coil on a 140 farmall

Is that coil supposed to get hot?

It is almost too hot to touch when its running.

No it should be 20 degrees or so above ambient, or less than block temp. If it is hot, it is either failing, or it needs a dropping (ballast) resistor. to change the applied voltage to ~6volts. That only if it is a 12v coil for use with an external resistor, or an actual 6 volt coil. Jim

the system is 12 volt.

(quoted from post at 06:49:14 05/01/12) the system is 12 volt.

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12 volt would require a ceramic resistor to drop the coil input voltage to 6 volts, or the use of a coil that is internally resisted.

A standard 6 volt ignition on a 12 volt system WITHOUT a resistor WILL get hot.

I think this is a factory 12 volt. Where would that resistor be?

If its factory it would be 12v but how do you know that it is original.

I don't know that the coil is 12 volt. is there a way I can tell?


Thanks guys.

(quoted from post at 07:13:44 05/01/12) If its factory it would be 12v but how do you know that it is original.

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Didn't IH locate that resistor up under the dash when they went to 12 volts? Or maybe it is built into the ignition switch?

If it has a Key start, there will be a Starter relay on or near the starter. This relay might have 4 terminals. one of the small ones will be labeled "I" this terminal should go to the coil side of the resistor if so equipped. Some have an ignition wire from the key to the coil that is a resistor in and of itself. This would then have that I wire connected to the coil directly. (on the terminal not connected to the points) The resistor in a wire type may not be tha case on your tractor, I do not know) if you measure the resistance of the coil with it disconnected) from smapp terminal to small terminal it should be around 2.5 to 3 ohms if 6volt, or 12v for use with a external resistor. If it is a 12v no resistor required, it will measure about 4 to 5 ohms. If it is a failing coil it will have low resistance. (it could be a 12v coil failing and looking like a 6v in resistance. If you measure the voltage at the coil within 1 minute of turning it on (but not starting it), and the points are closed (normally comes to rest in that position) the voltage will read 8 or less if it has an internal resistor, and about 12.3 or more if it does not. Do not leave the key on for more than 2 minutes with it not running, it will heat the coil even if it is the correct coil. Jim

My 1966 140 (born with 12V, a generator, and big button start) had no resistor from the factory. At one point, while I was overseas, the guy mowing fields with it put an 'external resistor required' coil on it. Not only did it run hot, but it managed to explode when I accidentally left it on for a few minutes when I was having a no-start problem. Hot oil all over the side of the engine. I am glad I was elsewhere when it went (had to answer the phone). Now I have the resistor in place with same type coil and it gets warm but not hot. When this one goes--been several years--I will upgrade to a true 12V coil with no external resistor.

great info thanks!!!