About all the coils Ive seen that are labeled "for use with external resistor" MEAN JUST WHAT THEY SAY and can overheat if you do NOT do as they say......They are typically 6 volt rated coils, thus on a 12 volt tractor they indeed need the external series voltage dropping (12 to 6) ballast AS THEY SAY.
HOWEVER the size of the ballast needs to be matched to the coil used, so if the ballAst is the wrong size (like maybe that Chrysler) YOU MAY GET A WEAK SPARK
If I were sizing a ballast on a 12 volt tractor in order to use a coil labeled requires a ballast, I would use one that has around the same ohms as the coils LV primary (+ to -) that way the ballast drops 6 volts leaving 6 for a 6 volt (one that requires ballast) coil. Another way to size it is use a voltmeter and with ign on and points closed, you wanna see around 6/7 volts on a coil thats labeled "requires ballast'
Check coil and ballast resistance, they need to be near the same ohms or spark may be weak (or coils too hot if ballast is too small ohms)
A typical old farm tractor 6 volt coil has a LV primary resistance around 1.25 to 2+ ohms,,,,A typical 12 volt is more like 2.5 to under 4 ohms.
The points shouldnt switch much more then 4 amps thus on a 12 volt system 3 ohms is typical ign primary resistance (12/3 = 4 amps) and on a 6 volt 1.5 ohms (6 /1.5 =4)
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