Originally Posted by
rickair7777
One mag powers the top spark plugs, the other the bottom plugs so I suppose that might make a difference?
This caught my eye, don't mean to be a persnickety A+P, and I'm not going to vouch for all installations, but on all the horizontally opposed engines I've worked on this ain't true.
An example- an IO-520's Right mag runs the top right plugs, and bottom left plugs. Left mag runs top left and bottom right.
If for some reason you replace a wiring harness, the easiest way to avoid screwing this up is to search out a wiring diagram for your exact installation.
Good advice above in that you want to make sure there is a drop in rpm when doing mag checks, if you don't get one, could mean the magneto or magnetos are not grounded, and when turning a prop by hand to do stuff like hook up a tow bar, could lead to a mishap.
Another bit of unsolicited advice- never turn the master on with people or objects in or near the prop arc. A sticky starter solenoid can cause the prop to turn just as if you'd hit the starter intentionally. Not fun. Seen it happen. Nobody hurt but they got scared pretty bad.
Cheers,