ohms for 6v verses 12v coil?

I have a round can coil without external resister from an Allis tractor. Was wanting to figure out if it is 6v or 12v.

I was wondering what the ohms reading should be for a 6 and 12 volt coil.

I checked it and think it was 2.9

(quoted from post at 18:15:38 10/11/12) I have a round can coil without external resister from an Allis tractor. Was wanting to figure out if it is 6v or 12v.

I was wondering what the ohms reading should be for a 6 and 12 volt coil.

I checked it and think it was 2.9

Click to expand...

don't know about the "think" part, but if 2.9 then ok on 12. Typically closer to half that for 6v.

Heres the deal, many BUT NOT ALL typical old circa 40's to 60's tractor (NOT talkin high performance or after market or elec ignition mind you) Kettering Ignition points and condensor type ignition coils designed for operation at 12 volts nominal that will produce a spark when battery voltage is lowered to maybe 10 volts or raised when charging to 14

ARE AROUND 2.5 TO UNDER 4 OHMS (3 Typical)

Many typical old tractor coils designed for use at 6 volts nominal (and all the above but half the values mentioned)

ARE AROUND 1.25 TO UNDER 2 OHMS (1.5 typical)

Many 6 or 12 volt tractors ALL USED THE SAME COIL its just that if used on a 12 volt tractor they added a series voltage dropping (12 to 6) Ballast Resistor which droped 6 volts leaving 6 on a 6 volt designed coil.

Now before anyone has a calf and can find a coil somewhere different from above note: THESE ARE TYPICAL VALUES to which like everything else there are indeed exceptions!!!!!!!!!

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.

I haven"t checked every coil made but the ones I have checked showed a primary reistance of ~1.6 ohms for 6v systems and ~ 3.2 ohms for use on 12v systems. Both of these yield ~4amps max primary current which appears to be a design point for long point and coil life.

The ballast reisitors I have seen have a positive temperature coeficient which means they have a low resisitance at cold temperatures and the resistance increases as the resistor heats up. This gives max current and a higher spark potential for cold starting and then causes the current to reduce for running to increase point and coil life.

Your 2.9 ohm coil appears to be for use on a 12V system.

You raised a good point that I forgot to mention, in the Kettering ignition points and condensor coil ignitions systems used on "many" typical OLD TRACTORS (NOT talkin elec ignition or high energy or after market or more later cars mind you) A MAJOR LIMITING FACTOR WAS TO LIMIT THE CURRENT THE POINTS HAD TO SWITCH TO 4 AMPS AND UNDER SO THE POINTS DIDNT BURN UP PREMATURELY.

THEREFORE 12 volts/3 ohms (coil) = 4 amps
6 volts/1.5 ohms (coil) = 4 amps

Of course, the system can be designed to have coils of different primary resistance values different from the above, Im ONLY talkin typical and general values here (for many tractors not all) for stock old tractor coils in the 40's to 60's vintage.

Good post and fun chat, God Bless yall

John T