What's up with these 8N coils????!!!!

I have from time to time jumped it using a 12-volt running truck, and some say this is bad, some say no problem. Any how, coils seem to have failed without having endured a 12-volt jump and people tell me they've fixed all kinds of old and new tractors and have NEVER had to replace a coil. Can anyone help me before I give up?

Gary Clark

• the real truth about ignition coils

Either you are running a coil that requires an external resistor on 12v, or you have an incompatible coil/condenser.

While the engine is running check the voltage at the positive side of the coil - it should be in the neighborhood of 6v. If it is, then I would replace the condenser (again if necessary) being sure to use the correct part by production date as your coil, and in your case the wiring harness.

If you have 12v at the coil while running there are a number of possibilities. One is that your tractor was designed to use a coil with an INTERNAL resistor. If this is the case there will be no external resistor in the circut, and from your description it sounds as if there is. The second possibility is that there is a problem in the wiring, possibly a by-pass wired in by a previous owner. The third possibility is that whatever component serves to switch the voltage feed from 12v to 6v is malfunctioning. On some applications the ignition switch has contacts to provide this function, on others either the starter or starter relay provides the switching function. A good wiring diagram will be helpful - hopefully one came with your new harness.

The reason for all this is that ALL ignition coils are essentially 6v coils. (Yes I know there are 12v coils but all these really are is a 6v coil with an internal resistor). On 12v vehicles that don't use an internal resistor coil, some form of external dropping or ballast resistor is used. Chrysler favored those big white porcelean resistor most of us are familier with. GM used a dedicated wire from the ignition switch to the coil which had a high resistance built in (if this wire is replaced with regular wire the problem of short coil life will be the result). Ford used both systems. Engineers took advantage of this situation by creating by-pass circuts that provided 12v to the coil when the engine was being cranked. This provided both a hotter spark and insured that even with a low battery a minimum of 6v would reach the coil. In some instances this altenate circut is energized in the ignition switch, some are done in the starter relay, and others use a contact in the starter.

Hope this helps

Gregg

When I replaced my harness yesterday I also replaced the resistor, so perhaps that will do it. I have a basically un-
tampered with 6V system (although the harness was in very rough shape and there had been some jerry-rigging over the years), and it sounds as though some of the repsonses favored upgrading to 12? I'd like to keep it as is if I can ... will get one more coil tomorrow and give it a whirl.

Could I be using the wrong coil? It's the kind in the black plastic casing that fits on top of the front-mount and is attached by a bail. Everywhere I've looked it's been the same thing, made in Taiwan.

Thanks again for everyone's help and if you or anyone else has any other ideas, I'd love to hear them!

Gary Clark

With a little electric knowledge and the DMM, this could have been solved in a matter of minutes. Basically you test for voltage across the coil with the engine running. If the tractor has a ballast resistor (or should have one per the wiring diagram) you should see about 4.5v at the coil (about 9v for a 12v system). If no ballast is present or called for, you should see a full 6v (or 12v) at the coil. I.E.: with the switch in the "S" position, you should see full battery voltage at the coil; when the switch is in the "I" position, you should see the ballasted voltage at the coil. Ballasting is usually done to allow the engine to start more easily in cold weather... full voltage gives you a hotter spark and makes up for the affect of cold temperatures on the battery.

Having said all that, if you have the engine tuned on the money and don't live above the Artic circle, the ballast can be removed and a coil marked "for use without external resistor" can be substituted. I think you can still get such a 6v coil at NAPA or Tractor Supply, etc. This is a bulletproof solution as there can be no mismatch between coil/ballast.

Let us know how it turns out.