240 Not charging

1991 240 with 2jz swap, shouldn't make a difference. Happened all of a sudden: Started car, no issue, alternator working and volt gauge showed 14+v. Moved car close to the house so I could vacuum it out, parked car, cleaned the interior real good. Started car and battery light stays on, volt gauge now shows 12ishv and dropping. What the hell? I start trying to figure it out. Fuses all good. Battery connections cleaned and tight.

Battery light comes on when key is on, stays on when car is running. Time to follow the circuit. Alternator showing same reading, 12.3v, as battery. Followed big red positive wire to distribution block on driver fender and all of those have 12.3v. Pulled cluster and knee boltster, 12.3v at the big red wire coming into the ignition switch. 0.4v at the small red wire in the circle plug that goes into the back of the cluster. 0.4v at the red wire at the bulkhead connecter. 0.4v on the little red wire coming into the alternator (signal wire).

So that is the entire circuit I do believe. The way I see it, I am not getting the same 12.3v coming out of the ignition switch as is going in. Ignition switch randomly failed after 200k?

Did I miss anything / do y'all want me to get you anymore info?

Thanks as always for sharing your experiences and thoughts.

Sounds like just a bad voltage regulator.
I had a lot of issues like that recently with used but supposedly good alternators, bought a new alternator and a Exide Dual AGM battery and it's been reliable ever since.

I got a lifetime warranty reman Denso 100 from AutoZone, never looked back at jy alternators. They just replaced the alternator on my Toyota matrix for the 2nd time gratis on my original purchase in 2008.

Ah hell, I was thinking about this charging circuit incorrectly wasn't I? So I suppose that the alternator should be sending 12+v to the battery light and turning it off. Since it is not doing that, the light is staying on. Basically, the alternator isn't doing anything right now > bad regulator? Am I seeing it correctly now?

Thanks for the heads up on the rebuilt alternator deals, I'll check that out.

Actually, the battery should provide B+ to the ignition switch then to the battery light on the dash which feeds the exciter circuit in the alternator. Yes, if your light bulb is burned out in the instrument panel the alternator will not start producing voltage. Once the alternator begins providing voltage back to the light rather than a ground the light goes out.

I have had an ignition switch suddenly fail internally but a bad regulator is a more common problem behind an alternator not charging.