I replaced my original alternator not because it didn't work, (it still does) It's just that the rubber bushings had turned into some kind of gelatinous slurry that had mostly flowed out, so no matter what i did the pulley was misaligned and barely any tension on the belt..
So I put in the new alt and everything was good for a few days, so wtf would it stop charging?
I attempted to pull the regulator & at the re-man factory they used external star head screws (WTF). I don't own any star head sockets & tried a pliers, then promptly gave up and pulled the entire alternator again(I'm an expert now!).
With the alternator out, I took it to oriley to test.. no dice, their machine is throwing some dongle error and wont run the test.. Then I go to Advance auto. their machine doesn't have this alternator in the database and also wont run the test. So I hit oriley on the other side of town & the re-manned alternator tests good. He ran the test twice. It's good.
Next ensued a 30hr deep dive on this & other forums for troubleshooting e28 alternators, which apparently never fail... And lo, there was the battery light in the instrument cluster.
When I did the key-on-don't-start check, I had no battery light. I know I've seen it before & i definitely had good charging prior to this (at least some of the time)
Nothing in my cluster works anyway, so i hadn't given it much thought.. At least some of the time when we first got the car, it was charging.. even after swapping in the new alternator, it was charging.. sometime between the swap and now, the light stopped working..
i pulled the cluster and the battery bulb cartridge is BADLY corroded.. So next I disassemble the whole cluster and find that my SI board is also BADLY corroded and the corrosion has also spread to the main cluster PCB & to the PCB on the speedo assembly. Oof.
The PCB contacts for the battery light were completely crusted over with green gunk. I used baking soda, water & a toothbrush to clean all the acid/grime I could from the surface of the main PCB, especially around the battery light socket and its traces back to the big blue connector. At least that part of the PCB traces are in tact and after re-assembly i have a strong/bright battery light at the key-on-no-start check.
I fire her up and she's purring now. I'm thinking with the alternator not alternating, the system voltage was so low that the fuel pump was under powered, and possibly the ignition as well.. Odd how more rev's would keep her running & after dying shed still turn over...maybe only for so long until the battery was totally dead?
Anyway now the idle is very strong now and seeing 13.7-14.1v at the battery. Fixed!
Adding SI board replacement to the list & fingers crossed I don't need a new speedo assembly as well!