Info Found here came from many WebPages, GMC Manuals & just my own knowlage.

The 12SI Delco Alternator is a very good up-grade for your GMC Truck, even if you already

have a stock alternator setup. In recent years it seems that the term “One-Wire” has

become a much-used buzzword. Among the GM/Delco SI series Alternators, there is both

the “One-Wire” & the “Three-Wire” type. Many Folks do not know that all alternators

with a built-in voltage regulator are “One-Wire” types. Many Folks simply refer to all

alternators with internal voltage as “One-Wire”. You want the Three Wire so you Gen.

light will work as it should. This will eliminate some wires & the external regulator.

You can get a lot more info on the “One-Wire” vs. “Three-Wire” at
Mad Electrical.

Pic & Following text from a Demolition Derby by Tony Hartin.

1. This is the main power wire that goes directly to the battery. Make sure it is a

heavy gauge since it has to carry the full output of the alternator (up to 100 amps).

2. This is the field wire that energizes the alternator. It must be switched, or else

it will drain the battery overnight. Do not wire this into the coil power wire that you

use to switch the engine on & off (a seemingly elegant solution), since once the engine

is running the alternator will feed power back through this wire, which will then power

the coil, & you won’t be able to shut your car off.

3. This is the lead for the sensing wire that goes to the idiot light in your instrument

panel. When this light lights up you know that your the alternator has just stopped charging.

4. This is the regulator bypass hole. Sticking a long thin metal object about 2 inches deep

into this D-shaped hole causes the alternator to go to full charge. If you do this while it’s

dark out & the main power wire is disconnected, the whole alternator will emit a cool X-Files

type glow. I don’t recommend that you try this since you’ll likely end up toasting your alternator.

Now that you know how to wire the 12SI Delco Alternator, here is what you need to do

to make it work in a 6066 GMC Truck. First thing to do is Find a donor vehicle to

remove Alternator & wires from or get the ALT-1
Alternator wiring kit from Mad

Electrical. Next remove the old Alternator & regulator. You’ll need to save you old

wiring & do the following mods. Next install the 12SI Delco Alternator.

Now for the wiring. Install the field wire to post #2 of the Alternator plug you removed

from the donor vehicle. Next install Reulator wire to post #3, this will become your Warning

Light wire. Now install plug into the Alternator. Next move to the old Regulator Plug.

You’ll be cutting this off One Wire at a time & spilcing the wires as follows.

There Should be four wires on the Regulator, the one at “Field” should be wired to the

one at “Battery”, the two wires on Gen should be hooked together.

OR if you want the easy out, just order the
Alternator
Conversion Kit Part # 52-895

from Classic Parts of America {Chevy Duty}.

Trucks With Alternators 1963-66

There Should be four wires on the Regulator, either numbered 1,2,3,4 or lablebed F,R,V,L.

The wire at 1 or F should be wired to the wire at 3 or V. the wire at 2 or R should be

wired to the wire at 4 or L.

Last install a Cable from post #1 on the 12SI Delco Alternator to the Positive Post

of the battery. Make sure it is a heavy gauge since it has to carry the full output

of the alternator to the battery.

OR if you want the easy out, just order the
Alternator
Conversion Kit Part # 52-896

from Classic Parts of America {Chevy Duty}.