coils going out

yall been so helpfull in the past I gotta ask another question. I got a 1953 WD Allis with a one wire alternator and distributor with a 12v coil. theres also a balast resister hooked up as well. The tractor runs great but the coils keep going out. I think there over heating because ive noticed oil leaking from the last one. this last time I changed the balast resister and coil and so far no trouble but I havent really used it since then either.

The guy at the parts store told me that all coils are the same but I dont think that they are. First off Im sure a 6v coil is different than a 12v coil. then I think some coils are required to have a balast resister while some will work with or without an external resister. I also think different coils have different ohms of resistance. Im not 100% sure though maybe somebody on here can share their wisdom and educate me some.

my question is of course why would the WD be burning up coils?

Should I be using a 6v coil? currently ive got a 12v on it.

Thanks for any and all information

HEI coils used in the 1970's are different, yet, the basic 6v coil is effectively the same as the 12v coils. the difference being the primary current coming in, which effect the secondary current going to the plug, but the actual coil is the same.

Get a "true" 12 Volt coil that needs NO external ballast resistor (NAPA IC14-SB or equivalent).

6 Volt or 12 Volt "use with external ballast resistor" coils have a primary resistance in the .75 to 2.75 Ohm range, "true" 12 Volt coils needing NO external resistor have a primary resistance in the 3 to 4 Ohm range. (ALL these values are approximate.)

First of all, 6 volt and 12 volt coils ARE NOTTTTTTTTTTTTTTT THE SAME what the parts dude told you ("all cois are the same") IS WRONG AS RAIN. A full true 12 volt coil has "around" 3 ohms of primary resistance while a 6 volt coil has about half that, "around" 1.5 ohms. THESE VALUES VARY SOMEWHAT BUT THIS GIVES YOU THE BIG PICTURE.

One of the main design limitation parameters in old tractor Kettering points and coil ignition systems is limiting the current the points must swtich to the 4 amp and less range, otherwise the points burn up too fast.

THEREFORE:

In a 6 volt system, the 6 volt coils low voltage primary winding resistance is around 1.5 ohms, meaning the current (the points must switch) is 6/1.5 = 4 amps

In a 12 volt unballasted system, the coils low voltage primary winding resistance is more like 3 ohms, meaning the current (the points must switch) is 12/3 = again 4 amps.

Now, some 12 volt tractors still used a 6 volt (1.5 ohms) coil BUTTTTTTTTTT they added a series 1.5 ohms EXTERNAL BALLAST VOLTAGE DROPPING (12 to 6) RESISTOR which dropped 6 volts leaving 6 on the coil as designed for, so the points still only switch 4 amps and alls well.

NOTE: Coils labeled "12 volts" or "12 volts NOT for use with ballast resistor" are full true 12 volt coils. HOWEVER a coil labeled "12 votls requires ballast resistor" or a coil labeled "6 volts" ARE 6 VOLT COILS and require the ballast on a 12 volt tractor but not if only a 6 volt tractor.

OLD WIVES TALES: Some mistakenly refer to some 12 volt cois as being "internally ballasted" HOWEVER if you disect that coil, on most, you WILL NOT FIND A DISCRETE STAND ALONE RESISTOR HIDDEN AWAY SOMEWHERE INSIDE THE CAN. The primary winding resistance of around 3 ohms is achieved due to more windings/turns or higher resistance wire NOT A RESISTIR HIDDEN INSIDE THERE.

If a coil overheats its because its drawing more current then it was designed for and cant effectivelty dissipate the heat.

IF YOU HAVE A 12 VOLT TRACTOR AND ARE USING A 6 VOLT COIL AND IT OVERHEATS, YOU MIGHT WANNA CHECK THE BALLAST RESISTOR OHMS VALUE (around 1.5 ohms or a bit more) OR INSURE ITS WIRED RIGHT AND EFFECTIVELY IN THE SERIES CIRCUIT BETWEEN IGN SITCH AND COILS INPUT.

Or else you can toss the ballast and use a full true 12 volt coil (around 3 ohms primary resistance) subject to any starting ballast by pass circuitry some tractors use

CHECK THAT BALLAST OHMS AND ITS WIRING

PS Modern solid state switched electronic ignitions are NOT the same animal, some use more of a higher voltage switched pulse to fire the coil instead of the old tractor mechanical points systems

Nuff said, hope this helps

John T retired Electrical Engineer

I really want to thank yall for the information. I put a true 12v coil on it actually went to NAPA and got the one that was suggested. I left the external resister on it though but I cut a 10 acre field with it today and the ole tractor ran like a champ. I'm going to run her some more tomorrow and if all goes well I'll feel pretty safe that the problem has been fixed. Thanks again everybody I sure am glad there are sites like this out here with smart people to answere my dumb questions. Thanks again