Quote Originally Posted by Shawn McGinnis View Post
Are you seeing this on your mag check procedure? Because you are the first aircraft in our 20 years that it doesn't work this way.

If your engine is running both mags are spinning on the same gear. If they are spinning on the same gear then the magnet attached to the shaft on both mags must pass by the hole use use for RPM pickup no matter which mag you place the sensor on. Grounding the mag just keeps the coil from building charge and has nothing to do (besides keeping engine running) with the RPM output.
I'm an idiot. 38 years as a mechanic and I ask this stupid question. Sorry to waste your time. I was overthinking the problem.....