't run if you just left the key on without starting. Some styles of voltage regulator look at this terminal to decide whether or not to turn of the idiot light.
If you look at the waveform with an oscilloscope it looks like the back view of a rat race starting grid.