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Old 05-01-2017, 10:04 PM   #15
VetteVet
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Join Date: Jan 2005
Location: Kalamazoo, Michigan
Posts: 8,437
Re: External to Internal Voltage Regulated Altenator Conversion

This was one of the first conversions posted on the forums and it made it's way to the FAQS where most people can access it. While it works, there are better ways to do it. It does not take into account the voltage drop down stream of the alternator, because looping the no.2 voltage sensing wire directly to the alternator output wire does not account for the voltage drop down stream in the circuits away from the alternator. This can result in as much as a 2 volt drop in volts loss to the system loads.

It is easily remedied by the method I use and recommend shown in the diagram below.
Simply extend the brown wire from the external voltage regulator to the no. 1 terminal on the SI alternator, and extend the red wire from the external regulator to the no. 2 terminal on the alternator. The red wire connects to the main system junction of the truck circuits and is able to sense the voltage draw on all the truck loads combined. This allows the alternator to output voltage for a longer period to deliver a full 15 volts to the truck circuits.

Here is the wiring for the stock OEM truck

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Here is my conversion for the SI internal regulated alternators. Use the 12 SI from the mid eighties GM vehicles. following the directions I gave, you can delete the regulator harness and the blue and white wires, although you can use them to extend the brown and red wires. Of course you'll have to use an adapter to connect the wires to the alternator.

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And if you want a real improvement you can put a 75 to 300 ohm resistor in the brown wire and run it to the L terminal on a more modern CS alternator, and run the red wire to the S terminal on that same alternator, and you have a more powerful alternator, and a much more available unit than the SI models.
The 12 SI is a very good unit and even the 10 SI is bettter than the old externally regulated stock unit.

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For the curious the alternator pulls cooling air from the rear so it's a bolt on and go deal. The CS models just require the resistor and changing the front pulley to belt drive.
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VetteVet

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