Farmall H problems, cooking coils

Have a rebuilt engine and carb. Was on an hour long tractor drive and it started running badly. I let it cool down and ended up plowing for about 30 minutes when it quit. I put in a new coil and it started up just fine. Weeks later I had it running full throttle, after a couple of hours of running just fine it just quit. Pulled the coil wire, dead and the coil was very hot. Put on a new coil and it started right up.

It has a 12 volt converstion, with 38amp alternator with internal regulator. The first coil was 12v with an external resistor. The second coil was 12v with an internal resistor.

Question: Is there anything with the electrical system that might be cooking my coils, or did I just happen to get a couple bad coils in a row?

Was the second coil brand new and marked on the outside as internal resistored?

Just had the same problem with a Bobcat 610 with a Wisconsin V4. The coil that was in there was a ext. resistor. The spark wouldn't jump .030. Replaced it with a new internal resistance coil and it runs at the present time.

"The second coil was 12v with an internal resistor."

NOT exactly. NO such thing as a modern 12 Volt coil with an INTERNAL resistor... they are simply wound with more wire, making them compatible with 12 Volts WITHOUT the need for a resistor, internal OR external.

Trouble is, it looks to me like the folks from "the land of almost right" are getting it WRONG sometimes nowadays and cheating a bit on the extra wire, making the coils draw more current and run hotter.

Also, if the charging system is putting out over 14 to 14.5 Volts, the coil will draw correspondingly more current.

A breaker point gap that is LESS than "spec" will result in more dwell = more coil heating, as well.

Are you set up to accurately measure primary ignition current... engine "OFF", ignition switch "ON", breaker points CLOSED?

If so, check that out. A typical measurement should be in the little-over-three Amps to about 4 Amps range.

A reading much lower than that will probably mean weak spark, hard starting, and possible misfire, and a reading OVER about 4 Amps is telling you your gonna have a HOT coil and shortened breaker point life.

If the reading IS over 4 Amps, you can try adding a ballast resistor (there are lots over different values out there) to get the current in the desired range. This MAY solve the problem IF the coil hasn't been permanently damaged by the overheating.

If so, you'll have to start over with another coil, and repeat the Amperage draw test to see where you're at.

Well probably been getting them cheap from the land of not always right coils so you never know. That said it can also be not having it set up right when you had the 6 volt coil and ballast resister. Incorrect resister equals coil going bad. The 12 volt one may have been a new out of the box junk part which happens a lot now days. Buy a good brand not a cheap brand of coil with a good warranty

yes, the second coil was new, brand name was "Standard" and it was marked 12v with internal resistor.

Part #?

Standard Auto Parts #UC15
"12v no external resistor required" is what the auto parts guy just said was printed on it.

Ah but did you read that on it or did he just say it was there. I have found that some times they say one thing and you end up with the wrong part because he grabs the one next to it and not the one he was suppose to grab.

First off I agree with Ol Bob there below. That being said, Id feel more comfy if I actually knew the coils primary winding resistance as measured between its little + and -terminals. If its in the vicinity of at least 2.5 prefferred 3 and over ohms, Id say it should be suitable for 12 volts no ballast required. Some lay persons call those "internally ballasted" or with internal resistor
(which isnt so, see

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=ac&th=138584 )...........

HOWEVER if its less then 2.5 ohms, especially if its more like 1.5 THEN ID CALL THAT A 6 VOLT COIL OR "12 Volts for use with or requires external ballast" and Id expect it to overheat plus the points burn up quickly..

The other possibility is if the coil is only marginally suitable for 12 volts (say its internal primary resistance is 2.5 ohms) and the alternator is taking the battery up to over 14.8 volts THEN THE COIL MIGHT OVERHEAT

Or it may just be a new but still bad coil

John T
Coils with Internal Resistor BUNK

have you checked the voltage that the alternator is putting out? hook up a voltmeter on the alternator with the tractor running at about half to 3/4 throttle and see what you are getting. i'd also try an external resistor in the coil and see if that eliminates the problem. napa has one that it the mid 70's chrysler automobiles.