Secondary resistor with 6v coil

CJarvis

Member
Since my 12v coil went bad after 12 mo., I decided to take a different tack and go back to a 6v coil with a secondary resistor inline.

I picked up this resistor and spliced it in. https://smile.amazon.com/gp/product/B01CV607H6/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

The resistor gives me a combined resistance (coil+resistor) of 3.7 - 3.8 ohms.

So far, it's working like a charm, and the tractor has never started so easily.

I'll keep you posted on how it works out.
 
How did you calculate 3.7 to 3.8 ohms?

That new resistor is 2.5 ohms.

Your oem ballast resistor is 1.7 ohms hot.

Your 6v coil is probably 1 ohm unless it's really old.

That's 5.2 ohms.
75 Tips
 

The resistor was 2.5 - 2.6 ohms, as measured with my Klein MM1000. Coil was 1.1 ohms. I did not include the OEM ballast resistor.
 
I should add, that the secondary resistor was added downstream of the OEM ballast resistor. I knew better than to omit that one. Been hanging around here too long... :wink:
 
You've got 5.2 ohms resistance in the ignition circuit meaning you have starting current of around 3.6 amps and running current around 2.8 amps. Your coil will last forever......but you may experience a weak spark as a result.

I fully agree w/ the importance of an external resistor on a 12v conversion and a 6v coil; you are indeed keeping the heat out of the coil.

But most folks who are aware of that (tip # 30) will calculate the resistance accordingly. In your case, I'd use a 1 ohm external resistor. That gets you a starting current of 6 amps and running current at 3.8 amps.


But that's just me ??.. ;)
75 Tips
 

Bruce, I must be missing something. The 3.7 ohm resistance plus the 1.7 ohm HOT resistance of the ballast resistor = 5.4 ohms.
That seems about right.

Admittedly, electron theory makes me a bit cross-eyed, so help a fella out. :)
 
5.2 or 5.4 is still too much resistance.
It's the current that counts. You need
higher current to start. That's why the oem
ballast resistor is .3 cold. As long as
running current is below 4 amps, you will
have a strong spark and the coil won't
overheat. In your case, at 5.2 or 5.4
ohms, your coil will last forever but at
the cost of a weak spark. My thinking is
that you have 1 ohm more resistance than
you need.
75 Tips
 

Quick update: I picked up a 1.5 ohm resistor and installed it, but I haven't had time to do more than install it and let the tractor idle for a while. I'll post another update when I get a chance to give it a good workout.
 
(quoted from post at 08:11:04 08/06/18)
Quick update: I picked up a 1.5 ohm resistor and installed it, but I haven't had time to do more than install it and let the tractor idle for a while. I'll post another update when I get a chance to give it a good workout.

1.1 ohms on the coil, 1.7 hot on the ballast resistor and 1.5 ohms on the new resistor gets you 4.3 ohms.

14.5 volts / 4.3 ohms gets you 3.37 amps.

Which is below 4 amps.

All is well.
:D
 

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