Try it with an analog voltmeter
Alright I'll give ya that would probably work with a analog meter, but I don't know many people who have them any more particularly when you can go get a cheap DMM for $10 and though I haven't tried it I can bet it would make these go crazy. My uncle taught me the method I described and it does work, and I'd venture to say he's rewired close to 100 antique tractors installing or checking the ballast resistors like this.
However I'm confused about your last comment.
Yes, the external-resistor 12 volt coils have a different primary resistance than 6 volt coils. The resistor cannot drop enough voltage to protect the points if it's used in series with a 6V primary winding.
Are you saying there are three types of ignition coils. a 6V, 12V, and 12V with external resistor? Cause I've only seen two. The 12 Volt coil (with internal Resistance) and the 6V. And saying that the 6 Volt coil works fine attached in series with a resistor. I've got 2 1947 WC's running with a ballast and a 6V coil and a 1947 2-ton running with a ballast and 6 volt coil and my uncle has about 15 tractors running this way. I have to change the points maybe once every 3-5 years and that's not chronic failure, according to manufactures your supposed to replace points yearly. You do have to make sure the dummy's at the auto-parts store gave you the right resistor through. They make two types one that is meant more for accessories (less resistance I believe) and the other that is for ignition coils.
