primary circuits

True on the primary connections being positive and negative.

The round coils and the stand alone E-core coils are true auto-transformers, the primary is tied to one end of the secondary (can be primary + or - depending on the specific coil), testing with an ohmeter is primary to primary and one primary to the secondary.

The GM coil in cap HEI coils are true transformers in that there is no common connection between the primary and the secondary, ohmeter testing is primary to primary and secondary to secondary. FYI, one end of the secondary is usually tied to ground (though there were variations in the mid 70s) DIS ignition coils are also true transformers with the primary and secondary separated from each other.

Thanks for the info. I was thinking about checking the voltage on the coil primary circuits, with a test-lite. One side goes hot when you turn the key on, actually, I guess both sides go hot, some makes I believe, the engine has to be cranking to get any voltage. Then with the engine cranking, one side should have a switching voltage. I guess on a DI system whether the vehicle is new or fifty years old, you still check for primary voltage the same way? thanks---I always thought that was interesting stuff.

Here is an interesting article you might like:

Thanks for the article, it was interesting.