M24 Chaffee Ignition Coil Question

[Malcolm Towrie]

I have tested the coils on our Chaffee and found something I don't understand. Every other coil I have tested, the secondary coil resistance is measured between one of the primary coil terminals and the HT post, usually measuring in the 7-11 kohm range. And measuring between the HT post and the casing shows infinite resistance. Clive Elliott specifically says this is normal in his ignition system paper.

But the coils on the Chaffee are quite different. The resistance between the primary terminals is 2.4 ohms, which is normal. But the resistance between one of the primary terminals and the HT post is infinite, so there is no connection between the primary and secondary coil. I can only measure the secondary coil resistance by measuring between the HT post and the coil casing. One coil shows 8.8 kohms, the other 10.0 kohms. I thought the coils were bad, but I bench tested both coils and they both work well, making strong sparks on a tester set at 3/8" gap.

From what I have read, having separated primary and secondary coils does not take advantage of the back-EMF amplification effect.

What am I missing here? Did coil design change in the 1940's?

Malcolm