Using a 12 volt coil

cannondalerob

cannondalerob

Junior Member
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?
 
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.
 
feelsharp said:
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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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....:eek:
 
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?
 
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.
 
feelsharp said:
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.
Click to expand...

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?
 
Most of the new 6volt coils are labeled 12 v. Just wire it like normal positive to the points.
 
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.
 
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.
 
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.
 
the only problem with NOS is availability and my customers would rather pay the $20 compared to the NOS probaby $40+
 
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.
 
ive never had a problem with the ones sold. generally the biggest problem ive had is people wiring them backwards.
 
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