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12 Volt Conversion

by John Milliman

Tired of Dim Headlights?

      Ever wanted to convert to 12 volts, but didn't want to sacrifice your original engine compartment? Well, here's a few tricks to help you solve your problem while staying original at the same time. Thanks to Patrick's Antique Cars and Trucks for information quoted from their catalog.

Wait! I have a positive-ground GMC, what do I do?

      This so easy, you'll kick yourself for not having figured it out on your own. All you have to do to convert that Jimmy to negative ground is:

  1. Disconnect the battery (VERY important first step)
  2. Reverse the wires on the coil
  3. Reverse the wires on the ammeter
  4. Reverse the battery cables. While you're at it, why not switch the ground cable from going to the frame to a starter mounting bolt or the transmission? It's better than the frame.
  5. Put all your tools away and sweep the garage floor, because you are DONE. Oh, yeah. Power up the system to see if you did it right.

    Easy job. By the way, this method works for Fords, too (should you have one laying around).


12 volts, but looks all original

       Okay, this one is a little harder, but still pretty easy. Even a Marine can do it. This method assumes you have either the original wiring harness, or you have replaced the original harness with a new 6-volt harness. Is your 6-volt harness adequate to convert to 12 volts? Yes, because the 6-volt system produces twice the amperage of the 12-volt system. So, the 6-volt harness is actually heavier than needed for the 12-volt conversion. With that out of the way:


12 volts, but with an alternator

      This one isn't quite so easy. I tried this the first time around and it took a few tries to get it right. At any rate, this approach is pretty widely used and has a lot of advantages, but you lose originality -- you have a big, ugly alternator sticking out like a sore thumb. Yuck. If you can live with that, here's what you do:       *  Check with your parts dealer for the correct part numbers for these items. I went back and checked my records and I actually bought my voltage reducers from Jim Carter's Antique Truck Parts, 1508 East Alton, Independence MO (816) 833-1913. Specifically, they were:

      The part numbers may have changed since 1995 when I bought mine.

Alternator Installation

      One low-buck way to install an alternator on a stovebolt six is to use All-Thread, six nuts, & six lockwashers. Using the original generator bracket, cut the All-Thread to size so that the length is a little longer than the bracket. Use a nut and washer on either side of outer bracket; a nut and washer on each side of the alternator; and a nut and washer on each side of the inner bracket. Using All-Thread also allows you to adjust the alternator to line up with the harmonic balancer and the water pump pulleys. It also acts like a shaft on which the alternator can rotate to be adjusted to tension the fan belt. Make sure that the All-Thread, nuts, and washers are the same size as the alternator and generator bracket holes. If you would like a ready made alternator adapter one can be ordered from the fine folks at Buffalo Enterprises.        

v. November 05

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