Cracked coil, why?

Working with a Ferguson TO20 that was converted to 12v neg ground system. It has an alternator, 12v coil and seems to be wired correctly according to a diagram for 12v systems. This tractor ran fine for a year or so, but this year, it wouldnt fire right up. After rebuilding the carb, checking fuel flow and spark, still would not run, just backfires and pops. Then at the end of the day, I somehow noticed a crack in the top of the coil, which I knew had to be the problem. After replacing it with another 12v coil, and rechecking wiring, we got about 3 hours on it running really well, then it did the same thing to a brand new coil. Could the alternators regulator be bad and letting too much into the primary, or possibly just overheating although the engine does not overheat. I tried getting it to fire back up tonight after the failure to check the output from the alt, but couldnt get it running long enough to do so.

You might have a bad coil wire (open connection either carbon or metal wire) causing the coil to build up too much secondary voltage which arcs over the insulated tower. doubt primary voltage would have anything to do with this.

Are you using a TRUE 12 Volt coil that needs NO external resistor or a "wanna-be" 12 Volt coil that NEEDS and external resistor and you are running without one, overheating the coil.

What make and part # coil are you using? Are you using an external resistor ahead of the coil?

I have seen this happen with a 12 volt conversion where there was no resistor in the circuit. I would also expect to have problems with points burning up. But sometimes coils simply fail, due to heat and vibration. If you can't find a resistor between the coil and ignition switch, replace the coil with one that specifies "12 volt no external resistor required".

At the risk of starting a flame war, I'll mention a myth that's often repeated here: Supposedly a "true 12 volt coil" is superior to using a coil with an external resistor. Fact: it makes no difference whether the resistance is internal to the coil or external to it. The coil's energy is the same in either case.

Thanks for the replies guys, the coil that was on it had no markings but it ran fine for a year or two with it. But just last week i found the points
were burned up like they had been stuck, and they werent touching center to center, kinda offset so the fixed point was half gone. I replaced those,
set the gap, then noticed the coil was busted. I picked up a coil from
Tractor Supply(closest store) althought the package said it was for IH, but marked for 12v, Not 3 hours of use later she began running terrible
especially under a load it would basically stall out, then noticed the new
coil was cracked and leaking fluid/oil. Not sure of the part number on this coil, but everything seems wired correctly, and the terminals to the coil were correct. I need to get it running again to check alt output voltage. Not using a resistor that i can see, but it ran the other coil without one and not sure if this 12v coil for IH is a "true" 12v coil. Thanks

Unless the battery is vigorously boiling over, you have a COIL/RESISTOR mis-match problem, NOT an alternator problem.


If you don't understand the whole resistor thing, save yourself a lot of headache and get a TRUE 12 Volt coil. NAPA part # IC 14SB is ONE example.

Thanks Bob, I guess it is possible that the first coil was not the right one either. After I get a new one, I gotta check the alt output, I know the voltage regs in those do go bad.

Today, I actually took a day off work to try and figure this out. I stopped at Napa, got the 12 volt coil that is stamped "For use without external resistor", so that problem is solved. It fired right up. So I checked the voltage at the battery to check the alternator, and it is driving my meter crazy flashing up over 1000 volts, so I quickly disconnected and shut her down in fear of cracking another coil and damaging the meter. I disconnected the alternator wires, started and ran off the battery, and still had wild readings. So thinking the alt could be somehow shorted throwing voltage on the ground, I completely loosened and took the belt off. I know some of you may worry I ran it like that for long, but I didnt even run it for 10 seconds, so I think Im okay without water pump for that long, but STILL had crazy readings at the battery. Decided to throw a meter on the condensor, it wasnt reading what it should, and the points i put in worked, but not quite right. So I headed back to TSC and they must have just got in an ignition tune up set for the to-20. Changed both condensor and points and rotor button. Same thing. Runs perfect, but I get crazy high voltage on either side of the coil, whether its at the dist connection or the keyswitch. I checked for shorting between the dist. body and points connection and it checked out except of course when points are closed. New dist. cap does same thing. I cant find copper plug and coil wires, but tried others I had from the parts cabinet, no change, and the dust cap under the rotor button I tried from the old fergie thats been sleeping for a few years and nothing changed. I know this is a long post but so is the grass. Any ideas you got Id love to hear. Thanks.

It's heat and perhaps vibration.
A 6V system uses a 1.5ohm coil.
A 12V system either uses a 3.0 ohm coil or........................... A 1.5 ohm coil plus a ballast resistor.
This situation looks like a 1.5 ohm coil that is labeled 12V. With twice too much current flowing through it, the coil will rapidly fail.

I checked the voltage at the battery to check the alternator, and it is driving my meter crazy flashing up over 1000 volts,


If you are using a digital meter to check things while the tractor is running you will get strange readings. Use a analog meter (One with a dial face and needle) and see if it looks better. David.............

Analog meter. David...............

I have an old Simpson 260 myself, but I never use it. If your digital multimeter won't give stable readings in a noisy environment, it's time to chuck it and buy a Fluke.

Are you sure the coil tower is cracked and sure it's not a carbon arc?

A carbon arc is created in a dist cap or any where on a high voltage part of the ign system. A carbon arc looks like a crack but is actually carbon left behind where spark jumps on the surface of an object for a number of times or a continous amount of time thus making it easier for the spark to travel on this carbon arc than jump all the given gaps withing the ignition's system such as from the rotor to the cap then to the spark plug gap.

A carbon arc will look like someone took a lead pencil and drew a crooked line on the part where spark is jumping.

I find it very had to believe that your coil is cracked. If a coil gets that hot it will usually push or swell the bottom of the coil outward! See if the bottom of your coil is rounded instead of being flat.

Like the others say, check voltage at the battery with an analog meter.

A point type ignition system operating perfectly normal, will put voltage spikes in the system that will sometimes drive a digital meter nuts.

A few months ago I was working on my Big Mo 400 Moline, a system voltage check with a digital meter showed totally bonkers system voltage readings. I laid the digital meter away and grabbed my cheap battery load tester with an analog meter. system voltage now read a rock steady 14.4 volts from the Delco 10SI alternator.
Nothing was wrong with the tractor, just Voltage spikes from the ignition system confusing the Digital meter.

As for carbon arc, no Im sure its a crack, the second one that cracked I actually could see the oil leaking from it. Good idea though. Dave, thanks for the pic. I know very well what the difference is between analog/digital, but Im sure some dont. Strange enough about the pic of the Simpson meter, one of my girlfriends uncles who passed away about a year ago, his name was Dave, first showed me how to work on the TO-20 and the 8N, and has that exact meter and I thought about asking his daughter if I could borrow it. But I found one in my shop to do the job. Didnt get a chance to go down tonight, but I gotta get this done. Thanks again guys.

Checked things out today with the analog meter, everything is checking out ok. I think Ill brushog a bit with it, and check temps and voltages every now and then. If I find anything, Ill post just to close this case. Thanks again.