12 volt coil conversion

Manitoba · Mar 13, 2009

When I install the new coil, should I leave the OEM resister in line?
Thanks in advance.

Bruce (VA) · Mar 13, 2009

It's a frontmount coil, right? If so, yes, you need the OEM resistor because it is a true ballast resistor. Depending on the resistance of the coil, you may need an additional as well to limit coil current to 3.5 amps. What is the resistance of the coil?
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Dell (WA) · Mar 14, 2009

Manitoba dude.......the new modern 12v squarecan ignition coil NEEDS the "infamous ballast resistor" or else you will MELT yer new $40 12v squarecan ignition coil. Understand? ........Dell, a 12v advocate for the right reasons

NOXJohn · Mar 14, 2009

You may or may not need an external resistor depending on the internal resistance of the coil you are using. I have measured several and they seem to vary substantially. If you have a side mounted coil I would suggest that you go to NAPA of some other good auto parts store and ask for a 12 volt round coil that does not need an external resistor. It should have this written on the outside of the coil. If you have a front mount coil you can buy the same 12 volt round coil but you need to modify an old square front mount coil to adapt the connections to use the round coil. HOBO's web page has instructions on how to do this. I have never seen a square front mount coil that did not need a resistor even if was label for 12 volts. The resistance has always measured low enough that it would allow to much current to flow and burn up the coil in a short time period.

gregy · Mar 14, 2009

I just installed a 12v system, the 12v coil measures 2.7 ohms. Do I need the extra resistor ?

Bruce (VA) · Mar 14, 2009

Front or side distributor? It makes a big difference! If it's a front distributor, the answer is yes.

Ohm's Law, I=E/R

E = 14.5 volts
R = 3.7 ohms (1 ohm ballast resistor, 2.7 ohms coil)

14.5 volts divided by 3.7 ohms gets you 3.9 amps.

You are pushing the envelope; I'd put a 1 ohm resistor in it. That gets you to 3 amps.

The coil was designed to operate at 3 amps (7.5 v & 2.5 ohms total)
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gregy · Mar 14, 2009

It's a front distributor.

gregy · Mar 14, 2009

Front mount distributor. The 12v coil is 2.7 ohms.
The extra resistor that came with the kit is 3.1 ohms. 14.5 divided by 5.8 ohms is 2.5 amps. Too low?

Bruce (VA) · Mar 16, 2009

The coil was designed to start at 4.1 amps (7.5v / 1.8 ohms) As the ballast resistor heated up to 1 ohm, the coil operated at 3 amps (7.5v / 2.5 ohms). But......if you use the spec resistance of the ballast resistor of 1.7 ohms the tractor would be running at 2.3 amps.

In your case, w/ the ballast resistor, you would be starting it at 14.5 v / 3.4 ohms, or 4.2 amps which is fine. As the ballast resistor heats up you will be running at 14.5 v / 2.7 ohms coil, 1 ohm ballast resistor & 3.1 new resistor, or 6.8 ohms....14.5v / 6.8 ohms gets you 2.1 amps. That's close to the "spec" current if you use 1.7 ohms on the ballast resistor instead of the usual 1 ohm.

Try it & see what happens, but I think it's going to be to low. Just get another resistor to replace the one that came w/ the kit.

12 volt coil conversion

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