Re: Coil 6v or 12v??

Posted by John T on December 30, 2010 at 08:59:48 from (66.244.97.31):

In Reply to: Coil 6v or 12v?? posted by Mr. Mayor on December 30, 2010 at 08:26:57:

When I was a used tractor dealer I accumulated a ton of coils, heres what I suggest that ALWAYS worked for me.

Stick an ohm meter on the Rx1 scale across its primary, the little + and - terminals.

If it reads around 1.25 up to 2 ohms (many average around 1.5 ohms) it can be used at 6 volts nominal and produce a decent spark when starting drags the battery voltage down to say 5 volts or its under a charge and up to 7 volts I CALL THOSE 6 VOLT COILS and they are usually labeled "6 Volts" and what NAPA may give you if you ask for a 6 volt coil as opposed to a 12 volt coil. Of course, you can still use it on a 12 volt tractor simply by adding an external series voltage dropping (12 to 6) Ballast Resistor.

If it reads more like 2.5 to 4 ohms (many average around 3 ohms) it can be used unballasted at 12 volts nominal and produce a spark if starting drags the battery voltage down to 10 or under and still if under a charge at 14 volts. I CALL THEM 12 VOLT COILS and they are labeled "12 Volts" or perhaps "12 volts NOT for use with external Ballast" and are what NAPA may give you if you ask for a 12 volt versus a 6 volt coil. Some call those coils Internally Ballasted.....

Many tractors used the same coil for 6 or 12 volt tractors, its just that if on a 12 volt they added the external voltage dropping (12 to 6) Ballast Resistor.

If you put 12 volts on a coil designed for 6 (and no ballast) it will overheat.....If you only put 6 on one designed for 12, the spark is weak. THEY ARE NOT ALL THE SAME due to current ratings and their capacity to expel the heat generated to their surroundings.

Hope this helps

1.5 ohms likely a 6 volt coil, 3 ohms likely a 12 volt

John T

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