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Here's the "straight dope" When GM switched from the older style 10DN alternator to the "10SI series", a certain number were made WITHOUT an internal regulator, and were used with the old-style external regulator like the 10DN's used. IF you have one of those alternators, the terminals will be parallel to each other, like this: . If the terminals are like that, an EXTERNAL regulator is needed, and you go by the (F)ield and (S)tator designations, and IGNORE the #1 and #2 designations. MOST LIKELY, though you have a 10SI WITH an internal regulator, and the terminals arranged like this: - - . 
If so, #1 is "switched excite"... powered only when the ignition switch is "ON", through an "idiot light", resistor, or diode. #2 is the "Voltage Sense" terminal... jumpered over to the large output stud, as shown, or wired
back to the battery cable side of the starter solenoid, for even more accurate voltage sensing, near the battery. I hope this clears things up for you... Bob (Alternator repair guy)
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