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Allis Chalmers Discussion Forum

Coils 101

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John T

10-28-2011 07:47:00

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First of all, 6 volt and 12 volt coils ARE NOTTTTTTTTTTTTTTT THE SAME what the parts dude told you ("all cois are the same") IS WRONG AS RAIN. A full true 12 volt coil has "around" 3 ohms of primary resistance while a 6 volt coil has about half that, "around" 1.5 ohms. THESE VALUES VARY SOMEWHAT BUT THIS GIVES YOU THE BIG PICTURE.

One of the main design limitation parameters in old tractor Kettering points and coil ignition systems is limiting the current the points must swtich to the 4 amp and less range, otherwise the points burn up too fast.


In a 6 volt system, the 6 volt coils low voltage primary winding resistance is around 1.5 ohms, meaning the current (the points must switch) is 6/1.5 = 4 amps

In a 12 volt unballasted system, the coils low voltage primary winding resistance is more like 3 ohms, meaning the current (the points must switch) is 12/3 = again 4 amps.

Now, some 12 volt tractors still used a 6 volt (1.5 ohms) coil BUTTTTTTTTTT they added a series 1.5 ohms EXTERNAL BALLAST VOLTAGE DROPPING (12 to 6) RESISTOR which dropped 6 volts leaving 6 on the coil as designed for, so the points still only switch 4 amps and alls well.

NOTE: Coils labeled "12 volts" or "12 volts NOT for use with ballast resistor" are full true 12 volt coils. HOWEVER a coil labeled "12 votls requires ballast resistor" or a coil labeled "6 volts" ARE 6 VOLT COILS and require the ballast on a 12 volt tractor but not if only a 6 volt tractor.

OLD WIVES TALES: Some mistakenly refer to some 12 volt cois as being "internally ballasted" HOWEVER if you disect that coil, on most, you WILL NOT FIND A DISCRETE STAND ALONE RESISTOR HIDDEN AWAY SOMEWHERE INSIDE THE CAN. The primary winding resistance of around 3 ohms is achieved due to more windings/turns or higher resistance wire NOT A RESISTIR HIDDEN INSIDE THERE.

If a coil overheats its because its drawing more current then it was designed for and cant effectivelty dissipate the heat. IF YOU HAVE A 12 VOLT TRACTOR AND ARE USING A 6 VOLT COIL AND IT OVERHEATS, YOU MIGHT WANNA CHECK THE BALLAST RESISTOR OHMS VALUE (around 1.5 ohms or a bit more) OR INSURE ITS WIRED RIGHT AND EFFECTIVELY IN THE SERIES CIRCUIT BETWEEN IGN SITCH AND COILS INPUT.

Or else you can toss the ballast and use a full true 12 volt coil (around 3 ohms primary resistance) subject to any starting ballast by pass circuitry some tractors use


PS Modern solid state switched electronic ignitions are NOT the same animal, some use more of a higher voltage switched pulse to fire the coil instead of the old tractor mechanical points systems

Nuff said, hope this helps

John T retired Electrical Engineer

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