resisters and coils

redgems · Mar 20, 2011

Can someone please explain why in converting a smta and 400 from 6volt to twelve volt we need resisters. I converted coil,lights,geny,starter. 12 volt coil without resister will eventually get hot and tractor begins to flutter and run rough.I purchased from oem. a 12volt coil that looks like original they say has resister built in and still had to put resister due to getting hot and fluttering. T.I.A. Tony

Janicholson · Mar 20, 2011

The coils you have been sold are not 12 volt. There are 6 volt coils and these always need ballast resistors when used on 12 volts.
There are 12 volt coils for use with external resistors, (truly a six volt coil in every functional way)
and 12 volt coils for use without external resistor.
NAPA has all three types. Also make sure your charging systems are at about 14.4 to 14.8 volts. If a lot more, the coils will heat, and battery life reduced. jim

pete 23 · Mar 20, 2011

I will just add that IH used to mark their coils as 12 volt or 12 volt R, with the R meaning to be used with a resistor. This confused a lot of people as IH used a lot of R suffix's in their part numbering system so some though it was just part of the number. Of course it is easy to tell with an ohmeter if it is 12 volt or not. Most 6 volts will run about 1.5 ohms on primary and 12 volt will be about double that . I remember when IH switched to using a 6 volt coil on the 450 diesel tractor and no resistor so as to give a hotter spark for starting and as it only ran a short time on gasoline and then was shut off on diesel it was not a problem. However, so many of those manifold switchs failed and were bypassed and then the key had to be shut off, so guess what, lot of coils and points had a short life.

504 · Mar 20, 2011

Most points can not take 12V. Somebody will post but I am thinking you only need 4.8V or the coil will over heat. Even Acel high performance coils from the 70s used resistors.

ScottyHOMeY · Mar 20, 2011

Battery ignitions of the era of our tractors run on six volts.

I fthe rest of your electrical system is 12V, you have two choices.

One is an inline resitor that effectively reduces the input to a coil designed for a 6V input to the 6v it will take.

The alternative is to have a coil that is wound in a way to take a 12V input, but put out the same final voltage as the 6V coil. It's not a matter of having an internal resistor to get to that point -- its internal windings are desigend differently to accomplish the same end.

Those coils that take a direct 12V input are clearly marked as such.

resisters and coils

redgems

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Janicholson

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pete 23

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504

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ScottyHOMeY

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