6 Volt system with 12 Volt Unballasted coil

I have a 1953 John Deere Model 40 with a 6 Volt electrical system that has a 12 Volt non ballasted coil. I've been told that is correct. It doesn't seem correct to me. Can anyone enlighten me on this?

If the coil has a 1.5 ohm primary coil winding, it would be correct for a 6V system when used without a ballast resistor.


The same coil would be correct for a 12V system if a ballast resistor was connected in series with the same 1.5 ohm coil primary winding.

Sorry I did not add in my first post, if the 12V coil has a 3 ohm primary winding, it would not be correct for a 6V system, but would be correct for a 12V system if installed without a ballast resistor. A good quality digital ohm meter connected across the coil primary winding terminals will tell you what you have. Disconnect all wires from one coil primary terminal befors checking the coil.

If you are using it and dont have any problems quit worrying as it just mite be the correct coil now. If it fails get a new one made for 12v systems.

In simple terms you have 2 types of coils one is a true 12 volt no ballast resister needed and will only work on 12 volts because it is of a higher resistance then the normal 6 volt coil. Then you have a coil that is 6 volts and all coils that are marked ballast resister needed are in fact true 6 volt coils. Yes this is just a simple way of saying things that could be gone into in terms many would not understand unless your an E.T. like I was years ago in the navy

The correct answer is: IT DEPENDS ON THE COIL

1) If the coil is labeled something like "12 volts for use with ballast resistor" or "12 volts requires external ballast" its in reality a 6 volt coil and will work okay on your 6 volt tractor.

2) If the coil is labeled something like "12 volts" or "12 volts NOT for use with ballast resistor" its in reality a 12 volt coil and the spark could possibly be weak.

3) If it starts n runs fine it must be okay.

Many (not all) typical old tractor circa 40's thru 60's 6 volt ignition coils were around 1.25 to under 2 ohms (1.5 typical) primary resistacne while many (not all) 12 volt coils were around 2.5 to under 4 ohms (3 typical) primary resistance. Many tractors still used the same coil on 6 or 12 volts, its just that if on 12 a series voltage dropping (12 to 6) ballast resistor was added in series with the coil.

John T

Six volt systems typically use primary windings in the coil of .6 to 2 ohms.

Twelve volt systems (with breaker points) tend to use the same coils with external resistors giving an additional 1.2 - 1.4 ohms in primary resistance.

Twelve volt systems with NO external resistor tend to use the same total primary resistance built into the coil-can. I.e. .6 - 1.8 ohms in the can plus 1.2 - 1.4 ohms external = 1.8 - 3.2 ohms total (and sometime higher).

12 volt breakerless systems often use "six volt" type coils with no external resistor.

1992 Subaru 12 volt breakerless: 2.4 ohms

NAPA IC-12 Echlin 2.1 ohms primary, 8.9 ohms secondary

NAPA IC-14 Echlin 3.25 ohms primary resistance (no external resistor used) $43.49 2010


A500 12 volt 2.6 ohms primary, 9.8 ohms secondary
no external resistor used.

Echlin 4.2 ohms, 4.82M secondary (no ext resistor)


John Deere 6 volt (as used on 430, 1010, etc.) 2.6 ohms prim, 7.12 secondary. Same coil used on Deere tractors with 12 volt systems with added resistor.

Hot Spark brand - HS06HEC - 12 volt HEI, CDI and electronic ignition systems requiring a coil with 0.6 Ohms primary resistance. 11.6 K ohms secondary. HEI, CDI and electronic ignition systems requiring a coil with 0.6 Ohms primary resistance. Can also be used as a 6-volt coil for many applications.

Mallory 29219, 12 volt coil - primary 1.4 ohms, secondary 9.8K ohms. 58K volts

Mallory 29217 12 volt breaker-point coil, primary 1.4 ohms, secondary 9.8K ohm, 58K volts,

Accel coil for points, 1.4 ohms primary resistance, 9.2 k ohms secondary resistance, 42,000 volts maximum voltage

Ignition resistors

IC23 1.2 ohms mostly Mopar
ICR11 1.35 ohms rated. I checked and it reads 2.8 ohms.