Don't forget that the P-lead is almost shorted to ground all the time. When the points are closed it is shorted and when the points are open the resistance is something like 0.7 ohms. Easy to misinterpret that as being "shorted." That might be your problem. If the mechanic timed the mag with a buzzer, it shows the mag was certainly good.
Gary Casey
As part of bringing my Lycoming O-290-D2 into AD compliance, My A&E had
the mags
rebuilt at a shop in Ohio. When they returned, he reinstalled
them on the engine and timed them. Today, after I hooked up the new
Bogert P-leads, I was ohming them out to insure that I had the ignition
switch wired up correctly when I discovered that the Left mag was
shorted to ground where the hex nut P-lead connects to the mag.I
eliminated all possibilities other than something internal to the mag.
What could cause that? The A&E mentioned something about a metal finger
down in the bottom of the hole. Anyone know what that does? Could a
shorted capacitor- the one that was supposedly just replaced - cause
this problem? My logs are at the hangar but I think this is a S4LN-21
mag. Any advice would be appreciated, 'cause I'm getting real tired of
going backwards on this project.