John,
A number of possibilities, but first and easiest to check, is to be sure the p lead is not grounded where it exits the mag. A washer or wire lug that touches the mag case when tightened will short it, as will leaving off the internal insulating washers and tube that keep the threaded post away from the case. Also, the mag coil has very little resistance. It will show as "grounded" if the points are closed. You can separate the electrical components by disconnecting them where they attach to the points. Hope this helps...

BOB

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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.


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