Replacing a WD Coil

The coil I bought says it needs an external resistor. Did I buy the wrong coil?

Also, the old coil had no polarity markings, but it did have one of the 2 terminals marked "Bat" for battery. A wire went from there to the battery, of course, and the wire on the other terminal went to the rubberized terminal on the side of the distributor. When I had checked the voltage from each terminal to the chassis, both showed 12V.

The replacement coil they sold me at the auto parts store has one terminal marked + and one marked -. Just wondering whether this coil will work and if so, whether I wire it the same as the other and whether I need a resistor. Thanks for any help!

NO you did not buy the wrong coil, yours will work fine IF YOU ADD A BALLAST RESISTOR.

1) If the new coil states it requires an external ballast, that means its a 6 volt coil (around 1.5 ohms primary) so you need an external series voltage dropping ballast resistor which drops 6 of the batterys 12 volts (in the ignition primary circuit) leaving 6 volts available across the coil as it was designed for. The ballast should ideally be the same resistance of your coils primary winding (as measured between its lil + and - terminals) which is likely somewhere around 1.5 to 1.85 ohms and rated around 25 watts minumum. They are available from most tractor or auto supply shops.

2) On the old coil the BAT side simply meant that was the terminal which received battery voltage (via the ignition switch when ON) while its other terminal wired to the distributor.

3) If youre now at Negative ground, your new coil receives power from the battery and ignition switch and series ballast on its + terminal, while its - terminal is what wires to the distributor where it gets to ground via the points when they are closed. It still gives a spark if wired in reverse, but its not as efficient and may run warmer.

Sooooo oo, Yesssss sss it WILL WORK and YES you need to add an external series voltage dropping ballast resistor (around 1.5 to 1.85 ohms) after the switch and before the coil or else it (though she will fire fine) will overheat and the points n condensor wear out sooner cuz you woule be switchingn twice the current they were designed to switch.

PS when you switched polarity, did you reverse the ammeter leads??? and re polarize the Generator?? Need to do both.

John T Norhoff in Indiana, retired electrical engineer

The other thing that"s confusing me is that on a negative ground system I think of negative as ground. The negative on this coil must not be equivalent to ground, right? On the old coil, I could measure +12V between EITHER terminal and ground. Pardon my ignorance!

The ballast resistor limits primary ignition current to keep the coil from overheating, and the points from burning prematurely. A coil designed for use with a ballast resistor will have a lower-resistance primary winding than a coil that doesn't need a ballast resistor.

The supplier of your new coil (NAPA or similar???) should be able to come up with a ballast resistor that matches your coil, as there is SOME variation in ballast resistors.

It is desirable to have the leading edge of the secondary ignition pulse at the spark NEGATIVE, and that is where the coil polarity markings come in.

For a system with the battery's (-) terminal connected to the machine's chassis (negative-ground), the coil's (-) terminal goes to the distributor, where it "grounds" through the breaker points. The coil connections are opposite for a positive-ground system.

Your original coil was a bit more "idiot proof", with it's "BATT" and "DIST" markings. The problem with that, though, is that those markings are correct only when the coil is used with an ignition system of the ground polarity it was designed for... switching a coil with "BATT" and "DIST" markings from a machine with positive ground to a machine with negative ground and connecting the coil as-marked will result in a "positive" spark at the plugs, which is less effective, due to the "Edison Effect".

The polarity of the spark at the plugs can be verified with an ignition oscilloscope, or the old "lead pencil test".

So, your new coil, with it's (+) and (-) markings takes a bit more thought to install, but it can be used on both positive-ground and negative-ground systems with the correct results.

Now, for your last question "The other thing that's confusing me is that on a negative ground system I think of negative as ground. The negative on this coil must not be equivalent to ground, right? On the old coil, I could measure +12V between EITHER terminal and ground. Pardon my ignorance! "

Well, when you were measuring the voltage at the coil terminals, the engine must have been stopped at a point where the breaker points were "open".

When you measure voltage at the coil's two primary terminals, with the breaker points "open", they will indicate the same voltage.

Rotate the engine a bit (safely, ignition "off", bodily appendages out of belts, fan, etc., and transmission in NEUTRAL), until the points close.

Then, the voltage at the distributor side of the coil will drop to (almost) zero.

I hope this helps a bit with your questions!