Hi I ex neighbor, good question "How to tell if coil needs a ballast?' here's the answer
If its a 6 volt tractor the coils LV primary resistance as measured between the two small terminals should be somewhere around 1+ to 2 ohms, many around 1.5 ohms, and IF SO NO BALLAST IS REQUIRED thats a coil for a 6 volt tractor AND YOU DONT NEED ANY EXTRA BALLAST on a 6 volt tractor.
If its on a 12 volt tractor, the TOTAL primary resistance (thats coil + any ballast) needs to be around 2.5 to 4 ohms THEREFORE
a) If the coil has ONLY 1 to 2 ohms resistance (like a 6 volt coil does) THEN IT NEEDS A BALLAST otherwise it will overheat
b) If the coil has 2.5 to 4 ohms resistance THEN NO BALLAST IS REQUIRED
Typical 6 volt coils are around 1+ to 2 ohms (many 1.5) while typical 12 volt coils are more like 2.5 to 4 ohms (many 3). If you use a 6 volt coil on a 12 volt tractor THEN YOU NEED TO ADD BALLAST or else it will overheat.
To measure resistance place an ohm meter on the low ohms scale across the two small coil terminals thats NOT the big high voltage terminal tower
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