Ignition Coils, Still Confused?

There is a trick involved -- when the power is first applied, the resistor will have a lower value than after the power has been on for a while. The resistor heats up, increases in value (more resistance) and as a result, drops more voltage. When cold, this allows more power to the coil when starting and helps make up for the voltage drop caused by the starter drawing lots of power.

If you convert to a 12-volt system you can use the old coil if you add more resistance to the primary circuit -- either by replacing the OEM resistor or adding another resistor.

You can also use a 12-v coil (which may or may not need any resistor). If your 12-v coil needs a resistor and you don't use one, the life of the coil will be greatly reduced. If your 12-v coil doesn't need a resistor and you use one, the intensity of the spark will be reduced and may cause hard starting.

The type that is used with an external resistor is really a 14v coil. The tractor's ignition system will either have a resistor mounted to or near the coil or use a shunt in the wiring harness at the starter to cause the full 14v to flow to the points only when starting the engine. This produces a hotter spark and helps with starting in cold weather. Once the engine starts, the shunt causes only 12v to flow to the points which prevents them from burning up.

The type that is used without an external resistor is a true 12v coil and must never be used with a resistor or shunt.

The reason that you may want to use a ballast resistor is to prolong the life of your points. When a ballast resistor is not used there will be more arcing across the point gap when it is opening. The resistor will reduce this problem. Put in a resistor anyway. If it starts OK leave it in. It only costs $5.

The next item I plan for my gas tractor is to use solid state ignition. I won't have to concern myself with burning the points and the ignition spark will be much more intense. Thus, I can run wider gap in the plugs, more complete burn, less missing, and easier starting in cold weather. With solid state ignition the engine will "jump" to life. It'll be running before you can get your hand off the starter switch.

Be careful about using a coil that doesn't use a resistor. It may be one designed for solid state ignition. You'll burn your points.

Ken McWilliams

I had some electrical schematics to use the points, but run the wire from the coil through a MosFET to do the switching. The spark of any coil will greatly be increased if the points are not used to make and break the primary circuit on a coil. The points will only be used to signal the MosFET to do the switching. If done this way, a home solid state ignition can be made for less than $15 in parts.

With principle of inductance as in a coil, the faster the current is shut off, the greater the intensity of spark. When mechanical points are used the arcing begins before totally opened, and the "shut off" is not as sharp. That's why you can only gap the plugs at .025" with points. When solid state switching is employed, the gap can be geatly increased to .040" and upwards to .060".

Using a solid state off of an auto I think is too bulky and awkward. I think I would only consider using an HEI off of a GM product if I would ever try it. The other manufacturers used auxilliary boxes. For my John Deere 3 cylinder, I would have to one off of a 6 cylinder. I would probably take the whole 6 cyl distributor and make it fit onto the cam gear and engine block of the John Deere. Just use every other spark plug wire location and remove every other pickup projection on the shaft.

Ken McWilliams
Dayton, Oh

Years ago when I was racing, I would use dual point distributors. (I still have one somewhere in the garage).

The postion of the points assemblies would be slightly staggered to increase the dwell time, and still have large enough gap to break the circuit properly. When I tuned it, I would have to insert a piece of plastic between one set of point to hold them apart while I set the dwell on the other. Then do likewise on the other set.

Back then that was the only way to get enough dwell time to saturate the ignition coil for high RPM's. Then the other problem at high speeds was point bounce. Solid state took all that away.

KRM