A good while back I posted this response to a question if ALL coils, 6 or 12 volt, are the same. Due to all the fun discussion below I thought its a good time to copy n paste it here again
First of all, 6 volt and 12 volt coils ARE NOTTTTTTTTTTTTTTT THE SAME what the parts dude told you ("all coils are the same") IS WRONG AS RAIN. If you go to a reputable parts vendor like NAPA they will ask “you want a 6 or 12 volt coil” NOT “Aww shucks, they are all the same”. A TYPICAL 12 volt nominal rated coil has "around" 3 ohms (maybe 2.5 to 4) of primary resistance, while a TYPICAL 6 volt coil has about half that, "around" 1.5 ohms (maybe 1.25 to 2) . THESE VALUES VARY SOMEWHAT BUT THIS GIVES YOU THE BIG PICTURE. A 12 volt nominal rated coil is designed to produce a spark when starting reduces battery voltage to maybe 9 or 10 and also when charging up to 14 or so volts AND STILL HANDLE THE CURRENT AND DISSIPATE THE HEAT. I’m talking typical old STOCK farm tractor coils here NOT after market, high energy such as Accell or Mallory or Pertronix etc. If you put 12 volts to a 6 volt coil with no external series voltage dropping (12 to 6) ballast resistor IT WILL OVERHEAT because its passing more current then it was designed to handle. SO AGAIN ALL COILS ARE NOTTTTTTTTT THE SAME there are 6 volt nominal rated and 12 volt nominal rated. Ive NEVER seen a coil labeled 6 OR 12 volts period, although you can use the 6 on a 12 volt tractor WITH A BALLAST
NOTE the design of the coil points and condenser allllllllllllllll comes into play soooooo it may be possible a SYSTEM was designed that has different then the TYPICAL figures noted above so don’t anyone have a calf if a coil is a bit different then described above. ALSO it takes a decent quality ohm meter to measure such small values and be accurate.
The parts dude may be right in that “many” old farm tractors, 6 OR 12 volt use the same coil HOWEVER if it’s a 12 volt tractor you add the series voltage dropping (12 to 6) ballast resistor which drops 6 leaving 6 on the coil AS IT WAS DESIGNED FOR.
One of the main design limitation parameters in old tractor Kettering points and coil ignition systems is limiting the current the points must switch to the 4 amp and less range, otherwise the points burn up too fast.
In a 6 volt system, the 6 volt coils low voltage primary winding resistance is around 1.5 ohms, meaning the current (the points must switch) is 6/1.5 = 4 amps
In a 12 volt unballasted system, the coils low voltage primary winding resistance is more like 3 ohms, meaning the current (the points must switch) is 12/3 = again 4 amps.
Now, some 12 volt tractors still used a 6 volt (1.5 ohms) coil BUTTTTTTTTTT they added a series 1.5 ohms EXTERNAL BALLAST VOLTAGE DROPPING (12 to 6) RESISTOR which dropped 6 volts leaving 6 on the coil as designed for, so the points still only switch 4 amps and alls well.
NOTE: Coils labeled "12 volts" or "12 volts NOT for use with ballast resistor" are true 12 volt nominal rated coils. HOWEVER a coil labeled "12 volts requires ballast resistor" or a coil labeled "6 volts" ARE 6 VOLT COILS and require the ballast on a 12 volt tractor but not if only a 6 volt tractor.
OLD WIVES TALES: Some mistakenly refer to some 12 volt coils as being "internally ballasted" or that they have a Resistor inside them HOWEVER if you disect that coil, on most, you WILL NOT FIND A DISCRETE STAND ALONE RESISTOR HIDDEN AWAY SOMEWHERE INSIDE THE CAN. The primary winding resistance of around 3 ohms is achieved due to more windings/turns or higher resistance wire NOT A RESISTOR HIDDEN INSIDE THERE. Of course all wire (and coils of wire forming an inductor) has some resistANCE but its still called wire (or a coil if many turns/coils) NOT a resistor.
NOTE they did however make a few early automotive coils that DID have a discrete stand alone Resistor inside the can.
IF YOU HAVE A 12 VOLT TRACTOR AND ARE USING A 6 VOLT COIL AND IT OVERHEATS, YOU MIGHT WANNA CHECK THE BALLAST RESISTOR OHMS VALUE (around 1.5 ohms or a bit more) OR INSURE ITS WIRED RIGHT AND EFFECTIVELY IN THE SERIES CIRCUIT BETWEEN IGN SITCH AND COILS INPUT.
Or else you can toss the ballast and use a full true 12 volt coil (around 3 ohms primary resistance) subject to any starting ballast by pass circuitry some tractors use
CHECK THAT BALLAST OHMS AND ITS WIRING
PS Modern solid state switched electronic ignitions are NOT the same animal, some use more of a higher voltage switched pulse to fire the coil instead of the old tractor mechanical points systems
Nuff said, hope this helps
John T retired Electrical Engineer