Id check the LV primary winding between the small + and - terminals which may well (Cant guarantee perfect or for every coil ever made mind you) indicate if its a 6 or 12 volt coil, see below:
If youre talking typical old circa 40's through 60's stock farm tractor ignition coils (NOT talkin after market or high voltage or performance or electronic ignition coils or many Mallroy or Accel mind you) MANY (NOT all) coils:
1) If labeled "6 volt" ORRRRRRR "12 volts for use with ballast resistor" ORRRRRRRR "12 volts requires balalst resistor" had a LV primary coil winding resistance in the neighborhood of: 1.25 to 2 ohms, many around 1.5. They were designed to operate at 6 volts "nominal" and could still produce a spark if starting dropped voltage down to 4.5 and if at 7+ volts while charging still spark and not overheat.....
NOTE if on a 12 volt tractor you can use EITHER a 6 volt coil PLUS external series voltage dropping (12 to 6) ballast resistor ORRRRRR a "12 volt" coil no ballast required. That way many (NOT all) old tractors, 6 or 12 volt, still used the same 6 volt coil, its just that if on a 12 volt tractor they used the voltage dropping (12 to 6) ballast resistor between/after Ign switch and before coil
2) If labeled "12 volts" ORRRRRRR "12 volts NOT for use with ballast resistor" ORRRRR "12 volts NO ballast required" had a LV primary coil winding resistance in the neighborhood of 2.5 to under 4 or so ohms, many around 3 ohms. They were designed to operate at 12 volts "nominal" and could still produce a spark if starting dropped voltage down to 9 and if at 14 volts while charging still spark and not overheat.....
SO GET OUT YOUR OHM METER AND GO TO WORK MAY IDENTIFY IF ITS A 6 OR 12 VOLT COIL
NOTE FYI HERES SOME EXTRA CREDT INFORMATION
Contrary to what many lay persons believe who call them coils that have an "internal ResisTOR" YOU WILL NOTTTTTTTTTTTT FIND A DISCRETE STAND ALONE RESIS"TOR" TUCKED AWAY SOMEWHERE INSIDE THE CAN. The way a "12 volt" coil achieves its necessary 3 or so ohms of LV primary resistANCE is by enough wire coil length (more wire or more coil turns) orrrrrrrrr using wire with certain sufficient resistANCE per unit length so the resistANCE end to end (+ to -) ends up in the range of 2.5 to 4 or so ohms so the points dont burn up and the coil doesnt overheat and handles the current when 12 volts is applied.
SORRY CHARLIE, DONT X RAY OR DISECT ONE AND EXPECT TO FIND A STAND ALONE "RESISTOR" HIDDEN INSIDE SOMEWHERE!!!!!!!!!!!!!!!!!
NOTE they actually did make some very early automotive coils that DID HAVE a stand alone discrete ResisTOR in a seperate part of the can!! Many had like a ring where the internal can portions were seperated, coil in one end resisTOR in other end all by itslef!! HOWEVER in alllllll my years as a used tractor dealer (older tractors) I never saw a coil with such an internal resisTOR inside the can!!!!!!!!
NOTE: For those who arent familiar with electronics terminology and definitions:
Wire and the coils and turns of wire used in the coil, not being a "perfect" conductor, sure, has some (but very very low) inherent ResistANCE, BUT WIRE IS NOT CALLED A RESISTOR, ITS CALLED WIRE which of course contains some small degree of ResistANCE. If you go to Radio Shack and ask for "wire" they show you wire NOT ResisTORS. lIKEWISE, if you ask for a ResisTOR, they will show you resisTORS NOTTTTTTTTTT wire lol Sooooooo when I say theres no stand alone discrete "ResisTOR" hidden away somewhere inside the coil can, you know what I mean and what standard definitions of WIRE and RESISTORS are, theres WIRE inside there but NO stand alone discrete RESISTOR
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