OOPS MY BAD, I thought I covered it all below but I forgot to mention something else which may help....
Yet another reason why typical old farm tractor coils have the LV primary winding resistances they have (6 volt around 1.2 to 2 ohms, 12 volt 2.5 to 4 ohms) IS TO LIMIT THE AMOUNT OF CURRENT THE POINTS CARRY AND SWITCH SO THEY DONT BURN PREMATURELY !!!
And that max current is typically somewhere around 4 amps
THEREFORE in a 6 volt system the total LV primary resistance (coil + any ballast) is 1.5 ohms as 6/1.5 = 4 Amps in a 12 volt system the total LV primary resistance (coil + any ballast) is 3 ohms as 12/3 = 4 Amps
Again a typical old farm tractor coil DOES NOT HAVE A DISCRETE STAND ALONE RESISTOR TUCHED AWAY AND HIDDEN SOMEWHERE INSIDE THE CAN the LV primary winding resistance is a function of the wire used, its resistance per unit length, and the length.
On a 12 volt tractor if the coil plus any ballast used doesn't have around 3 ohms total resistance the coil will overheat and the points burn prematurely. Of course extreme high charging voltage can also cause overheating as can leaving the ignition on when not running
YOU HAVE TO INSURE YOurCOIL (since you have no ballast) has at least 2.5 to 4 ohms LV primary winding resistance or it can overheat. If it has more like 1.2 to 2 ohms IT NEEDS AN EXTERNAL BALLAST RESISTOR
I think I covered most of it now lol post back any questions
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