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Positive or negative ground

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  • Positive or negative ground

    just bought a Model a which had the 6v battery grounded to the negative terminal instead of the positive. The car seemed to run Ok, but I switched the polarity to a positive ground, but now there seem to be a short to ground as the 30 amp fuse at the starter blows. What issues may have been caused by the original incorrect grounding? Is there something else, other than a shorted wire, that I should be looking for?
    I should mention that this is my first model A so am not real familiar with it, so any info would be greatly appreciated.
    Last edited by Dreess; 04-16-2018, 01:05 PM.

  • #2
    First, let me WELCOME you to the VFF!
    Second, did you by chance repolarize the generator after turning the battery around? That shouldn't cause the fuse to blow, but I am thinking that the PO had something else added to the car that had a polarity protection circuit that could be blowing the fuse. It may be easier to just turn the battery around and leave it as it was, after all, no one except your hairdresser will know for sure.`
    Other things need to be done as well, like swapping the wires on the ammeter, but that won't cause the fuse to blow, unless one of the wires touches the gas tank. Polarize the cutout.

    All this is speculation as long as we don't know if or how the car was wired when the fuse was added. Additional information is needed to make a better call.
    Last edited by DaWizard; 04-16-2018, 01:29 PM.
    VFF Recruiter

    YOU JUST CAN'T FIX STUPID!!

    There is way too much month left at the end of my retirement check.

    "CENSORED" over there!!
    Not just Censored, BANNED!!

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    • Mitch
      Mitch commented
      Editing a comment
      Welcome Dreess!!!!

  • #3
    As Wiz stated....
    But especially, Welcome to the forum. Congrats on the A. Let the fun begin. Jeff

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    • #4
      Does it have the original generator,or did somebody change it over to an alternator?Has anybody added any accessories,maybe something polarity sensitive?Usually if that is the case,something just won't work,or fries the offending component.

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      • #5
        Thanks for the help...it had been changed over to an alternator. I added the fuse, after I had switched the battery around. So I am not sure if the fuse would blow with the battery connected the way it was when I got it. Will it hurt anything to put it back to where it was with the negative ground and run the car that way? I went through the wiring and could see no obvious issues, but will need to do a deeper dive into it. There are no other accessories on the car and it looks like the wiring colors are correct for each placement

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        • DaWizard
          DaWizard commented
          Editing a comment
          Turn the battery around and hope that you haven't hurt anything in the alternator. That is what would blow the fuse if it is hooked up behind the fuse. Not knowing how you installed the fuse.

      • #6
        If you keep the alternator and if it worked OK with negative ground, then you MUST go back to negative ground.

        Also be sure the fuse is wired correctly. NO starter current goes through the fuse.

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        • #7
          There is a good chance you blew out the alternator diodes when you switched the ground.

          Switch it back to neg. ground (which it has to be if there is an alternator as Tom W stated above). If it still blows the fuse, then disconnect the alternator, tape the ends of the wires, and then try again. If the fuse still blows there will be more trouble shooting needed. Meaning, clip in a 12V lamp in place of the fuse. If there is a short somewhere, the lamp will light. Go thru each circuit, disconnecting and reconnecting one at a time, until the light goes out. When the light goes out, you have found the grounded circuit.
          You are using the short to light the test bulb. Remove the short, the bulb goes out.

          Also, please post a pic of how the fuse is mounted/wired, since that was added at the same time as the problem arose
          Last edited by tbirdtbird; 04-16-2018, 03:44 PM.

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          • #8
            Thanks to all for the input, I will try the various suggestions and let you know. The fuse is connected in series with the yellow wire going from the switch on the starter (battery connection on starter) to the connection box ( the typical fuse block offered by the various suppliers)

            Comment


            • tbirdtbird
              tbirdtbird commented
              Editing a comment
              I should add that for whatever reason, posters seem to find shorts more often than not in the brake light circuit

            • DaWizard
              DaWizard commented
              Editing a comment
              Specially in the '28-'29's

          • #9
            It would also be helpful to add some location as we are world wide and who knows, one of your neighbors could be me and willing to help you figure it out in person with experience.
            VFF Recruiter

            YOU JUST CAN'T FIX STUPID!!

            There is way too much month left at the end of my retirement check.

            "CENSORED" over there!!
            Not just Censored, BANNED!!

            Comment


            • BNCHIEF
              BNCHIEF commented
              Editing a comment
              He is in Michigan Wiz.

          • #10
            3 OR 4X on what others have said. If the battery was connected with negative ground and had been changed from a generator to an alternator, chances are that the alternator was made for negative ground. The battery hookup has to be matched with the alternator type. Hopefully, you did not blow the alternator diodes by connecting the battery and the alternator at cross-purposes. Alternators have a tendency to be kind of touchy when it comes to their diodes.

            Like others have said, I would connect it back for negative ground on the battery and see if it goes back to behaving itself. Take a look at tbirdtbird's post in #7. Keep your fingers crossed. It does sound like your fuse is connected correctly, and it should not blow if all else is working well.
            Last edited by Bill G; 04-16-2018, 05:17 PM.

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            • #11
              Don't let a little wiring bo-bo get you down. If you blew up a few things, no big deal. The nice thing about a Model A is you can easily get replacement everything. Complete replacement wiring harnesses, rebuilt generators, new alternators, etc. My only suggestion here- Always nice to have a fire extinguisher around, class "C" rated. Both in the car and in the garage.

              A few details-
              1) The + and - terminals on a car battery and the cables are actually slightly different sizes. If you were able to easily reverse them there is a problem. I've seen them stretched, bent, over tightened to distort them into fitting. The result is always the same. Damaged battery posts and reduced current carrying capacity when starting.

              2) The ignition coil will work with reversed polarity but with a weaker spark. Whether the coil polarity was corrected when the previous owner went to neg ground or not is unknown and needs to be checked.

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              • #12

                Maybe this wiring diagram will help. 6v alternators are available for both Positive & Negative Ground. Sounds like yours was for Neg Ground.
                Hopefully the fuse blew prior to damaging the alternator diode bridge. You received good advise from others in this thread.
                With the original Generator, there are also new Diode replacements for the original voltage cutout that you will need to observe polarity.
                Other than that, there's not very much on a Model A that is sensitive to Polarity.
                Alternator / Diode Generator Cutout / Coil Polarity (if reversed, you likely wouldn't notice).
                Regarding Coil Polarity, lots of threads on the subject. Don't believe half of what you read regarding what color wire goes on what coil terminal. It's meaningless unless you know how the coil was wound and if it's an original coil designed for a Pos Ground vehicle. Get an inexpensive tester from one of the dealers. Join a local Model A club. Someone there will already have one to loan and there are always members willing to help. Congratulations on your purchase and welcome to the forum.

                Model A Wiring Diagram.jpg

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                • #13
                  If you have an old Delcotron converted to one wire,you might be lucky,they can take a lot of abuse and misuse and survive.A lot of other brands won't tolerate it at all.

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                  • #14
                    You could take that alternator to a local place and have it checked to first of all find out if it is ok so you know where you stand.

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                    • #15
                      Best place to start.

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