Tractor coils

Can someone explain to me in laymans terms the difference
between a 12v coil no external resistor required that is rated
at 3 ohms vs one rated at 1.5 ohms. Yes I understand 1.5 is
half of 3. Im not sure what it means when one or the other is
on a tractor. Ive been fighting with our 1650 Oliver. Has
electronic ignition. I have the correct wires, and want to make
sure I have the correct coil. Petronix says to use a coil rated
at 3 ohm for 6 cylinder applications unless its a racing
application which the you would use a 1.5 ohm coil. Ive been
fighting this tractor and have several posts on the Oliver
board. Im posting this here because of a little more traffic.
Currently it has a 12v coil on it from Napa which says no
external resistor required. Not sure of the ohm rating of it.

Go with the Flame Thrower coil. That is what I do when installing electronic ignitions.

That is an easy question. They even wrote a song about it. Something like
OHM, OHM ON THE RANGE
Second verse. I NEED the HOMES limit my current to 4 amps.
Third verse. 12 VOTES and 4 hamps, I needs a total of 3 HOMES.
Fourth verse. I don't really care where the HOMES come from, just need a total of 3 HOMES.
Put all 4 verses to music. IS that simple enough???
Ohm even wrote a law about it 12v = 4 amps and 3 ohms

The NAPA coil that requires no resistor is a NAPA IC14SB.

There are others, just different applications. If this is the one you have, it will work. Be sure the part number is actually printed on the can of the coil, not on the box. They are often resold and reboxed, mixed up by unfamiliar counter people. If you have a different number, research what you have, it might be OK. Again, go by the number on the coil, not on the box it came in or any printing on the coil, strictly by the factory number!

To test it, put an ohm meter across the primary terminals, out of circuit (nothing connected to the terminals). Set the meter on the lowest setting, read carefully and be sure the test leads are making good connection without touching them with your fingers. It should read close to the 3 ohms.

Another test, run the engine long enough to get it up to temperature. Check the spark output. From the coil tower to ground it should give minimum 3/4 inch spark, and the coil can should not be noticeably hotter than the surrounding components. If it is too hot to hold, it is the wrong coil.

The Pertronix kits are tough. But they do require the correct coil. Running a 6v coil without a resistor will eventually burn up the electronic trigger and/or the coil. Running a 12v coil with a resistor will not hurt the trigger or burn up the coil, but will have trouble starting and always have a weak substandard spark.

(quoted from post at 18:10:35 10/18/21) Can someone explain to me in laymans terms the difference
between a 12v coil no external resistor required that is rated
at 3 ohms vs one rated at 1.5 ohms.

Click to expand...

I'll try

Coils are designed to run on approx 4 amps of current. Much more than that and they overheat and will fail. Much less than 4 amps and you get a weak spark.

Amps = Volts / Ohms, or said another way, Volts / Ohms = Amps (This is one iteraration of Ohm's Law)

So, 12 (volts) dived by 3 (ohms of resistance) = 4 (amps of current) Good!

12 (volts) dived by 1.5 (ohms of resistance) = 8 (amps of current) Bad!

If you were to use a coil with 1.5 ohms of resistance in a 12 volt system, you would need to add about another 1.5 ohms of resistance. This is done by adding a resistor with that value in the circuit (between power supplying the coil and the coil itself)

A coil with 1.5 ohm's of resistance is designed for a 6 volt system
6 (volts) dived by 1.5 (ohms of resistance) = 4 (amps of current) Good!

Think approximate when figuring the amount of resistance needed, as voltage varies due to charging systems and other loads.

1.0 to 2.0 ohms of resistance is ok for a 6 volt coil,
2.5 to 4.0 ohms of resistance is ok for a 12 volt coil

Is there any pans to repeal the OHMS Law?

300 GREAT QUESTION Ive answered this probably a hundred times on here the past 20 years but Im happy to help again !!

1) A 12 Volt tractor can use EITHER:

a) A full true 12 Volt rated coil which has a LV primary winding resistance between small + and - terminals around 3 Ohms
Some mistakenly call those internal ballasted HOWEVER there's no actual discrete resistor hidden inside the can, the 3
Ohms is the resistance of the LV primary winding/coils NO EXTERNAL ADDED BALLAST RESISTANCE IS NEEDED for a full true
12 volt coil. It already has 3 ohms resistance

b) A 6 volt coil with around 1.5 ohms LV Primary PLUS an external series voltage dropping (12 down to 6) Ballast

The thing is you need to limit the coil current the points switch to not much over 4 amps else they burn too quick

THEREFORE 12 Volt /3 Ohms (6 volt 1.5 Ohm coil + 1.5 Ohm ballast orrrrrrrrr 12 volt 3 ohm coil no ballast) = 4 Amps

