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Farmall & IHC Tractors Discussion Forum
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12 Volt coils

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GeorgeH

09-25-2004 16:50:11




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I read many posts lately trying to ubderstand coils and ballast resistors. I Have a Farmall H that has 12 V battery, 6 V coil, and a ballast resistor built into the ignition switch. I think switch went bad so got a new 12 V coil and new on/off switch. Coil stamping said "requires external resistor" but nobody knew why (surprise?). Tractor started fine but after 5 minutes of run time, coil was too hot to touch. Back to local parts store and got a 1.8 ohm ballast resistor and installed (fabricated a heat sink). After 10 minutes of run time, coil barely warm. My vote is to use the resistor - even on 12 volt system.

Next step now is to debug generator.

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Jokke

09-26-2004 07:25:46




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 Re: 12 Volt coils in reply to GeorgeH, 09-25-2004 16:50:11  
Well, what I"m gonna say next is considered to be a rule of thumb.

The current flowing thru the coil will cause it to heat so:
Four cylinder engines should never go over 4 amps
and six or eight cylinder engines should stay under 8.5 amps.
Thus volts/resistance=amps. eg.: 4 cyl: 12volt 12v/4ohm = 3amps => coil or coil+resistor must be 4ohm at least.



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Dale in Ar.

09-25-2004 19:30:13




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 Re: 12 Volt coils in reply to GeorgeH, 09-25-2004 16:50:11  
I just went through the same thing on my Super C.I asked around for some information on the right coil for my 12 volt system,got pretty much the same info.Went to my parts house for some adviceand I was told that all point systems run on 8 volts or less,he couldn't remember the exact voltage.He suggested a 12volt coil with an internal resister that had 3.3 ohms of resistance.I read somewhere that this is the amount of resistance that was needed with the combined resistance that a resister and coil has for a 6 volt.Took his suggestion and put in the 12 volt coil,every thing works fine and no resister to look out of place.

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terryj

09-26-2004 20:45:53




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 Re: 12 Volt coils in reply to Dale in Ar., 09-25-2004 19:30:13  
I know on my dads M that was converted to 12 volt before he bought it he had a coil and a ballest resister. He switched that for a coil with an internal resister and took out the ballest resister about 5 years ago and it has worked fine since. Myself I am going to be looking into how to convert a C to 12 volts.



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scotc

09-27-2004 18:47:51




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 Re: 12 Volt coils in reply to terryj, 09-26-2004 20:45:53  
i got took the resistor off my 85 dodge donor truck and fabbed a little bracket to mount it beside the coil. looks like it belongs there with good wire terminals. People on here say that the old generators are good for up to like 14 or 18 volts, I know my SM is pushing about 15 volts into its 12v batt. One of the mopar mags had a simplified wiring diagram for the older pre-computer altenators that I think I can go from to move the charging system from my pickup to the B.

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Doug in OR

09-25-2004 18:57:17




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 Re: 12 Volt coils in reply to GeorgeH, 09-25-2004 16:50:11  
Let me complicate your life, if I may. (smile) To create a true 12 volt coil, they would need to double the number of windings on the primary side. The problem becomes one of the extra inductance from adding turns to the coil. You couldn't use a standard condensor and points if they did this. Most true 12 volt coils are merely a 6 volt coil with the ballast resistor already included internally in the coil. These would not be marked "use with external resistor". You can tell a 12 volt coil by putting an ohmmeter across the terminals. A 12 volt coil will read about 3.2 ohms.

To complicate this, many Fords used an 8 volt coil. They used a piece of resistance wire between the ignition switch and the coil to drop the extra 4 volts. If the coil tells you to use an external resistor, it is probably just a 6 volt coil that the manufacturer doesn't wish to have branded as obsolete.

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Hugh MacKay

09-25-2004 18:18:11




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 Re: 12 Volt coils in reply to GeorgeH, 09-25-2004 16:50:11  
George: Some coils designed for 12 volt require a ballast resistor and some don't. They are marked indicating this.



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GeorgeH

09-25-2004 18:45:23




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 Re: 12 Volt coils in reply to Hugh MacKay, 09-25-2004 18:18:11  
Guess the moral of this story is "believe everything you read and nothing of what you hear".



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scotc

09-27-2004 18:49:56




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 Re: 12 Volt coils in reply to GeorgeH, 09-25-2004 18:45:23  
thought it was only 1/2 what you read



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