A little progress:
It's a Hitachi alternator with internal regulator, as shown by the designations 'S' and 'L' for Sense and Lamp.
I found some info on a site for 240/280Z owners with photos that match my unit. http://www.atlanticz.ca/zclub/techti...wap/index.html
The diode is mentioned here too- on Z cars, when an internal-regulated alternator is fitted, it's fitted in series with the lamp wire to stop the engine continuing to run when the key is turned off. Apparently the alternator can power the ingintion through the charge lamp when the key is off and the engine running.
I can't see how this can be a problem on the GTV though- for starters, the diode is the wrong way round. current flow should be only into the alternator to start it charging, after which the internal wires take over.
The 'voltage regulator' box remains a mystery, since its cables go back into the loom that runs across the front of the car to the fusebox. They are not on the wiring diagram (thick red and brown, thinner black). I will have to trace the wires. I don't think this VR box (if that is what it is) actually does anything with an internal regulated alternator, but I want to be sure before I pull it.
The Nissan site implies the sense wire ought to go direct to the battery positive, and not into the loom- to compensate for the volt drop along the main charging cable. Presumably, though, it could also go to the fusebox and keep the voltage up to 14V there? Which is best?
This car has also has a Bosch electronic ignition with Hall effect sensor in the distributor, but the wiring to that looks stock.