A few months back my alternator went out, and I replaced it with a used sr20det s14 one, which tested fine at the time. Now, a few weeks ago my battery light in the cluster starts to flash on and off every now and then, nothing consistent. A couple days ago it started staying on most of the time unless I was over 5000 RPM. If the car idles and doesn't charge, the battery starts to drain down, but if I'm over 5000 RPM it will charge just fine. This doesn't sound like a standard dead alternator problem, is there a seperate voltage regulator?

Sounds like the regulator is on it's way out.
You might as well start looking for a Quest alternator or something newer.

Mine isn't quite gone yet, but it won't start charging until I rev past 2000RPM... And after that it's running 15volt.
So far I haven't had a major failure on it... Yet.

I've been keeping an eye out for a place that sells the internal regulator on the S14 alt, but no luck in the last 3 years.

If you are looking for parts to rebuild your alt, go to rock auto.com. My experience though is the voltage regulator is usually the same price as a remanned alternator. Brushes and bearings and simple stuff like that are pretty cheap though. I personally think that the quest alt is overkill unless you have a serious system in it. other then that a healthy stock alternator will do just fine, even with electric fans and dual walboro pumps (which is what I run) and yes, it sounds like your alternator is toast and the voltage regulator is internal. the last external voltage regulator I've heard of was like 1970 something. Lol.

Sounds like it's dying. I'd have this one rebuilt at an alternator shop instead of buying another used one. If you have to get a new alt, buy a Quest alt and Codyace's adapter. It's just as much hassle as swapping pulleys from a KA alt and gives you room to grow should you install a decent stereo or tons of electronics.

Yeah it does sound like the alt. Only thing is it is different than last time because then I did not get the battery light at all, it just slowly drained out til the car died. And last time revving the car did nothing. And when the battery light flashes I hear a kind of electrical buzzing under the dash. I'll double check everything soon and see what the results are. Hopefully I won't have to change out the alt again. If I do I'll just get the KA one and use the pulley from this one. Only thing to do for KA alt is just widen one hole and use sr pulley right? Is the quest pulley a direct bolt on with just an adapter?

The Quest alt is like the KA alt in that you have to drill out one set of mounting tabs to a larger diameter hole (I can't remember the size to be honest, but I want to say 15/32)? The pulley is the same number ribs just like the stocker (just need a different length belt) and with the adapter Codyace sells, it lines up perfect and also has adjustability so you can adjust tension on the belt. With the KA alt, you'll need an impact gun to remove the pulley for swapping. You might have a short or something in the wiring if you're hearing a buzzing noise.

Figured it out. The + connector for the alt was loose, so it was sometimes charging and sometimes not. Good to go now. I guess the lesson to be learned is for 89-90 if the battery light goes on it's between alt and battery, or just a dead battery, but if the alt goes bad the battery light will not go on, there are no gauge lights for that.