12v conversion coil question

SolaceGardens · Oct 4, 2016

Hello! Many posts about the 12v's (thanks Dell and others!!), but this question I couldn't quite find an answer to and I don't want to burn out another coil.

I have a 1948(ish) Ford 8N and I got a new 12v coil, measuring 1.4 ohms resistance. I measured the ballast resistor and it's 0.7 ohms cold, as expected. The ceramic resistor that came with the conversion kit measured 4 ohms though. So, if my math is right, using the 4 ohm ceramic resistor will drop the voltage down too much to spark reliably. I think (though I may be wrong) that I need about a 1.6 or 1.8 ohm resistor, which is what I kind of gleaned from other experts on the forums. I'm new to tractoring, so apologies in advance for my newbie questions and lack of knowledge.

With 14.5 v on alternator, and 2.4 ohm currently, i get about 6 A to the coil, which is too much. I think I need to get to about 4 ohms, so a 1.6 or 1.8 ohm resistor should do it. I found, in another forum, this resistor:

ICR13 NAPA 1.8 ohm resistor

My question is, does the above make sense and should this be the kind of resistor I'm looking for? Thanks in advance!!!

Bruce (VA) · Oct 4, 2016

First off, that's not a 12v coil. That's problem #1. A new front coil will have at least 2 ohms of resistance, most get 2.25.

Next, if you use that coil, do NOT add a 1.8 ohm resistor.

Do the math.

That gets you 2.9 amps running current (not bad) but only 3.5 amps starting current (not good)

With a 1.4 ohm coil, in a perfect world, you would only need a .5 ohm additional resistor to get 4 amps running current & 5.6 amps starting current.

But, for the problems of adding a resistor compared to the benefits, I'd leave it alone. (see tip # 30)

Or find a coil w/ more internal resistance.

NOTE:

You meter might be off; the OEM ballast resistor should read .3 ohms cold & 1.7 ohms hot.
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12v conversion coil question

SolaceGardens

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Bruce (VA)

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