In my 77 280Z I've read 13 volts on the meter for over 40 years.  Just lately it has started spiking upon starting the vehicle. It will shoot up to the pin and the car runs but the audio system sounds pretty funky.  So, something is going on. I can restart the car and most often clear the problem. Sometimes not.

Edited February 9, 20223 yr by 77Z
Misspelled

Bad regulator or alternator.  Better check things out before damage is done.

@Zed Head is steering you in the right direction. The voltage regulator is likely ignoring (or can't detect) the battery sense voltage. Do you have an external VR or has the car been converted to use an internal VR? If it still has an external VR, is it a stock VR, or a solid state one? (Hint, look for a manufacturer/part number on the VR).

By the way, if you do happen to have a stock VR, they are adjustable. The cover should be screwed on. A solid state VR probably won't have screws on the cover.

You can download a copy of the factory service manual from this website. The VR section starts on EE-18 (195 of the 681 page PDF). If you or a friend know how to use a multimeter, there are some good diagnostics there. It's usually cheaper to diagnose than to fire the parts cannon randomly. 

I don't think he can adjust away a spike on the meter.  But maybe the mechanical points can be cleaned up.  Should be fun, usually they just crap out completely.  Worth a shot.

I had a problem with my external VR alt on my 75, would spike intermittantly, the fix was a new alternator. I never got a chance to dig into the problem as I was on the road and turned in the bad one for the core.

That being said I would suggest you make sure all the cables from the alt to starter positive are in good shape, and check engine harness ground (pass side near the strut tower). The alt or the VR for external regulated needs a good battery and ground reference to know what it should regulate too. Check the battery cables too.

Edited February 9, 20223 yr by Dave WM

Thanks guys.  Its stock and the original VR.  How can I confirm internal or external?  I've followed the harness from the alternator through the harness apparently to the area under the fusible links and the large black plate where they mount.  Is the VR under this plate?

I've got the service manual and section EE18 and reading that I see the adjustment.  But, usually, it reads spot on 12.8vdc. So, maybe its ready to crap out and spiking to the pin is its failure mode.  I have a large audio load wired directly to the battery and fused properly but its been flawless for several years.

Looking at parts I see alternators go from 75 to 2/77.  Mine's a 3/77.  Perhaps I have the first month of an internal VR in which case I just need a new alternator.

Zed and Dave, yeah, its going to stay parked for a bit while I check cables and such.  Good point.

20 minutes ago, 77Z said:

Thanks guys.  Its stock and the original VR.  How can I confirm internal or external?  I've followed the harness from the alternator through the harness apparently to the area under the fusible links and the large black plate where they mount.  Is the VR under this plate?

I've got the service manual and section EE18 and reading that I see the adjustment.  But, usually, it reads spot on 12.8vdc. So, maybe its ready to crap out and spiking to the pin is its failure mode.  I have a large audio load wired directly to the battery and fused properly but its been flawless for several years.

Looking at parts I see alternators go from 75 to 2/77.  Mine's a 3/77.  Perhaps I have the first month of an internal VR in which case I just need a new alternator.

No you should have the externally regulated 60A alternator, 23100-N3303, in your car. The round plug in your last photo is the VR.

Your ground wire at the alternator is looking a little green. You might want to find some Caig Deoxit and spray it down good. Heck it wouldn't hurt to do that on the connector for the VR and the T connector at the alternator, too.

27 minutes ago, 77Z said:

Is the VR under this plate?

Yes,  There are two round plugs but I think SteveJ means the big one.  You can see the fastening screw on the bracket in your picture.  Can't remember but I think that the big plate is removable, exposing all of the goodies underneath.

Viola!  Its a VR hiding under the plate. Never had cause to be there except for re-terminating the vacuum hoses when the heater doesn't work (pro tip).  But, its a 23500-B8000, not a 23100-N3303. I see a couple of sources for a new VR out there.  Anything I should look (watch out) for?

Thanks!

I have 2 new, never used ones available.   The one on the right if I remember, came from ZcarDepot  :  $45 plus shipping 

The one on the left nos :  $60 plus shipping  (only because it says Datsun on it ... lol)