Here is the response I got from Champion Aerospace. It doesn't mean much without my note to them, so I also include it. I wish he had distinguished between the center P-lead wire and the shield.
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Bill, Your IA is correct, we do want the mag end ground floating and the P-lead should be grounded at the ignition switch.
From: Bill Garnett <
interstellardust@yahoo.com>
To:
slicksupport@champaero.com,
Date: 04/04/2014 02:41 PM
Subject: Proper grounding for Slick 6364 Mags
I own a 1956 Cessna 170B with a Continental 0-300-A engine equipped with Slick 6364 magnetos. During my annual inspection I discovered that the engine runs fine with the switch in the off position. In replacing the (original) P-leads I discovered there are many opinions about how the mags should be grounded. Can you please tell me what the proper method is and or point me at the appropriate technical bulletin/wiring diagram? I am not an A&I but there is disagreement among those I have consulted.
My current A&I, (no pun intended) insists that the P-lead shields should only be attached at one end, either to the ignition switch ground terminal, or to the ground screw on the mag casing. This seems wrong to me since new P-leads (ordered from Aircraft Spruce) come with ring connectors at both ends for both the P-lead and the shield. It would seem the intent is to use the shield to ground the switch directly back to the mag. Several mechanics I poled agree.
A little history. The airplane came equipped with Bendix mags. The original equipment P-leads had an insulated center wire surrounded by a woven metal shield. the shield was connected to the ignition switch at the G terminal and to the hex nut fitting at the mag end. There was no ground wire from the G terminal of the switch to the airframe. In other words, the shield did indeed provide the ground from the switch back to the mag.
Apparently, when the Bendix mags were first replaced with Slicks in the 70s someone (not me) just clipped off the hex nut connection, skinned the woven metal shield back an inch and reused the original P-leads. This should not have worked as the switch itself is not grounded and the shields were no longer attached to the mag. It looks like it only worked as the metal shields had been zip tied to the instrument panel bracing which provided the ground. Now that's a scary thought!.
My A&I installed a ground wire from the G terminal of the ignition switch to the back or the instrument panel. The shields are attached to the G terminal as well. He actually cut the Shield connectors off at the mag end so they could not be connected. This setup works but depends on multiple connections from the switch to the instrument panel to the firewall to the ground strap around the Lord bushing to the engine mount, then via ground wire around the forward starboard isolation mount to the engine block to mag case. That's a lot of connections.
What is the correct method?
Bill Garnett
Cessna 170B N2974D SN: 26917
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I should point out that while the woven metal shields on my original P-leads were very robust and could certainly have grounded the primary back through the switch to the mag ground screw, my new certified P-leads seem extremely wimpy by comparison. I'd be surprised if the shields are even 22 gauge wire. Just looking at them I don't trust them.
While I doubt if it would hurt to add another wire from the G terminal of the switch back to the mag ground screw, it would only be providing a redundant ground path
