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Let's talk coils

grandpa Love

grandpa Love

Well-known Member
We bought a new 12 volt coil, no resistor needed, for the Farmall B. But......I remembered we had a good 6 volt battery from my dad's cub (we made it 12 volt) no charging system on the B. What happens to the 12 volt coil if I run the 6 volt battery??
 
What happens to the 12 volt coil if I run the 6 volt battery??

Using a 12 volt coil with only a 6 volt battery will result in a weak spark.

A 12 volt tractor (like the B if its 12 volt) can use that full true 12 volt rated coil no resistor needed PROVIDED THATS WHAT IT ACTUALLY
IS ??

A full true 12 Volt coil (no ballast required) has a LV Primary in the 2.5 to 4 ohm range, typically around 3 ohms.

A 6 volt coil (or one labeled 12 volts for use with or requires ballast resistor) has around 1 to 2 Ohms LV Primary, typically around 1.5 ohms

John T
 
GP
I think the rule of thumb is most coils need 4 amps to make a good spark.
Too many amps and you will complain they don't make points like they used to.
Too few amps and you will complain tractor won't start.
My Jubilee has a ballast resistor. There is 6v across the resister, 6 volts across the coil and 4 amps in the primary.

Can anyone confirm the rule of thumb, 4 amps??
 
The spark will be so weak it won't start.

If you're keeping it 6 volt, everything will have to be right or it will be ongoing frustration!
 
Steve,it's started right up! Ran for a minute then remembered I had the 12 volt coil on it. 6 volt battery started it great
 
Then chances are it is not a true 12v coil.

Check the part number on the coil itself, if there is one, good chance it has been reboxed under the wrong part number.

If you can check the amp draw, not running and the points closed, and get about 3 to 4 amps, it will work. Or check the ohms across the primary terminals, out of circuit (nothing else connected to it) should get about 1.5 ohms for a 6v coil, 3 ohms for a 12v.
 
Steve,so it won't hurt to run it? A little around the yard? Need to pressure wash tractor and prep for paint.
 
No it wont hurt to run a 12 volt coil on a 6 volt tractor (IF thats actually what you have a 12 volt coil???) other than weak spark and some carbon build up.

John T
 
Grandpa, FYI A coil labeled 12 Volts or 12 Volts not for use with ballast or no ballast required that has around 3 ohms LV primary resistance is a full true 12 Volt coil no ballast required. If ran at 6 volts the spark will be weak

HOWEVER a coil labeled 12 volts for use with ballast or 12 volts ballast required etc that has around 1.5 ohms LV primary resistance is in effect a 6 volt coil.

What is the LV primary resistance of your coil ??????

Hope this helps

John T
 
The 4 Amps current has to do with so the points will not burn up prematurely which is true regardless of 6 or 12 volt coil. 12 Volts/3 Ohms = 4 Amps as 6 Volts/1.5 Ohms = 4 Amps

John T
 
So my rule of thumb is right, 4 amps.
Is it true?
Connect 6v to a 12v coil and you will get about 2 amps.
Connect 12v to a 6v coil and you will get 8 amps.
So why don't people recommend measuring amps and not worry about ohms or how many volts a coil is? Shoot for 4 amps. Add ballast as needed.
 
Only way to know for sure what the coil is, is to ohm it out. The way things are made now days one never knows what you are getting or if it has to wrong writing on it that says one thing but isn't as it says
 

OK, so help me out here, as I'm new at this part. By checking the resistance, does that mean putting the ohm meter across the + and - terminals of the coil? And then the readings should be as noted above, correct?

I miss the days when 6 volt systems used 6 volt coils, and 12 volt systems used 12 volt coils. The coils were clearly marked as such and we didn't have all this resistor/non resistor crap. You didn't have to go through all this hoopla to try and figure out what you had.
If you had spark across the points the coil was good; if not, you replaced it.
Times were much simpler then and life was good!
 
(quoted from post at 13:58:30 09/26/21)
OK, so help me out here, as I'm new at this part. By checking the resistance, does that mean putting the ohm meter across the + and - terminals of the coil? And then the readings should be as noted above, correct?

I miss the days when 6 volt systems used 6 volt coils, and 12 volt systems used 12 volt coils. The coils were clearly marked as such and we didn't have all this resistor/non resistor crap. You didn't have to go through all this hoopla to try and figure out what you had.
If you had spark across the points the coil was good; if not, you replaced it.
Times were much simpler then and life was good!
Click to expand...

"and 12 volt systems used 12 volt coils"

You are pining over a time that never existed!

There never was such a time in the 12 Volt era.

While it is true that there are some "true" 12 Volt coils out there probably half the ignition systems (or more) used 6 Volt coils along with a resistor.

Ford, GM, Chrysler, IH, and John Deere to name a few used the 6 Volt coil with a resistor system on many of their products in the 12 Volt era.

Even in the six Volt era some manufacturers (Ford is a notable example) used resistors along with their coils to operate at 4.5 Volts or so once warmed up!
 
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