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Wiring the magneto

K

Kato's 8

Well Known Member
Hello friends!
I have the p-leads run and terminated with ting terminal. The shields are ready to be grounded too. I see where the p-lead attaches to mag ok but not sure about the shield. Do I just attach it yo the grand screw on mag or do I need to run it yo firewall? The grand screw on mag feels like it gets tight as I try to unscrew it and I don't want to mess up threads or mess something up trying yo remove it if it's not supposed yo be removable. Here is a pic of mag just for a visual. The main wire gives to the bigger screw and the grand screw is just beside it.
72A4F548-3118-46F8-ABD8-DD8B8B06EE7D-4514-000003EE122A3F9F_zps8ee97288.jpg
 
It is just a screw - remove it. It is a hex though, not phillips. I am doing this now as well. I am following DanH suggestion, but flipping the wire paths to allow for the recessed mag post. It is powder coated under that ground screw and I plan to clean down to the metal case for good conductivity.

I am having a little trouble locating an all steel "adel" clamp. I might have to make one.

http://www.vansairforce.com/community/showpost.php?p=711896&postcount=15
 
Thanks Gil. The photo pretty much confirms what I was gonna do anyway. I feel better now. That smaller screw for the shield pig tail does come out all the way I hope. It just feels like its binging up dome as I loosen it.
Another question: the shield in photo is terminated on both ends. In Vans wire harness, the ignition switch end has the shield cut off and not terminated. But shows in the drwg to ground shield at the magnito end. I know a similar thing was done with the intercom wiring but does this way work well with the ignition?
 
Kato's 8 said:
Thanks Gil. The photo pretty much confirms what I was gonna do anyway. I feel better now. That smaller screw for the shield pig tail does come out all the way I hope. It just feels like its binging up dome as I loosen it.
Another question: the shield in photo is terminated on both ends. In Vans wire harness, the ignition switch end has the shield cut off and not terminated. But shows in the drwg to ground shield at the magnito end. I know a similar thing was done with the intercom wiring but does this way work well with the ignition?
Click to expand...

The smaller screw is, as noted, just a screw, BUT...IIRC, it's a Torx head, so make sure you're using the correct tool to get it out (NOT a Phillips or cross-point screwdriver). Might be a hex head, but I seem to recall that it's Torx.

I don't know about Van's harness, but you need to understand how the mags work. The circuit is from the mag (the larger connector), through a switch of some kind, and back to the ground (the small screw). Using the shield as one "wire" in the circuit just a) makes it simple, and b) helps keep RF noise out of your radios, etc. (if I understand correctly). The key point here is that to turn the mags OFF, you need to have a complete circuit from mag, through the switch, back to the ground. That is, the mag MUST BE GROUNDED to be off.

To turn the mag "on" (make it "hot"), the switch actually opens the circuit (the mag is ungrounded). Not connecting the other end of the shield to the switch will ensure that that the mag is hot *all the time*, a dangerous condition.

If this is unclear, get someone to show it to you. It is essential, and a critical safety item, to make sure your mags are grounded when the mag switch (however you do it or whatever switch you use) is in the "Off" position.
 
RV7A Flyer said:
The smaller screw is, as noted, just a screw, BUT...IIRC, it's a Torx head, so make sure you're using the correct tool to get it out (NOT a Phillips or cross-point screwdriver). Might be a hex head, but I seem to recall that it's Torx.

I don't know about Van's harness, but you need to understand how the mags work. The circuit is from the mag (the larger connector), through a switch of some kind, and back to the ground (the small screw). Using the shield as one "wire" in the circuit just a) makes it simple, and b) helps keep RF noise out of your radios, etc. (if I understand correctly). The key point here is that to turn the mags OFF, you need to have a complete circuit from mag, through the switch, back to the ground. That is, the mag MUST BE GROUNDED to be off.

To turn the mag "on" (make it "hot"), the switch actually opens the circuit (the mag is ungrounded). Not connecting the other end of the shield to the switch will ensure that that the mag is hot *all the time*, a dangerous condition.

If this is unclear, get someone to show it to you. It is essential, and a critical safety item, to make sure your mags are grounded when the mag switch (however you do it or whatever switch you use) is in the "Off" position.
Click to expand...

