How to properly ground a dual magneto?

We have been batteling a recurring issue with a customer of ours, with burnt contact assemblies and capacitors long before they were due to be replaced. Recently we visited the customer, and inspected the installation as part of other issues with that aircraft, to find that the P-lead shielding was peeled off about an inch from the bead nut, and there was no actual grounding between the p-lead and the mag. Now from what we understand, the grounding path in the airframe is from the capacitor to harness housing, through the magneto clamps to the engine, to the engine frame, then to the airframe, but if there are any issues in that chain, the magneto would not ground out properly, like a magneto pressurization gasket, correct?


We would simply like to know what is the best and most efficient way to ground the magnetos to the airframe, and what resources would you suggest.



Scrape away the paint from the harness cap (outside) where the capacitor mounts and this should solve your problem.

D-3000 magneto cap showing capacitor mounting towers

This has solved the same problem others have had. I can't explain why the capacitor isn't adequately grounded to the inside surface of the cap which means I don't fully understand what is happening. Thus I have to rely on past reports of solutions to the same problem.

The magneto need not be grounded to the airframe to complete the circuit - engine will run without any p lead connections - however if the capacitor isn't being grounded through it's mounting then the shielding can provide an alternative grounding path.

I have also seen the capacitors fail from the magneto getting too hot. In this case the rubber capacitor legs were toasted.

There should be plenty of ground (return path) between the magneto and engine through not only the magneto mount clamps but also the shielded ignition leads so there is no reported problems of the engine not firing because of a faulty ground. The capacitor is another story - the magneto will run and fire the plugs without a capacitor even installed so I treat the capacitor as a closed-loop circuit internal to the magneto - or in this case magneto + harness cap. The cap, even if insulated from the magneto by a flange gasket and painted screw holes, should still have a ground path through the shielded ignition leads. But I have to admit it appears otherwise in cases like your's. I think you are on the right track - fix all the grounds, p-lead insulation grounded at both ends, remove the paint from under the capacitor, and possibly make sure the screws holding the cap to the magneto are contacting metal.

 

D-2000 / D-3000 magneto capacitors

Inside view of D3000 hoarness cap