OK, on a regular 3 wire late 12SI GM alternator, there is one large wire from the battery to a lug on the rear of the alternator. There are two other terminals on the alternator side, one marked 1 the other marked 2. The terminal 1 isd to be used for a charge light, connected to the charge light circuit. Alternators that are in circuits that have wither a voltmeter or ammeter don't need this charge light circuit to function.
Now, for terminal 2. This terminal is a power source for the internal regulator. In a 3 wire wireup, the number 2 terminal is literally connected to the larfe lug on the rear of the alternator, from the battery. The large wire to the rear lug, and a jumper fdrom there to the number 2 terminal will make the 12SI internally regulated alternators function.
That said, 1 wire 12SI alternators are simply the exact same alternator as a 3 wire, but they use a specially excited regulator designed for industrial, farm and fleet useage. These special regulatos have the large lug to terminal 2 jumper connection INSIDE the alternator. This is the ONLY difference between the 12SI 1 and 3 wire alternators. The number 2 term is connected to the BATT lug either inside or outside the alternator, NOTHING else is different.
12SI alternators are available in 65, 78 and 94 ampere versions.
There is a CS alternator that uses the same type of internally excited alternator, in the smaller overall size. My 1986 GMC Safari van has the 105 amp version.
Early 10SI remote regulator alternators can greatly benefit from using a Wells VR715 regulator. These regulators are electronigc and perform as the later internally regulated 12SI regulators do. Using the Wells regulator will also stop the light fluctuations at idle speeds, lights going from bright to dim to bright to dim. The stock regulators are servo actuated, in essence, they turn the alternator stator on and ott and on and off to regulate voltage production. What is seen in the fluctuating lights is the servos turning the alternator stator on and off and on and off, constantly. The Wells style electronic regulator simply turns the stator on and keeps it on, and when charge volt level reaches a set point of 14.60, the regulator drops excess charge voltage pas the cut off point, to ground, thereby keeping the charge rate and voltage constant, and the alternator gets much less of a workout to do its work.
The stock Delco regulator cap will fit the Wells, making for a stock appearance for restorers.