'70 Cuda with square-back alt and stock type VR.
Ran today for the first time in a month. Was charging at close to 16V. Thoroughly checked wiring and connectors and engine/body grounds. Ran it with VR disconnected and then with green field wire on alt disconnected and nothing helped.
Is this an internal alternator issue? I ran the 110V test lamp test (in the FSM) but it failed on every alternator I have. I don't trust it.
Anybody ideas? Please help. Thanks.

Brush grounded? Probably broken isolator?

2 field terminal alt with the blue wire feeding it switched 12V & the green wire pulled off of the other alt male terminal & it is full fielding? If so then what Nacho said, an alt problem. (if not holler back). eyeball the brush assys for any visual grounding then pull the blue wire off of the other alt male field terminal & ohm both the male alt field terminals to the nearby case & there should be NO continuity (& ohm em to each other). You might take the brushes/holders off & inspect/reinstall (something hot is touching ground). the field circuit is improperly dead shorted to ground makeing it full field. Do not let it FF for more than a few seconds at a time, just long enough to see if each of your corrective procedures is fixing it. & have all lights/accessories off during this

I just went out and rechecked the continuity between the 2 field terminals (individually) and the alt case and found they are both open or in the Megohm range.
If it sounds like an internal alternator issue, maybe I should go through the full battery of FSM checks.
Any other opinions?

Forgot to check between the 2 field terminals.
Just did that and found ~12 ohms.
What is typical here?
Thanks for the help.

"at rest" the field circuit ain't grounded (you have no continuity to the alt case) but if it is full fielding with the blue wire field lead plugged in & nothing on the other male alt field terminal then the field circuit has to be grounding/full fielding while it is "running". Am I seeing this correct per your description? If so I would take the alt in to O'reillys for a free check & let them spin it & tell them how it is acting so they can not only check max full fielded output (their std procedure which ain't the issue here) but see if they can get it to FF at a lower RPM (like it is doing now). We'll get it

ok, let's see... dual field alts get both brushes isolated, one for constant positive the other one for regulated negative. Hence both needs to be isolated.

positive ( blue ) comes straight from ign switch.

negative ( green ) is sourced by the regulator.

This is a non brain deal: an alt sourcing is somehow getting both fields to still work. With regulator unplugged, it is getting still both fields on whichever form. If blue wire is still plugged to field and green is unplugged and STILL is feeding, something is shorted to ground feeding on rotor.

Once I got 18 to 24 volts reading ( depending on RPMs ) and discharge on ammeter. After schatch my head for couple of hours just found vent vanes were getting some friction with diodes leads coming from stator while spining. Just bent a bit the stator leads to diodes bank and done. Maybe a diff problem than you but meaning with this a fail like this is a straight or internal short, somehow.

Now about ohms testing... allong the years the rotor ohms reading changed a bit from 12 to 7 ohms between leads. However no matter what, it must be isolated from chassis if reading straight to rotor tracks. Now, reading between brushes, a single field will be grounded JUST via the grounded brush, but on dual field alts they still must be show non continuity with chassis, no matter what.

If you get ground continuity between brushes, something is shorted to ground. It can be brushes itself or rotor ( I have got maybe half of dozen damaged rotor, is my main fail ). HOWEVER a Rotor grounded can get two kind of damages: continuty on windings and still shorted to ground OR broken wire and one of the leads shorted to ground. Both will cause diff fails. I have not analized that, but MY THOUGHT is the first case can keep still working but full fielding... second case not ( that's for sure ). MAYBE you are getting the first case ?

A damaged diode will cause still nice voltage but not enough charge

many other fails can appear on alts, but I have not analized everyone

on a note appart... IF the brush the green wire was connected is grounded YOU GOT LUCK the blue wire is/was not conected there because that will burn the wire up to ign switch and even blown the fuse link, burning all around.

If the blue field was shorted to ground you'd have a smoking wire, if the green field was shorted to ground you'd have maximum voltage output.

Situation update...
Guys, thanks for all of your responses.
I reported in error above. My volt-ohmmeter must have been frozen last night, but when pulled the alt and rechecked continuity between the field terminals and the case, I indeed had low resistance (10-12 ohms).
I pulled it apart and found that the slip rings were shorted to the rotor shaft (and, of course, ground).
Stator and diodes all ck'ed out OK.
I'm going to put in another (good) rotor and should be back in business.
Thanks again for the replys.

It was tooo much casuality the slip ring side was the opposite to the one running to the + field, because I think it could be shorted being a straight conection, with no resistance due the winding coils

There was a vendor on ebay selling those slip rings quite ago. If still, the slip ring can be changed by yourself

several in fact:

https://www.ebay.com/sch/i.html?_from=R4...ng&_sacat=0

its a good thing that the downstream side of the alt field circuit shorted as opposed to the input (blue wire side) shorting, so that it full fielded (which is bad enough) as opposed to a dead short going back to the fusible link.

Thanks for the info on the slip ring availability. I swapped in a good rotor that I had laying around and all is well now.
It's good to know that these piece-parts are still available here and there.

That's right as my buddies 71 Roadrunner had the alt rotor short to ground right after the brush fed by the blue 12 volt ign voltage and burnt his fuseable link out and started to melt some wires. I had to replace the alt and fuseable link and repair some wires on it. Ron

That's why I told it got luck there. If wires were conected the other way around ( which is posible without any problem since they work in both positions the same ), the short was to happen just turning on the ign 1