Alternator help

[Sonic Boom]

Delco alternator not charging. It’s a dual 12 and 24 volt unit. Has a good ground and 12.5 volts to large wire and 25 volts to the small wire. Tried exciting the field and that didn’t work. Wanted to ask the experts here if any other ideas and what is the small round capacitor looking piece on top of the alternator before replacing. Thanks in advance

[madhatter1]

I have the same boat. Call me crazy but my alternators never worked and I run gen which powers chargers. Only the port forward battery ever sees a load as the house battery. The 3 others sit to start gen or engines. I took the belts off the alternators.

[Captain Ralph]

That is the old tried and true Delco 30SI alternator.
The box on the back (Non-Delco), offers the dual voltage output. Usually a good 12V and a trickle 24V outputs.
That cap on the top looks like an extra noise filter. There is already one internally unless replaced with this larger one, outside mounted.
It is a brush-less alternator. Extra noise reduction confuses me.

I remember a SS engine where that was (smaller package) a time delayed diode.
Part numbers or any specs off of that potato would help more than guessing.

Because of its nature and later operations (nobody goes fast anymore), these units do not start charging automatically at low RPMs well. One of mine did not want to start auto-charging until 1700 RPM and when it did, threw out wonderful current back down at idle..
I have modified mine with a switched sense wire and they work great now. Rev up not required.