NOTE A coil labeled 12 Volts orrrrrrrr 12 Volts NOT for use with ballast is a full true 12 volt coil no ballast required HOWEVER A coil labeled 12 Volts for use with ballast in in reality a 6 volt coil and needs ballast as it says on a 12 volt tractor or it will overheat

DO AS PERTRONIX SAYS NOTTTTTTTTTTTTTTTTTTTT WHAT ANY OF US ON HERE GUESSES

If points burn too soon they are switching too much current so that needs to be limited to around 4 amps and on a 12 volt tractor thats a 6 volt coil plus ballast (1.5 + 1.5 total 3 ohms) orrrrrrrrr a full true 12 volt coil (3 ohms) no ballast required

NOTE these numbers may not be exact they are rough averages, Ive seen 6 volt coils 1 to 2 ohms and 12 volt 2.5 to near 4 and different accompanying ballast

Hope this helps, got it any questions ????????

John T

Ok John thanks ! I follow you ! I think for the most part! Does what you said hold true to the Petronix ignition system ? Thats where I am getting a little confused. Being it has no points is the 4 amp still correct ? Ive tried researching this and cant come up with a definite answer. On different forums or talk areas Ive heard both. Use a 3 ohm coil some say 1.5. Pretty confusing to say the least. I do understand about the differences in coils between resistor, 6 and 12 volt. Im just not so sure on what Im needing for this application. Thank you and red , white , and blue !

What happens if you run a 1.5 coil with an electronic system
with no points ?

So it was originally wired with a true 12v coil and resistor. I took out the resistor. Will I damage the system with a 1.5 ohm coil ? 12 divided by 1.5 isnt 4. Im just trying to figure out what this system needs. We have a similar setup on our Oliver 88 puller. Petronix electronic ignition, 1.5 ohm coil. It also has an Msd Al box on it. Has been flawless. Different application though.

1.5 ohm or 3 ohm ? They make both.

A 1.5 ohm coil needs a 1.5 ohm resistor in series with the + primary terminal.(1.5+1.5=3)

A true 12v coil is already 3 ohms, so no need for a resistor.

Running a 1.5 ohm coil and no resistor will eventually burn up the Pertronix unit and/or the coil.

I'm not sure what the MSD unit requires, possibly you get by because of short run time, but I suspect the coil gets hot!

With MSD, the points just switch on/off 12v signal to MSD box. Since in this scenario the points don't carry spark current, they last a long time.

(quoted from post at 20:41:05 10/18/21)Being it has no points is the 4 amp still correct?

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[b:e888074973][i:e888074973][u:e888074973]Coils[/b:e888074973][/i:e888074973][/u:e888074973] need to run with about 4 amps of current. Forget about points / no points.

(quoted from post at 20:42:26 10/18/21) What happens if you run a 1.5 coil with an electronic system
with no points ?

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Forget about points / no points. If you run a 1.5 ohm resistance coil on a 12 volt system, without additional resistance, you will eventually burn up the [b:e888074973][i:e888074973][u:e888074973]Coils[/b:e888074973][/i:e888074973][/u:e888074973]

(quoted from post at 20:48:13 10/18/21) So it was originally wired with a true 12v coil and resistor

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A true 12 volt coil has around 3.0 ohms of resistance. Should [b:e888074973][i:e888074973][u:e888074973]NOT[/b:e888074973][/i:e888074973][/u:e888074973] be any additional resistors in the circuit. If there was, the system would have a very weak spark.

I took out the resistor.

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Good. See last statement above

Will I damage the system with a 1.5 ohm coil ? 12 divided by 1.5 isnt 4

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Yes. Too much amperage. But you just said above

it was originally wired with a true 12v coil

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Again, a true 12 volt coil has around 3.0 ohms of resistance.
So is it a coil with 1.5 ohms of resistance, or a true 12 volt coil with around 3.0 ohms of resistance? Can't be both.

We have a similar setup on our Oliver 88 puller. Petronix electronic ignition, 1.5 ohm coil...... Has been flawless

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This either has additional resistance inline, or you have been very lucky, ..... so far.

I dont use modern view so Im trying to figure out your
quotes. Sorry. Originally when I started working on it was a
12v coil from Napa. Says on that coil no external resistor
required. New coil I have sitting here bought by my brother is
a Petronix flame thrower. 12v coil rated at 1.5 ohms. On the
Petronix box it states to use the 1.5 ohm coil is suitable for v-8
engines. Use 3 ohm coil for 6 and 4 cylinder engines except in
racing conditions where a 1.5 ohm coil may be used. Why the
difference ?

(quoted from post at 16:19:35 10/19/21) I don't use modern view so I'm trying to figure out your
quotes.

Click to expand...

Maybe you should.....