Sometimes it charges, sometimes it doesn't.
 

Ahhh... I enjoyed about 2 months of my car being trouble-free. It was nice while it lasted. But here we go again; this time it's just an annoyance, although it could leave me stranded somewhere so I'll ask. I found nothing like this doing a "alternator" search in the forum search, or a "intermittent alternator", "intermittent charge" or a "How fast can Tinkerbell fly". The latter one proved to be the one with the least results. Anyway, What's happening is that my alternator sometimes decided to charge normally as expected and sometimes it just won't charge at all. My lights will go really dim, my wipers really slow and when I turn off the car the alt light won't come on like it usually does for like half a second or so. Sometimes it will charge just fine, lights normal, wiper normal, alt light comes on when I shut off the engine. Sometimes, the alt will not charge and all of a sudden my lights come on bright, and everything seems okay until I shut the car off. Once I start it it goes dim again for like 2 minutes and then it will charge again. Why would it work intermittently? Is it the regulator (which I replaced about 6 months ago) or the alternator (replaced 2 months ago), or is there something else that I'm overlooking?

Sounds to me like your alternator and regulator are probably ok. I would get out the wiring diagram and start tracing wiring looking for a bad/loose/intermittent connection.

What is the measured voltage output when idling? It should be high 13 to 14V. If not, check voltage regulator but make sure you aren't glowing. Autozone monkey was ready to condemn alt when glow was on and acting as designed.

Since you replaced the alternator a couple of months ago. See if you can turn the cooling fan disk on the alternator by hand. If you can the belt is too loose.

Check the alternator cable as it passes the air box firewall, that place gets rubbed alot sometimes.

check and clean the connection at the distribution block where the cable connects to charge the battery, should be 3 bolts.

also check for wear or the wire right at the alternator plug

mine had something close to this and after replacing the alternator wire it works perfect now.

hope this helps

Thank you all for the replies. I'm going to check the wires and see who's loose and who's tight. The belt is tight and doesn't squeal or anything...

Intermittent electrical problems are usually bad connections. Look at the wires on the alternator, battery, and body ground from the battery, and the chassis ground at the transmission. Loose or corroded wires on any of these could be causing the issue.
Also make sure you're getting between 13-14volts when the car is running.

I had a wire break right at the alternator plug on my '71. Took a lot longer to find the problem than it did to fix it.

Hm, the wiring problem is certainly possible... The thing is that the car charges a little below normal when I just start it, for the next 2 minutes. It will charge if I rev it up, and it will go down again when idling, and then all of a sudden it will charge in idle... I don't know what to make of that. Why would it charge at 2000 - 3000 RPMs, drop at idle, and then after a couple of miinutes charge fine on idle?

because your glow plug controller is drawing 50 - 80 amps for the first 3 minutes of run time...

You may also want to tap the alternator lightly with a mallet when it's not charging, and see if it suddenly starts charging. If so, you have stuck or worn-out brushes and they aren't making good contact with the slip rings. I know you said you changed the alternator, but if it's a rebuilt unit they may not have put in new brushes. I've seen "rebuilt" alternators where I suspect the entire rebuild process consisted of applying paint from a rattle can.

vstech also reminds me of another possibility -- maybe the alternator is fine, and your glow plugs are intermittently coming on while the engine is running. I've seen it happen on VWs due to loose connections at the glow plug relay; I don't see why it couldn't happen on these cars, too.

My friend just cured this problem with a new to him used Voltage regulator i had lying around. His brushes looked ok but were a little short. But it still looked good.

Holy Cow! Is that normal? Why would it draw all that amperage for so long? But at any rate, there's something wrong with mine... nobody else complains of that symptom. If it's not normal, how do I stop it from doing it? If it is normal, what do I do?

3 minutes of glow time is not normal. I had to deal with an intemittent charge issue for about a year. I replaced the voltage regulator which worked for a while, then it would charge intermittently again, then I found the 3 pin harneed connector where the spades was loose and not making good contact. I always wanted a higher amperage alternator so bought a new one for $100 and have been trouble free knock on wood since.

Pull out your 3 prong connector and stick a single spade in each of the connections and if any feels loose, there's your problem!

3 minutes of glow time IS totally normal for SOME of the controllers.

they are called afterglow, and they do indeed hold the plugs in for up to 3 minutes to aid in smooth idle at cold engine temps.