6V or 12V coil for 57' 720 - Gas tractor

I'm in the process of refurbishing a 1957 720 GP Gas tractor. This tractor has a 12 volt electrical system, but I noticed the coil I pulled off shows 6V on the side of it. It does have a ballast resister in the wiring harness between the ignition switch and the coil. Am I ok to run the 6V coil, or should I replace it with a 12V coil? All input is welcome. TYIAFYH.

I'd stay with a 6 volt coil.

If you stay with the factory 6 volt coil with a ballast resistor and the factory original ballast by pass when starting system, it can improve cold weather starting. Id stay with the original 6 volt coil if it were mine. If you go to a 12 volt coil and toss the ballast it will still work, but the original system can improve cold weather starts.

THIS ASSUMES IT HAS A 6 VOLT COIL AND BALLAST WITH BY PASS WHILE CRANKING IN THE FIRST PLACE

John T

If your battery is low even in the summer it will start much more easily with the 6 volt coil & bypass system as it left Waterloo. Go to a 12 volt coil and a battery that barely cranks it over will likely not spark it to life!

With it running at 12.6 volts at the coil versus 11 volts at the coil when cranking, the POWER delivered to the 12 volt coil when cranking the engine is reduced by roughly 15%.

With the 6 volt coil that same change of system voltage (cranking at 11 volts) the coil is receiving almost 50% MORE power while cranking than it does when it's running! Lot hotter spark.

These percentages are not necessarily 100% accurate. A lot depends on how big the battery/batteries are, how good the cables & wiring harness, engine compression etc. The whole point is, that 6 volter gets an extra kick in the pants while cranking instead of being ham strung like the 12 volt coil. When cranking, the 6 volt coil gets MORE power from the battery making hotter sparks! When cranking, the 12 volt coil gets LESS than normal power from the battery making weaker sparks!

Hope that helps.

Thanks for responding. It sure is nice to have a resource like this as I've lost my own resources as the farmer I worked for passed on and I've lost touch with the neighborhood folks from that areaa that ran tractors like this.

I've just replaced the old (creatively modified) harness with a new one from Brillman. I hooked it up according to their directions with the help of my JD manuals. I don't recall any mention of a bypass circuit. There's a brown lead from the combo switch "GD" to "+" on the coil, then there is a green wire running from the ballast resister to the "-" side of the coil. At the ballast resister there is a black lead from the "IGN" post on the combo switch to the ballast resister going in and the green lead going out to the coil. I'm thinking I don't have the bypass circuit. The Brillman harness instructions talk about another lead from the starter to the out side of the resister, (same connection as the green lead). Would that be the bypass? I didn't add that lead of the harness kit as there isn't a 2nd wire going to the starter, there's only the large battery feed running to the starter. Without the bypass circuit, will the 6V coil still work as explained in the previous posts?