So John,

Is it possible that it’s the right mag that’s defective? You’d not be the first airplane that had the two P-leads connected to the wrong terminals at the switch, indicating that you were grounding the right mag when it was the left that was being grounded by your switch movements… Just a thought from Homer S… J

Bob

I'm awarding myself the Homer Simpson Award for Excellence in Magneto Troubleshooting. Doh!  On the Internet I found the Bendix S-20 Series Magneto Overhaul Manual . Page 17 of the .pdf reads:

"Primary resistance between breaker and ground leads shall be 0.2 to 0.6 ohms. Secondary resistance between high tension terminal and ground lead shall be 12000 to 16000 ohms."

Luckily, my A&E hadn't sent the mag back to the shop yet so I took an accurate measurement with my Fluke and the primary resistance turned out to be 0.4 ohms, smack in the middle of the proper range. I apologized profusely and slunk back to my hangar with the left mag.

But this saga isn't over yet, because I checked the right mag and it reads open at the P-lead terminal. To be continued...