Farmall M Ignition coil

So I have been fighting my M for eight months now I have tried everything I have read in this forum I currently have an internal resisted coil end it continues to get hot tractor starts fine and runs great for about 30 minutes and then starts missing sputtering and the coil is always too hot and I can't figure out what to do initial problem was the external resistor had burned out I change the resister and it took me a while to find the right one here on the forum after changing to the recommended NAPA part number I thought I had her fixed unfortunately she started missing again I then tried replacing the alternator and again it failed I am at a loss of what to do about ready to take it to a mechanic but it has become personal does anybody have any ideas she ran fine until the resister burned out I I am now on my third coil and have currently eliminated the X ternal resistor?

Buy a coil that says right on it NO EXTERNAL resister needed and then do away with the resister. The problem with coils and external resister is you do not know what the matching resister to that coil is it can be very hard to get things as they where really made to work.

There is no internal resistor in the coil. it is just wound with enough thin wire to operate on 14.2 to 14.7 volts found in a running and charging 12v system.
If a coil gets hot and fails, it is getting too much voltage. My first suggestion is to diagnose the charging voltage. Using a meter with a needle (not
digital) or a Fluke level digital meter with pulse DC averaging circuitry) connect the meter across the battery and start the tractor. run it at full throttle
and check the voltage. It should be as above, if higher, it will heat the coil. Excessive dwell will also heat the coil. Dwell is the proportional amount of
degrees of rotation of the distributor that the points are closed compared to the time they are open. It is usually about 50 % on, points closed< and 50% off,
points open. This means the coil is being fed voltage about 1/2 the time. If the dwell is 90% on and 10% off it will still run, but the coil will be on much
more than it is supposed to. Setting the points at .020" when the distributor cam has them as open as they can be (hand rotating the engine) is correct, and
will provide correct dwell. Measure the voltage and get back to us, Jim

First of all I doubt you have a coil that has an internal stand alone discrete "resistor" hidden away inside the can.

What some mistakenly call an internal resistor coil is more likely a full true 12 volt coil BUT STILL IT HAS NO RESISTOR INSIDE.

NOTE A coil labeled "12 Volts" or a coil labeled "12 Volts NOT for use with resistor" is a full true 12 volt coil HOWEVER a coil labeled "12 Volts for use with (or requires) ballast" is more like a 6 volt coil and needs external ballast or IT WILL OVERHEAT

If its a 12 volt tractor AND YOU HAD A FULL TRUE 12 VOLT COIL (instead of a 6 volt) YOU DONT NEED A BALLAST

If yours is overheating PERHAPS ITS A 6 VOLT COIL (or one labeled 12 volts for use with ballast) ON A 12 VOLT TRACTOR and if that's the case and you have no external ballast IT WILL OVERHEAT.

PS measure the coils LV Primary resistance between small + and - terminals. If its like 1 to 2 ohms its a 6 volt coil AND NEEDS BALLAST ON A 12 VOLT TRACTOR OR IT WILL OVERHEAT,,,,,,If its more like 2.5 to 4 ohms, its a 12 volt coil.

John T

So John how many coils have to held in your hand tha thad what you said on them?? ALL the ones I have had say either external resister needed or no external resister needed. The one I have on the Case VAC I got last month says 12 volt No External resister needed

As a used tractor dealer back in the day who had to repair tractors himself plus farmed for years with them, I've had my hand on hundreds of them lol

Anyway like I've said on here over 15 yrs, if its a 12 volt tractor you need EITHER a full true 12 volt coil orrrrrrrr a 6 volt coil PLUS an external series voltage dropping (12 down to 6) ballast resistor, but if not it will get too hot.

The coils labeled "12 Volts" or "12 Volts not for use with (or requires) ballast" are just what they say, 12 VOLT COILS NO BALLAST REQUIRED.

IF ITS NOT LABELED use an ohm meter to see if its a 6 or a 12 volt coil PIECE OF CAKE

The coils labeled "12 Volts for use with (or requires) ballast resistor" are more like 6 volt coils and mean EXACTLY what they say IE a ballast or else they will overheat.

Hope this helps

John T

My point John tha ti am trying to explain so yon can
use the right wording not your long winded Lawyer talk. LOL

I.E the coils all I have seen say 12 volt no external resister

Or external resister needed .

They are not worded as you have said

Rich, but I have seen several of them worded as I said !!!!!!!!!! but there are a ton of manufacturers out there so maybe you haven't. When I was a used tractor dealer and farmer I couldn't count all the different ones I saw lol

John T

I'm sorry you got jumped on, terms change on this forum every other day. Just a few weeks ago, the term(s) you used were perfectly acceptable.

Ok guys. I am back in the hunt. I was on a business trip, and hunting. I went back to work on the M, and it lasted about 3 minutes before it sputtered and died. I found the alternator belt had broken, so I got that changed and I am back to square one. I want to clarify that the coil I am currently using, says right on it no external resistor needed. I got the NAPA# of one of these forums. I am going to check the voltage out tonight and will report back. If I cannot figure this thing out I am going to take it to the dealer....Ouch!!

I checked the resistance on the original coil. 2.0 ohms that means it was a 6 volt coil. The new one I had on was NAPA IC 14SB and it had 3.5 ohms across the terminals. I put the original coil back on, along with the external resistor. It ran great for a while, and then did the same thing. Only now it didn't quite die, it sputtered and back fired and missed, and eventually it came out of it. It never did that with the 12 volt coil on? It just died. My question is are the points and condenser the same for 12 or 6 volt? The points and condenser I bought are by application at Tractor Supply. Is there a different set once converted, and if so does anyone know the NAPA #? The voltage across the battery was solid at 14.3. Also this old coil doesn't get hot like the 12 volt did. I am going to upload some video showing what it does. It has to be something someone else has seen before. Thanks guys!! I am off for Veterans Day, and I would like to get this thing figured out so I can get some work done.

(quoted from post at 19:50:02 10/20/16) Buy a coil that says right on it NO EXTERNAL resister needed and then do away with the resister. The problem with coils and external resister is you do not know what the matching resister to that coil is it can be very hard to get things as they where really made to work.

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YES! That is exactly what is needed. I have one on my 1940 M that is converted to a 12 volt, Delco alternator. I am NOT using a resistor. Ignition coil DOES NOT get hot. Engine starts easily and runs excellent.

Points and condenser are the same no matter what voltage it is they make one one size to fit. As for what it is doing sounds like the 6 volt coil once warmed up is shorting out a bit which will cause it to run poorly. I would just bite the bullet and buy a true 12 volt no external resister needed coil and give that a try. Veterans day is tomorrow LOL.
U.S.Navy 1974-1980 SSBN633 and JFK CVA67 both are long gone and could well be razor blades by now

I guess what you didn't understand is it is better with the original 6 volt coil? I bought a new 12 volt coil from Napa, and one from Tractor Supply. The coil would get very hot, but with the old one it doesn't? I was able to make several passes with my big blade yesterday before it started cutting out? I dont know what to think?

Well the one form Tractor supply is junk form the get go most of the time. As for the one form NAPA unless it says 12 volt no external resister needed it still need a resister and that would cause it to get hot and also burn the points fast either of which can cause the problem your having