Most people characterize their alternators as one wire, when in fact they are three wire. The 10SI or 12SI are three wire alternators, but the 12SI is better because it self excites and still gives you three wire functionality. It also doesn't have a min RPM and starts charging the second it starts turning. I just have a 10SI. Be sure to get an alternator that is UNDER 50 Amps. If you don't, you run into the problem of shunting. That entire subject is covered here:
http://devestechnet.com/Home/Native12VUpgrade#gaugesThree wire is better because you have idiot light functionality. While that isn't a big deal in itself, there are things you can do with that functionality to expand other things.
The three wire alternators are internally regulated so there is nothing else to buy and act the same as one wire alternators in that it only takes one wire from the main terminal to hook it up to the truck. There is a second wire that is jumpered over on the Alt itself. The AC Delco Part Number is 13-4010 for the standard 42 Amp version.
