Battery charging voltage

Mine is a mid 1990s Nissan and the alternator is very standard. One of the wires is definitely a 'sense' wire and is not connected directly to the alternator B+ post, but to some point that's supposed to be system voltage. Anyway I added a wire from that terminal direct to the battery, and the alternator no longer puts out 15V all the time, but after starting it settles down to a more reasonable 13.2V to 14.6V.

do you have the correct bulb in the alternator warning light? usually the 'sense' wire goes to the non powered side of the alternator warning light.

Yes, the bulb is the same as the other warning lights. The wire that goes to the warning light isn't what I'd call a 'sense' wire. It's there to light the bulb up when the alternator isn't charging, and to energise the windings initially. The 'sense' wire is a thicker cable with battery voltage permanently on it. Alternator is a Hitachi LR170.

A LR170 is not standard by any means, the alternator does not contain a regulator, the regulation is done by the vehicle ECU, or a separate Power Control Module.

I'm not sure what's non-standard about a Hitachi LR170. It definitely does contain an internal regulator.

Are the terminals on the 2 pin plug labeled 'P-D' or 'S-L'?

labelling is 'S-L'.