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Old 08-19-2019, 06:41 PM   #1
JohnB15632
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Default 12 VDC Wiring

What changes are required to the attached drawing for using a 12 VDC Negative ground vs the 6 VDC positive ground?

Does a similar drawing exist for the 12 VDC Neg Ground?

thanks,
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Old 08-19-2019, 07:07 PM   #2
Purdy Swoft
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Default Re: 12 VDC Wiring

Swap the wires to negative on the coil , ammeter and the battery . This will work if you install a twelve volt battery connected with negative ground . I run some of my model A's on twelve volt positive ground with all new original style wiring harnesses from Brattons and no changes to the wiring is needed .
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Old 08-19-2019, 09:58 PM   #3
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Default Re: 12 VDC Wiring

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Originally Posted by Purdy Swoft View Post
Swap the wires to negative on the coil , ammeter and the battery . This will work if you install a twelve volt battery connected with negative ground . I run some of my model A's on twelve volt positive ground with all new original style wiring harnesses from Brattons and no changes to the wiring is needed .
Wouldn't you need to change the coil and globes to 12 v versions and drop 12 volts to 6 for the horn? The horn will work on 12 volts but it sounds more like a scream, or was that my wife?
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Old 08-19-2019, 10:50 PM   #4
Purdy Swoft
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Default Re: 12 VDC Wiring

Usually a one point five OHM coil is used with six volts . A one point five OHM coil can be used on twelve volts if a ballast resister is used . I use a three OHM coil on my twelve volt converted model A's . A three OHM coil will run on twelve volts without the need of a resister for a cleaner more original look . To save typing . I figure that it should be mostly common sense that twelve volt bulbs would be necessary for twelve volt use . A resistor will be needed for a six volt horn that is used on twelve volts , same for six volt windshield wiper motors or any other six volt accessories that are used on twelve volts . Besides the replacement of the bulbs , the six volt battery will need to be changed to a twelve volt battery . I figure that most would know this but I will add the battery for those that wouldn't . John B asked what wiring changes to the wiring drawing would be needed . I listed the wiring changes to run twelve volts If fnegative ground . No wiring changes are needed to run twelve volt positive ground . No harm meant . I figure that anybody that didn't know that the bulbs would need to be changed to twelve volts when making a twelve volt conversion would probably never be able to make a twelve volt conversion , no matter what .
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Old 08-20-2019, 11:33 AM   #5
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Default Re: 12 VDC Wiring

Purdy Swoft covered most of the things that need to be changed to run on 12 Volts positive or negative ground. Would recommend that you change the Bendix on the starter to the newer type as the 6V starter motor really spins on 12 Volts. If you would like a list of the bulbs for 12V or 6V and other electrical info send me an email and will send it to you. My email is jrelliott42@gmail.com
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Old 08-20-2019, 08:21 PM   #6
Purdy Swoft
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Default Re: 12 VDC Wiring

Thanks jr . I did use the modern starter drive and forgot to mention it . The modern starter drive wouldn't be a bad idea even with six volts ..
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Old 08-20-2019, 10:17 PM   #7
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Default Re: 12 VDC Wiring

What changes need to be done to the starter if converting to negative ground ? Seems like the starter will rotate in the wrong direction if not modified. Would the field contacts just need to be reversed ?
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Old 08-20-2019, 10:35 PM   #8
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Default Re: 12 VDC Wiring

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What changes need to be done to the starter if converting to negative ground ? Seems like the starter will rotate in the wrong direction if not modified. Would the field contacts just need to be reversed ?
Purdy is right about the coil and I have no problem with anything else he said.
The starter will still turn in the same direction so there is no need to fiddle in there unless you want to change the field windings to all in series for 12 volts. Many people leave the starter unaltered although I switch the windings around jus to make life easier on the bendix.
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Old 08-21-2019, 11:26 AM   #9
Purdy Swoft
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Default Re: 12 VDC Wiring

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Originally Posted by Mulletwagon View Post
What changes need to be done to the starter if converting to negative ground ? Seems like the starter will rotate in the wrong direction if not modified. Would the field contacts just need to be reversed ?
Converting to negative ground will not require any modifications to the starter . Ground has no effect on direction of rotation . Some change the field coils . I do use the modern starter drive but have had no problems with the original setup .
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Old 08-21-2019, 11:49 AM   #10
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Default Re: 12 VDC Wiring

Install a Bosch 12volt blue coil, built in resistor
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Old 08-21-2019, 12:01 PM   #11
Purdy Swoft
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Default Re: 12 VDC Wiring

I use a Pertronix flame thrower coil on my twelve volt converted roadster . I use the three OHM version that needs no resister.
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Old 08-23-2019, 10:36 AM   #12
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Default Re: 12 VDC Wiring

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Originally Posted by Purdy Swoft View Post
Thanks jr . I did use the modern starter drive and forgot to mention it . The modern starter drive wouldn't be a bad idea even with six volts ..

Hey Purdy, which modern starter drive do you run/suggest? I'm running 12V on an original 6V starter and my bendix jingles a bit occasionally .
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Old 08-23-2019, 11:03 AM   #13
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Default Re: 12 VDC Wiring

Here's a 12 volt neg ground schematic off the net. Some discussion in other threads about pros/cons about relocating the condenser to the firewall. Make sure you have a polarity check for the coil terminal wires. I install the coils with the terminals up.
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Old 08-23-2019, 11:19 AM   #14
Purdy Swoft
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Default Re: 12 VDC Wiring

Bratton offers a made in the USA modern starter drive , part number 15961 price $36.95 , Snyders also offers the modern starter drive on page A-154 . This part can probably be purchased locally if you know what modern car it was used on .
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