8N 12v system

I have a front mount distributer and after 10-15 hours i loose spark. 12v coil is very hot so I believe it is burn out. Reading I think I need to add a additinal ballast resistor. Point and condensors are replaced even though not burnt. new rotor Dist cap look good.

Should OI add a additional resistor and at what ohm resistance

Check out tip # 30 at the link.

All you need is a 12v coil & the OEM ballast resistor as in the picture. Nothing else.

75 Tips

if you indeed have a 2.5 ohm coil, then that ballast resistor is what you need.

Bruce,

Some years ago before I installed EI on my 12 volt, front-mount 8N, I installed the resistor you showed in the picture in series with the ceramic resistor. I'm sure I did it because of a recommendation on this YT discussion; but I can't recall now why..., can you help refresh my failing memory? It ran great and points lasted a long time, and I never replaced the coil that came on it many years before.

Thanks,
Jim

Technology & materials being what they were in the 30's, that square coil would melt if it ran on much more than 4 amps for any length of time. (see tip # 38 for an example). In order to get a hot spark at the same time the starter was drawing max current from the battery, a ballast resistor was added in the ignition circuit. What that did was add about .3 ohms of resistance in the circuit, added to the 1.5 ohms of the coil. That got you 3.5 amps or so at start up. As the voltage increased when the engine was running to about 7.5 volts, the resistor heated up, adding more resistance in the circuit. 1.0 ohms hot, plus 1.5 ohms of the coil got you down to 3 amps or so to keep from melting the coil. The same rule (actually, Ohm's Law) applies to a 12v circuit. I= E/R. Current equals voltage divided by resistance.

It used to be before the "Land of Almost Right" started making coils that you could count on a 12v frontmount coil as having 3 ohms of internal resistance & the 6v coils as having 1.5 ohms or less. Thus, thanks to Ohm's Law, you could calculate what additional resistance you needed in the circuit to limit coil current to 3.5 amps.

So, it's very common to see a 6v front coil w/ an additional ceramic resistor. And, some "12 volt" conversion kits come w/ both a 12v coil & a ceramic resistor, and have instructions that say to keep the ballast resistor too. So, what happens is that the tractor runs fine until the battery gets weak. Then, that unnecessary resistor adds too much resistance in the circuit & the tractor won't start.

Bottom line......2.5 ohm coil & the OEM ballast resistor.
75 Tips