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6 to 12 volt

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  • Electrical: 6 to 12 volt

    I'm sure this has been hashed and rehashed... I'm sorry I couldn't find old posts that really answered it.

    The Speedster is coming up... shell and frame first. Every wire, blower motors etc., every bulb need to be replaced. So why not go 12 Volt?

    I have several alternators on the shelf that should still be good. A couple of Runtz resistors for the gauges, a resistor for the wiper motor, a later model ignition from a 12 volt... comments seem to say leave the starter alone... seems to simple! what am I missing?

  • #2
    If your question is only about the long as you don't crank it long enough to burn it and, since it will be turning a bit quicker under twelve volt that should not be a problem.
    Why not get Randy Rundle's 12-volt conversion guide. No questions unanswered.
    "All attempts to 'rise above the issue' are simply an excuse to avoid it profitably." --Dick Gregory

    Brad Johnson, SDC since 1975, ASC since 1990
    Pine Grove Mills, Pa.
    '33 Rockne 10,
    '51 Commander Starlight,
    '53 Commander Starlight "Désirée",
    '56 Sky Hawk


    • #3
      Will the starter turn the correct direction when converting from 6 volt positive ground to 12 volt negative ground? Or does some rewiring of the starter need to be done?
      Gary Ash
      Dartmouth, Mass.

      '32 Indy car replica (in progress)
      ’41 Commander Land Cruiser
      '48 M5
      '65 Wagonaire Commander
      '63 Wagonaire Standard
      web site at


      • #4
        If I'm not mistaken, all starters are designed to rotate in one direction regardless of the polarity. Motors with permanent magnets typically care about polarity...
        1948 M15A-20 Flatbed Truck Rescue
        See rescue progress here on this blog:


        • #5
          The starter will turn the correct direction, regardless of polarity. My'55 is proof.


          • #6
            Don't use a resistor for the wiper motor. Use a twelve-volt motor from a Hawk model. You will probably need a new wiper switch, too, because the two-speed feature may have been wired differently. You might get away with a resistor on a blower motor, because the load on blower motors is pretty constant. The work required of a wiper motor varies a lot with the amount of rain, and motor speed, etc. You could wind up stalling the motor and burning it out. The Runtz devices aren't resistors; they are actually small voltage regulators.
            Gord Richmond, within Weasel range of the Alberta Badlands


            • #7
              The starter will rotate correctly because both the fields and armature get their polarity reversed, so it spins the same way. If you reversed only the field or only the armature then it would spin backwards. The only way to make that happen is to rewire it internally.
              1947 M5 under restoration
              a bunch of non-Stude stuff


              • #8
                To: mmagic,----This is a conversion I accomplished on a lemon/lime Speedster which I owned several years ago, and on a Friend's Speedster last year. If You intend to drive Your Speedster much, converting
                to 12V really is a good move.


                • #9
                  Originally posted by gordr View Post
                  Don't use a resistor for the wiper motor.
                  Here's the resisters I have on Frosty's wiper motor. I have driven him in the rain several times and have not noticed any problems.
                  Attached Files
                  Jerry Forrester
                  Forrester's Chrome
                  Douglasville, Georgia

                  See all of Buttercup's pictures at


                  • #10
                    Well, Jerry, you probably don't have snow in your part of the country, or don't drive in it with Frosty. But if your wiper motor needs 15 amps at six volts, that resistor network has to dissipate 90 watts of heat in order to drop the voltage to the correct six volts for the motor.
                    Given that the wiper motor is so easy to change, and that the Hawk ones are readily available, I figure that's the best way to do it. With a blower motor, a dropping resistor might be fine, because you could stick the resistor into the air stream from the blower, and actually make use of the added heat.
                    Gord Richmond, within Weasel range of the Alberta Badlands