Zed Head

Also, never really noticed it before, but Nissan put a nice description of the regulator in the FSM. They even supplied a circuit schematic. It's not my field but still pretty cool to ponder.

Did O'Reilly's test the alternators before replacing, or did they just swap them without testing? Adding on to CO's point - maybe you have an intermittent connection that is actually damaging the alternators. The alternators really are bad but it's your car that broke them. They're already weak because the reman shops keep the old parts that still function. I think that the S wire might be connected to the WR fusible link wire by one of those metal butt connector splices buried in the harness. The ones that look like a PO's hack job. Might be worthwhile to peel the harness open and check it out. My 76 harness had a bunch of melty fused wires buried in it from some previous catastrophe.

Here's a source showing it on the rod side. They don't show one on the nut side though. https://www.carpartsmanual.com/datsun/Z-1969-1978/axle/front-suspension

I forgot the other advice often offered - take the bad OReilly unit to a shop that rebuilds alternators. See what they can do. Or, break out the soldering tools and replace the regulator. https://www.rockauto.com/en/catalog/nissan,1980,280zx,2.8l+l6,1209316,electrical,voltage+regulator,4884

Autozone's Duralast has the same part number as that new WAI Rockauto unit. https://www.autozone.com/batteries-starting-and-charging/alternator/p/duralast-gold-alternator-14592n/740842_0_0 https://www.rockauto.com/en/catalog/nissan,1980,280zx,2.8l+l6,1209316,electrical,alternator+/+generator,2412 Has a limited lifetime warranty. About $106 after shipping from RA. About $149 from Autozone. $43 from peace of mind knowing that Autozone is close by. I'd probably go to a salvage yard and look for a Frontier or Xterra. Just for fun.

Odd that new units for the Frontier cost less than reman. https://www.rockauto.com/en/catalog/nissan,2001,frontier,2.4l+l4+dohc,1374500,electrical,alternator+/+generator,2412

Remy seems to make good stuff. They make new alternators for some brands, I have a new (not reman'ed) Remy Gold on my vehicle, a 2003 Ford. It's been good for 35,000 miles, and I got it used from a salvage yard. Looks like only reman for your application though. But, if I was going to buy one I'd buy Remy. Check the other mainstream stores, maybe Napa or Autozone carry the brand. I had an OReilly Ultima on my Z and it failed. I got the warranty'ed replacement, put it on the shelf and installed a salvage yard 280ZX alternator. Those O'Reilly alternators tend to be junk. https://www.rockauto.com/en/catalog/nissan,1980,280zx,2.8l+l6,1209316,electrical,alternator+/+generator,2412 https://www.rockauto.com/en/moreinfo.php?pk=37579&cc=1209316&pt=2412&jsn=383

Here's a shot of a crusty old, that looks original, 76 compression rod end. Picture 187. https://bringatrailer.com/listing/1976-datsun-280z-59/

More from later.

Something different.

The display auction ends today, in about 2 hours, 40 minutes. One very early bid. Looks like some sort of odd attempt to generate interest in a bunch of scrap parts. E88 head on an F54 block. It doesn't even have value as a display engine. I bet that the bidder gets paid back by the seller and they both agree that it didn't work. Might also just be an advertisement for the shop. Using BaT for advertising. They got used, I think.

That is an odd first bid. Also odd that the seller is in the desert too. Hmmm.... Also also odd that the seller's chosen BaT name is harmonresto. Very similar to the old Harmony01 seller that puts stuff on eBay for inflated prices. All very strange! https://bringatrailer.com/member/deserthermit/ https://bringatrailer.com/member/harmonresto/

It was in a barn. There's potential. Looks like that back bumper was ripped off when they tried to pull it using a strap on the bumper. At this point, do no more damage while cleaning and inspecting seems in order.

Is this a real question or are you just showing the car? It's probably worth more as it sits than after parting it out. I don't know that I'd even try to start it. Post up more pictures after cleaning it up. Rust underneath is the big bad thing.

You could use a meter and see when it has power, if it does. Or if it's grounded or has resistance. I'd take a better picture from distance also. That's a great infocus closeup, but it's just a female bullet connection.

Just as vague in 1978. "engine accidentally stops while driving". ???

More -

Here's Nissan's view.

They could be counterfeits, who knows. Z Car Source is out of business now. Maybe that's part of the reason.

On a 76 one would be right around where the yellow is in this picture. An eyelet with a screw through it in to the manifold. The ground wire is in that harness. The alternator also has a dedicated ground wire which would provide grounding through the mounting points, assuming contact was good. Poking around with a meter is always fun. You could also use a headlight bulb to pull some amps through the various ground points. I like to measure things, but, adding a redundant ground wire won't hurt anything if you don't trust your measurements. https://bringatrailer.com/listing/1975-datsun-280z-45/

The premise of the added relay is that something along the circuit to the starter solenoid is drawing so much current that the solenoid can not actuate. The circuit has worked just fine for over 40 years I'd guess. Something is different now. The wire from R to the resistor bypass circuit is one of the few things that are different during Start. You have a "key at Start" voltage drop problem. Just offering troubleshooting paths to check. As far as the relays, there are several pictures on the forum of burnt/baked relay coils. They just get old and crusty with time. It's not just water that causes problems. How has it been starting with the new relay?

I missed the part above where you already installed the relay. Oh well. Don't overlook this part. Part of it is only in use at Start.

Nissan put the relays in an area that gets wet if the windshield leaks. If a crusty relay is causing excessive draw the extra bypass relay might just be delaying another problem. They're by the fusebox. I don't know if Nissan just changed names or did not use a standard ignition relay in 75. The 1976 BE chapter shows one but 75 only shows an "igntion interlock relay". They don't include the small diagrams either. 75 is one of the difficult years. This is the 75 drawing - Here's 1976, where that rusty picture above came from -

Interesting that the draw doesn't affect the other gauges, not even a needle quiver. But it does affect that horn noise. Not sure what that is, is it the seat belt warning buzzer? I have to say/ask - why are you avoiding the simple electrical diagnostic methods? It's a very straightforward circuit with a simple solenoid at the end. Low current, one device. I almost replaced my starter when the spade terminal lost contact but then I pulled back to the basics and figured out what was happening. Another time on a Ford Escort I was halfway down to the starter when my brain told me the symptoms didn't match what I was trying to fix. It was a loose battery terminal (not your case since the gauges don't move). Notice on the wiring diagram that there are other functions on that Starter wire from the ignition switch. You could check those to find the break.

Who is "they"? "Any or all of the above" doesn't mean anything. Z Car Source is out of business, AZC is more of a performance shop. Did you get them from eBay? Maybe you just bought from the wrong place.