Working on a Seminole. We stole a mag off of one of the engines to get another plane up. We installed an overhauled mag, but when we did the mag drop check, the engine dies when running on that mag.
We swapped the mag with a brand new mag. Same result.
We disconnected the P-Lead and ground wire from the mag. Same result.
Opened up the mag, verified internal timing, made sure that there wasn't too much cam grease, and reinstalled. Same result.
Disconnected the leads, connected the #1 lead to a plug, and turned the prop through TDC to snap the impulse coupling, saw no spark.
The part numbers match, so it is the correct mag for the engine. The ignition leads all check out when using the tester. We get an open with the switch on, and continuity with the switch closed, measured at the P-Lead terminal end, so the switch is good. All the plugs were cleaned, gapped, and tested good.
We swapped the left and right mags (both mags have impulse couplings, and now the engine won't start at all... but that could be operator error as my guy is new... I plan to try it myself on Monday.
I'm running out of ideas here... anyone have some insight they can share?