The car starts and runs fine for a short period of time, but the alternator won't kick out a charge. The voltage tested right at the alternator is only 11.5 volts when running. Removed the alternator and battery for testing, both are in perfect working condition.

What I'm thinking is that the voltage regulator isn't being "told" to charge. On the plug that goes to the voltage regulator there are two wires. One big white, one small white with red stripe. The big white wire has 12v when running. Red and white is unknown.

Now please note that this S13 SR20DET is in a Datsun 510 and the wiring harness is a homebrew creation (I purchased it this way). Where should the two wires originate?

http://i127.photobucket.com/albums/p149/whatthehellisapodcast/My%20SR20%20turbo%20Datsun%20510/Alternatorplugpic.jpg

make sure you got good ground that goes to the altenator also check the wiring..let us know what goes on..

Just for future research, the problem ended up being a wire separation of where the alternator plug was spliced into the home made Datsun/240sx harness.

My car had the same problem a few months back and it ended up being the same problem as yours. Lesson learnt, don't wire up a loom hungover lol.