I am working on a few coils for a friend. one coil shows a open circuit between the side contacts but still has 16 nice blue sparks on my hand crank coil tester. This is one of the junk coils with the square laminated core. How is this possible?
The open N the secondary windings is small enough that the spark jumps both gaps. The coil is still a paperweight because it won't last long as that gap in the secondary winding will get worse.
What Eric says. As the coil is used, the arcing across between the ends of the broken wire will gradually erode the wire so the distance between the break gets larger and eventually there's no spark.
It is practical to repair such coils, but only if the break is within the top couple of layers of winding, and you can see where it is - remove the pitch around the winding and see if you can see it arcing.
Or maybe your meter is on the wrong scale.