Pmag grounding

[jfr3]

Looking for info on grounding dual pmags on a new io-540. Emag manual said it should be grounded to the panel ground whereas general community says it should be to the engine case. Others say both. I understand the engine case would keep it always running if you lost ground from the neutral bus. Questions:
1. Is there a case for grounding to both and why?
2. If you ground to both should it be two separate splices on to the pmag harness to share the same splice?

 

[sansoneservices]

Someone will add to this but my opinion is they assume your interior avionics/panel ground is "cleaner" and possibly less likely to become intermittent. Theoretically ground is ground. One exception is running sensitive unshielded wires near a source of RF interference

 

[rv8ch]

The 4 cylinder mag shows ground to the engine case, and the 6 cylinder mag shows ground in the cockpit. I recommend doing it as they document.

 

[paulsmeds]

Just a thought, but is it possible that when they designed the 6 cylinder version they grounded internally to the mounting bolts for the coil while the 4 cylinder they didn't? The ground to the panel for the 6 version could be to make sure any communications, tach signal, etc. are clean to the avionics. This is just a guess why the wiring diagrams are different.

 

[sansoneservices]

When all other options fail, follow the directions. . apologies

 

[jfr3]

The 4 cylinder mag shows ground to the engine case, and the 6 cylinder mag shows ground in the cockpit. I recommend doing it as they document.


View attachment 118016


View attachment 118015

I called both Lycoming and EMags and asked the question. Lycoming said they can’t recommend deviating off the Emag manual. The Emag tech support said to ground to the engine case and he doesn’t know why they haven’t updated the 6 cylinder install manual to reflect this. He said the issue is if the grounding strap, ground gets warn or not replaced after service, the starter will use the other ground path and burn up the pmag circuit board causing complete failure. So only one direct ground from each pmag to the engine case.

 

[jfr3]

Just a thought, but is it possible that when they designed the 6 cylinder version they grounded internally to the mounting bolts for the coil while the 4 cylinder they didn't? The ground to the panel for the 6 version could be to make sure any communications, tach signal, etc. are clean to the avionics. This is just a guess why the wiring diagrams are different.

Good thought and I asked the emag tech support that also and he said that may be the case but he wouldn’t rely on that since many mechanics and also lycoming use paper gaskets on the mags which would nullify the ground. So it seems safest to just directly ground to the engine case.