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Old 09-10-2012, 03:20 PM   #1
cannondalerob
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Default Using a 12 volt coil

Please help verify an old wives tale told to me by a friend. I was told that you can use a 12 volt coil on a 6 volt system (flathead), if you wire it backwards. Is this true?
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Old 09-10-2012, 03:53 PM   #2
bige386
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Default Re: Using a 12 volt coil

I just went though the coil thing and I don't believe there is any truth to it.
Just my 2 cents.
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Old 09-10-2012, 04:10 PM   #3
feelsharp
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Default Re: Using a 12 volt coil

No, if you wire it backwards you have reversed the coil polarity, it will over heat and fail. If you are actually using a 6 volt battery and the coil is marked requires external resistance, you will be ok. A 12 volt coil that requires external resistance is in essence a 6 volt coil. It will have around 1.5 ohm resistance. If it is used with a 6 volt battery you end up with the correct amps. With a 12 volt battery the coil must have added resistance totaling around 3.0 ohm to keep the amp load on the point around 3 amps. That resistance will either be internal with no external resistance required, or added by running your hot lead thru a chrysler resister.
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Old 09-10-2012, 04:40 PM   #4
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Default Re: Using a 12 volt coil

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Originally Posted by feelsharp View Post
No, if you wire it backwards you have reversed the coil polarity, it will over heat and fail. If you are actually using a 6 volt battery and the coil is marked requires external resistance, you will be ok. A 12 volt coil that requires external resistance is in essence a 6 volt coil. It will have around 1.5 ohm resistance. If it is used with a 6 volt battery you end up with the correct amps. With a 12 volt battery the coil must have added resistance totaling around 3.0 ohm to keep the amp load on the point around 3 amps. That resistance will either be internal with no external resistance required, or added by running your hot lead thru a chrysler resister.
CORRECT !

Another issue is that a coil wired with the incorrect polarity looses approx 80% of ots output. Many times over the years coils hit the street wired backwrds the result was the vehicles need a tune up very often. Many service bulitens were written saying how to handle this problem and test etc. AMC was one i remember . The fix was to trow the coil away once driven incorrect polarity the coil cant be ran the other direction....
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Old 09-10-2012, 04:52 PM   #5
cannondalerob
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Default Re: Using a 12 volt coil

So, If I have a 12 volt coil, that is marked, use with external resistor, I can run it on a 6 volt system, wired for correct polarrity?
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Old 09-10-2012, 05:52 PM   #6
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Default Re: Using a 12 volt coil

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You use only a 6 volt coil with 6 volt battery,and 12 volt coil with 12 volts.
They are not the same.If your coil says pos+ goes to pos,neg goes to neg.
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Old 09-10-2012, 06:14 PM   #7
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Default Re: Using a 12 volt coil

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Originally Posted by feelsharp View Post
No, if you wire it backwards you have reversed the coil polarity, it will over heat and fail. If you are actually using a 6 volt battery and the coil is marked requires external resistance, you will be ok. A 12 volt coil that requires external resistance is in essence a 6 volt coil. It will have around 1.5 ohm resistance. If it is used with a 6 volt battery you end up with the correct amps. With a 12 volt battery the coil must have added resistance totaling around 3.0 ohm to keep the amp load on the point around 3 amps. That resistance will either be internal with no external resistance required, or added by running your hot lead thru a chrysler resister.
So to clarify. I can use a 12 volt coil on a 6 volt system if it is marked "requires external resistor"? Would that be wired like a standard coil, or backwards?
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Old 09-10-2012, 06:19 PM   #8
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Default Re: Using a 12 volt coil

Most of the new 6volt coils are labeled 12 v. Just wire it like normal positive to the points.
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Old 09-10-2012, 06:53 PM   #9
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Default Re: Using a 12 volt coil

If you are so sure go do it.
If a 12 volt coil had a resistence of 4.5ohms and a 1.5 ohm resister that would be 6 ohms.6 volts didved by 6 ohms would be 1 amps.
A 6 volt coil needs about 3 amps.
No don,t be cheap and buy a 6 volt coil.
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Old 09-10-2012, 07:02 PM   #10
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Default Re: Using a 12 volt coil

George/Maine, where are you finding specific 6 volt coils?
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Old 09-10-2012, 07:12 PM   #11
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Default Re: Using a 12 volt coil

They sell at collector parts stores,But I would buy from Epay new old stock
there are plenty around.
Pre 49 Ford coils best have rebuilt. and they need a .5 ohm resister.
All other cars and trucks use 6 volt coil only.
Most 12 volt colls need about 2 amps and are about 9 volts,3 volts across 1.5 ohm resister. Pos gound ,pos goes to points.
neg ground goes to points.
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Old 09-10-2012, 07:15 PM   #12
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Default Re: Using a 12 volt coil

what collector parts stores are you refering to? Macs, Snyders, and Brattons all sell 12v coils
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Old 09-10-2012, 07:22 PM   #13
George/Maine
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Default Re: Using a 12 volt coil

Macs has 6 volt A12000 $20.
I would buy NOS US made.
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Old 09-10-2012, 07:33 PM   #14
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Default Re: Using a 12 volt coil

those coils are 12v, they have a sticker that says 6v and when you peel it off it says 12v underneath. i have them in stock at my shop.
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Old 09-10-2012, 07:33 PM   #15
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Default Re: Using a 12 volt coil

Im refering to the MAcs ones
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Old 09-10-2012, 07:36 PM   #16
George/Maine
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Default Re: Using a 12 volt coil

I think maybe now you under stand why NOS is best.
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Old 09-10-2012, 07:39 PM   #17
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Default Re: Using a 12 volt coil

the only problem with NOS is availability and my customers would rather pay the $20 compared to the NOS probaby $40+
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Old 09-10-2012, 07:46 PM   #18
George/Maine
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Default Re: Using a 12 volt coil

I,m not in business and try to fix it the first time.
I'm using a 12 volt coil that is about 40 years old.NOS.
6 and 12 are not the same.
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Old 09-10-2012, 08:09 PM   #19
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Default Re: Using a 12 volt coil

ive never had a problem with the ones sold. generally the biggest problem ive had is people wiring them backwards.
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Old 09-10-2012, 08:19 PM   #20
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Default Re: Using a 12 volt coil

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Originally Posted by cannondalerob View Post
So, If I have a 12 volt coil, that is marked, use with external resistor, I can run it on a 6 volt system, wired for correct polarrity?

Yep we do it all the time. Just remember once wired and ran with a given polarity you cant go the other way..
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