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Tractor Talk Discussion Forum

12 v coil vs 12 volt coil with external resister

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davidlove1969

01-27-2008 11:30:28




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What is the difference between a 12 v coil vs 12 volt coil with external resister.??

Thanks Dave




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Jerry/MT

01-27-2008 17:29:34




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 11:30:28  
About 1.5 ohms of resistance! Most ignition systems are limited to ~4amps max current. The true 12V coil(NAPA IC-14SB~$15) has ~3 ohms of resistance built in. The so called "12V coil Use with a resistor" is really a 6 volt coil with ~1.5 ohms resistance and when you put it in a 12V system you have to put the additional 1.5 ohms in series with the primary wire to limit the current to ~ 4amps.



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Texasmark

01-27-2008 16:23:48




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 11:30:28  
My MF service manual said that ignition points are set up for 5 amperes of break current. Break current is when the points open and the coil dumps into the spark plug.

Assuming no inductance is part of that number, you take 12v (12.75 or 14 when the engine is running at moderate rpm's and the gen/alt is putting out) divided by 5 amperes and you have your wiring dc resistance measured in ohms; actual measurable resistance with an ohmmeter, from the battery to ground through all the ignition operating components.

Doesn't matter where it is unless you have a bypass circuit in your ign switch which bypasses an external resistor (usually half the circuit resistance) to put full voltage across the coil to aid in getting a hot spark for starting..... since the starter pulls the battery voltage down while cranking and you need all the help you can get....and since farm tractors may sit out for long periods in the weather and all, the battery charge could be pretty low. (I think Ford, for one, had it in their cars.)

That's it.

Mark

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El Toro

01-27-2008 14:13:37




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 11:30:28  
That coil from NAPA is P/N IC14SB and is around $14.00. Hal



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El Toro

01-27-2008 14:13:30




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 11:30:28  
That coil from NAPA is P/N IC14SB and is around $14.00. Hal



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TractormanNC

01-27-2008 11:57:06




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 11:30:28  
If you use a coil with an external resistor and do not have the bypass wire for starting, it makes for hard cold weather starting. BTDT on a Farmall M and 130 converted to 12 volts. Use a real 12 volt coil. NAPA has 'em. HTH



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Jon Hagen

01-27-2008 11:41:03




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 11:30:28  
A true 12V coil will have a 3 ohm primary winding.
a 12V with external resistor coil will Have a 1.5 ohm primary winding hooked in series with a 1.5 ohm external resistor to end up with the same 3 ohms total resistance.
The advantage of the coil that uses an external resistor, is that you can wire it to bypass the external resistor to make extra hot spark for starting, then having the resistor back in line for running to give long point life. This was pretty much standard equipment on all automotive 12V point type ignition systems.

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davidlove1969

01-27-2008 15:24:56




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 Re: 12 v coil vs 12 volt coil with external resister in reply to Jon Hagen, 01-27-2008 11:41:03  
Thanks for the information. That helps explain the with or not with resistor.

So if you are in a warm climate just use the 12 coil. Right?



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Jon Hagen

01-27-2008 17:21:40




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 Re: 12 v coil vs 12 volt coil with external resister in reply to davidlove1969, 01-27-2008 15:24:56  
If the engine starts easily,then a 12V (3 ohm primary) coil will do fine and you do not have an external resistor to mount/wire / have fail if exposed to moisture. If you want/need the extra hot spark for starting, then the external resistor type with the bypass circuit has a distinct advantage at the cost of being a bit more complicated.



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