Farmall M External Resistor Getting Hot

johnb5345

New User
On my M, the external resistor for the 12V coil gets very hot to the touch when I hook up the battery. It only takes about 30 seconds and the resistor starts smoking. The coil and all wiring are not heating up at all, just the resistor. I've tried replacing the coil and resistor but the problem persists. Any idea what the problem could be?
 
Sure they get hot but not intil the sw is on. When the sw is off there is no voltage there thus it will not get warm until the engine is started and runnng. Any time you have a resistor on a coil dropping voltage it will get warm. Just get a 12v coil and do away with the resistor they do make 12v coils for that dist.
 
Do you have the resistor connected correctly?

It should get hot, because that is what a resistor does, but it should NOT smoke.

The wire should run from the ignition switch to the first terminal on the resistor. From the second terminal to the + on the coil. From the - on the coil to the post on the side of the distributor.
 
Those ballast resistors usually get pretty hot, they are like 25 watts. Of course if you turn the ignition on and she sets there very long before starting NOT running they get even hotter then normal. It helps depending on where and how they are mounted. Mounting the resistors metal clamp to good heavy iron can help transfer heat away from the resistor and if its where it gets better air flow that can also help. Id guess it may be "normal" hot but take a look at how and where its mounted to improve heat transfer.

John T
 
Why are you using a resistor with a 12volt coil? Does
the coil require it. If so, just get a true 12volt
coil that doesnt need a resistor and be done with it.
 
New resistor will smoke for a bit until it burns off the factory coating.
Don't leave the switch on and engine stopped for any length of time; the points can be damaged (burned)if they are in the closed position.
PS: the resistor will not get hot if the points are open because no current is flowing in the primary circuit.
 
I have a 12 volt M where I am still using the 6 volt coil, and have the block ceramic wire resistor on it, probably the same as your setup.

Yes mine gets very hot too, and if it is a new resistor with maybe paint on it or if oil or grease gets on it, it can smoke.
 
Protect, no, limit current during use, yes The low ohm coil will draw 5 or more amps and get hot (as well as lower the life of the points. The coil is designed to operate at 6 to 8V in operation. Jim
 
The reason the resistor gets hot is as stated below (it needs to get hot to work) the reason the ignition should not be turned on while not running is that almost all engines stop with the points closed. Thus when turned on and not running, the full draw of the coil and resistor is placed on the system continuously. When running, the point closed time is about 50% on 50% off (Dwell). This results in the coil and resistor and points only having current flowing one half the time. This allows the devices in that circuit to cool half the time. Jim
 
you have a heating element ( resistor ).

just add something combustable.. like oil or dust.. and you get smoke.

VERY common.
 
(quoted from post at 05:45:49 09/26/14) Why are you using a resistor with a 12volt coil? Does
the coil require it. If so, just get a true 12volt
coil that doesnt need a resistor and be done with it.

Because 1/2 of the "12 Volt" coils on the market require a ballast resistor.

Someone somewhere along the way decided that people couldn't grasp the concept of a 6V coil on a 12V vehicle, so they started marking the coils as 12V so people wouldn't be confused.
 

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