Car: no mods, bought off the floor in 1989 garage kept since
Issue: 11.4 V and dropping at battery terminals while running. New Optima battery, second new alternator. First Alt I suspected POS rebuild, installed second from different vendor same issue. Terminals clean, fuses under dash all good. Where now? Thanks.

 

You have a short somewhere.

 

Unfortunately I dont have anything to add for your charging problem, but I see this is your first post here and I always like seeing members who have owned their Thirdgen this whole time since purchasing it new. Well done and thank you for keeping a cool car all these years. Wish I could go back in time and buy one new.

 

What's the voltage at the alternator terminal? A quick check to see if the alternator is working is to place a steel tool at the back bearing area while running. That area will be magnetic.

Do you have wiring diagrams?

 

More likely an "open" VS a "short"....

Short circuits draw too much current. This leads to blown fuses/fusible links in the best case, or melted wires, burnt components, and release of the magic smoke sometimes coupled with fire in the worst case. Open circuits simply cause stuff to quietly stop working, without all of the biblical fire & brimstone that shorts sometimes cause. If I were the OP I'd be taking voltmeter reading of all three* of the alternator's connections with the engine running and post up the results here .......

* the one big terminal with the fat wire and the two skinnier wires going to the plug .

 

I would recommend conducting a Parasitic-Draw Test, with a Multi-Meter connected to the Battery in Series.