Battery power goes to the coil directly (well, through the fuse, keyswitch, and kill switch) and the Dyna switches the ground connection to power up the coil. You will see 12V power at the junction to the Dyna wires when the "points" are open. But that's not much of a crank rotation with a Dyna S, you should measure a volt or less most of a turn, going to 12V when the timing marks align. Spark happens just when the ground connection is broken by the Dyna.
If the Dyna is not powered, you will always read 12V there... are you sure it has power?
