Using a 12 volt coil

[cannondalerob]

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?

[bige386]

I just went though the coil thing and I don't believe there is any truth to it.
Just my 2 cents.

[feelsharp]

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.

[BUBBAS IGNITION]

Originally Posted by feelsharp

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....

[cannondalerob]

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?

[George/Maine]

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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.

[cannondalerob]

Originally Posted by feelsharp

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?

[jkeesey]

Most of the new 6volt coils are labeled 12 v. Just wire it like normal positive to the points.

[George/Maine]

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.

[jkeesey]

George/Maine, where are you finding specific 6 volt coils?

[George/Maine]

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.

[jkeesey]

what collector parts stores are you refering to? Macs, Snyders, and Brattons all sell 12v coils

[George/Maine]

Macs has 6 volt A12000 $20.
I would buy NOS US made.

[jkeesey]

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.

[jkeesey]

Im refering to the MAcs ones

[George/Maine]

I think maybe now you under stand why NOS is best.

[jkeesey]

the only problem with NOS is availability and my customers would rather pay the $20 compared to the NOS probaby $40+

[George/Maine]

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.

[jkeesey]

ive never had a problem with the ones sold. generally the biggest problem ive had is people wiring them backwards.

[BUBBAS IGNITION]

Originally Posted by cannondalerob

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..

[jake197000]

with all due respect,why not?

[FlatheadTed]

Since 6 volt coils are hard to come by here we run a 12 volt with the ballast ,external resistance removed ,Seem to run OK .some we add the 2.5 volt Ford resistor ,

[Bruno]

NAPA has a 6 volt coil.