You say the coil has a resistance between 1 and 2 ohms (sounds like its a 6 volt coil)
IF THAT MEANS ITS 1.5 OHMS ????? in order to achieve a 50 50 voltage divider so the ballast drops 6 volts leaving 6 volts for a 6 volt coil,,,,,,,,,, THE BALLAST NEEDS TO BE THE SAME RESISTANCE AS THE COIL
Many typical old farm tractor 6 volt coils have around 1.2 to under 2 ohms (1.5 typical) LV Primary resistance, so if used on a 12 volt tractor the ballast would also be 1.2 to under 2 ohms.
In the alternative, its not good to switch over 4 amps of current using mechanical ignition points, and since I = E/R, use 4 = 12/R which means the TOTAL of coil PLUS ballast resistance needs to be at least 3 ohms..........IE If the coil is 1.5 then the ballast needs to be 1.5 as 1.5 + 1.5 = 3 ohms and that combination would draw 4 amps.
NOTE under charge the battery voltage may rise to 13+ or so !!!!!!!!!! The figures above are an illustration ONLY
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