Switch to Magneto Wiring

Looking for suggestions to run new wire from TYPE AA7 Mag Switch to Magneto's. Shielded wire has been suggested... looking for wire size, single strand or braided etc... etc...
Thx.

20 gauge, single conductor, shielded wire. Use Sumitomo solder sleeves to attach a 20 gauge jumper wire to the shield at both ends of the wire. At the switch, attach the center conductor of the right hand wire to the terminal marked 'right' and the jumper to the terminal marked 'ground'. At the right hand mag, attach the center conductor of that same wire to the P=lead terminal and the jumper to the mag body. Do all the same steps for the left hand wire.

And never bundle P-leads with any other wires.

Web

wireweinie said:

And never bundle P-leads with any other wires.

Click to expand...

Although prudent, could you expand on this statement just for my own education? (the wires are shielded, a wire failure would fail a mag open, etc..)

Noise. Shielded or not, P-leads will transmit noise generated from the magneto. If they are not bundled with other wires, there is zero chance of that noise being passed on to other circuits. This is also why the shields are only connected to the 'ground' terminal at the switch and to the mag body. If the ground path back to the switch was the airframe and not the shield on the wire, once again there is a path for noise to get into other circuits (like headset audio).

Shielding helps reduce noise, but the best defense against electrical noise is distance between the wires that generate noise and wires/components that need to be protected from noise.

Web

I was guessing that was going to be your answer, but thank you for the explanation.

I bundled my p-leads together and passed them through their own breech in the firewall keeping them away from the rest of the wiring loom. It felt a bit overkill at the time, but it does sound like that is the proper way to to do it.

That's the best way to route them.

Web

I wonder if that might cure my faint noise. The 12 volt line to the radio goes up the same post as my P leads. I shall re- route them and see . . .

Let us know what you find.

Web

wireweinie said:

20 gauge, single conductor, shielded wire. Use Sumitomo solder sleeves to attach a 20 gauge jumper wire to the shield at both ends of the wire. At the switch, attach the center conductor of the right hand wire to the terminal marked 'right' and the jumper to the terminal marked 'ground'. At the right hand mag, attach the center conductor of that same wire to the P=lead terminal and the jumper to the mag body. Do all the same steps for the left hand...

Web

Click to expand...

It seems like grounding only at the mag (as you indicate above) is the way to do it, but what if the wiring diagram is different? For example, the Cessna 180/185 diagrams show the p-lead grounds attaching to the switch GRD terminal, which then is connected to airframe ground under the panel.

Chris


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wireweinie said:

20 gauge, single conductor, shielded wire. Use Sumitomo solder sleeves to attach a 20 gauge jumper wire to the shield at both ends of the wire. At the switch, attach the center conductor of the right hand wire to the terminal marked 'right' and the jumper to the terminal marked 'ground'. At the right hand mag, attach the center conductor of that same wire to the P=lead terminal and the jumper to the mag body. Do all the same steps for the left hand wire.

And never bundle P-leads with any other wires.

Web

Click to expand...

Thx. Will report back.

airChris said:

It seems like grounding only at the mag (as you indicate above) is the way to do it, but what if the wiring diagram is different? For example, the Cessna 180/185 diagrams show the p-lead grounds attaching to the switch GRD terminal, which then is connected to airframe ground under the panel.

Chris

Click to expand...

The shield is also grounded at the magnetos in the Cessna 180/185 diagrams. Both ends of each P lead shield are grounded.

wireweinie said:

And never bundle P-leads with any other wires.

Web

Click to expand...

What about the pickup for RPM for engine monitors...etc?

skywagon8a said:

The shield is also grounded at the magnetos in the Cessna 180/185 diagrams. Both ends of each P lead shield are grounded.

Click to expand...

The point I was making is that many wiring gurus recommend AGAINST grounding p-lead shields at the switch end (ie: attach to the GRD terminal, but don’t attach the terminal to airframe ground), but if the aircraft wiring diagram says to do so, what approved data do you use to go against the wiring diagram?

Chris


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Electrically speaking, grounding either end of the P-lead to the airframe is NOT necessary for operations nor is it desirable due to noise considerations. As for the tach inputs for gauges, they are connected at the mag switches (not the magnetos) and are routed direct to the gauge. Short lengths and shielded leads are generally enough to eliminate or minimize any noise.

As for the 'approved diagrams' issue, it's a rabbit hole. Absolutely no one follows diagrams or even repair manuals down to the last detail. Techniques change and improve. If you were doing a sheet metal repair and used countersunk rivets instead of universal heads, or vice versa, you wouldn't even give it a second thought. Tight space behind a panel? Pull Cherry max's in place of the solids. If you follow this line of thinking, would it be an unapproved repair if I used nylon lacing on the wire bundles if the factory used zip ties?

If some fed comes along and questions me about wiring P-leads as I've described, my fight would be that a) this technique works better than using the airframe as a ground path and b) the system has not been redesigned as there is a direct, unbroken path from the P-lead terminal to the switch and from the switch back to ground at the mag body. It's a fight that I can win.

Web

You can MELT your pleads if you engine ground fails, and the plead shields become the ground.....also applies to primer lines and tach cable housings.


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