Interesting enough, I was testing my mag today and was puzzled by the operation of the mag. To note, I have wired my mag the same way, only the shield is connected only on one side (the magneto torx screw)

Here is what I don?t understand, if I do a continuity test from the P-lead to the block of the engine, the circuit is always closed. So, how does Mag operates that by closing the circuit on the wire between the p-lead and ground, it will shut down the Mag.?

I know my mag is operating normal the way it is wired since the mag test at the run up will result in drop of the RMP (both L&R) and turning the key to off will kill the engine. I just don?t understand how does grounding the p-lead shuts it even though the p-lead seems always connected to the ground. I tested this with my spare Mag and it behaved the same way, regardless of the position of the gear.

Can some one set me straight on how it operates.
 
RV7A Flyer said:
..........I don't know about Van's harness, but you need to understand how the mags work. The circuit is from the mag (the larger connector), through a switch of some kind, and back to the ground (the small screw). Using the shield as one "wire" in the circuit just a) makes it simple, and b) helps keep RF noise out of your radios, etc. (if I understand correctly). The key point here is that to turn the mags OFF, you need to have a complete circuit from mag, through the switch, back to the ground. That is, the mag MUST BE GROUNDED to be off. This is true

To turn the mag "on" (make it "hot"), the switch actually opens the circuit (the mag is ungrounded). This is also true

Not connecting the other end of the shield to the switch will ensure that that the mag is hot *all the time*, a dangerous condition.This is NOT true. The mag can be grounded without using the shield. The shield is to stop the RF from "getting out" of the P-Lead, and it works best if grounded on only one end, usually the engine end.

If this is unclear, get someone to show it to you. It is essential, and a critical safety item, to make sure your mags are grounded when the mag switch (however you do it or whatever switch you use) is in the "Off" position.Again, true. But the mags can be grounded without using the shield.
Click to expand...

Hope my comments above in red help.
 
Not connecting the other end of the shield to the switch will ensure that that the mag is hot *all the time*, a dangerous condition.
This is NOT true. The mag can be grounded without using the shield. The shield is to stop the RF from "getting out" of the P-Lead, and it works best if grounded on only one end, usually the engine end.
Click to expand...
Click to expand...

You're correct. I was assuming that the shield was being used as the "ground wire", and certainly one can wire up a mag without using it as I described. I was just taught that it's a clean, simple way to make a p-lead (using a single shielded wire, and using the shield as one of the pair). You are correct, though, that a mag can be grounded without using the shield that way (e.g., using a shielded pair of wires).

Incidentally, my aircraft is wired as I described, with the shield connected at one end to the ground screw of the mag and the other to the switch, and I have zero noise from the mags in the electrical system...nothing in the audio, nothing in the EFIS or EMS, etc.

In fact, when the mag is "hot" and the switch is open, the shield portion of the circuit is doing exactly what you describe...since it's "open" at the switch, the shield is *only* connected at the one end (the engine, via the mag).
 
Bavafa said:
Interesting enough, I was testing my mag today and was puzzled by the operation of the mag. To note, I have wired my mag the same way, only the shield is connected only on one side (the magneto torx screw)

Here is what I don?t understand, if I do a continuity test from the P-lead to the block of the engine, the circuit is always closed. So, how does Mag operates that by closing the circuit on the wire between the p-lead and ground, it will shut down the Mag.?

I know my mag is operating normal the way it is wired since the mag test at the run up will result in drop of the RMP (both L&R) and turning the key to off will kill the engine. I just don?t understand how does grounding the p-lead shuts it even though the p-lead seems always connected to the ground. I tested this with my spare Mag and it behaved the same way, regardless of the position of the gear.

Can some one set me straight on how it operates.
Click to expand...

This puzzled me, too...has to do with when the points are open, IIRC. When they're closed, you'll see continuity all the time.

Get a magneto timing "buzz box" and you'll get a better idea of how it works.

There's also an awesome video somewhere on-line that shows how mags work...it's a series of animations showing the electrical flow, magnetic fields, collapse of the fields, etc. I'll see if I can find it. Done by a university somewhere...it was the first time I truly understood how magnetos *actually* work.
 
Bavafa said:
To note, I have wired my mag the same way, only the shield is connected only on one side (the magneto torx screw)
...
I know my mag is operating normal the way it is wired since the mag test at the run up will result in drop of the RMP (both L&R) and turning the key to off will kill the engine.
Click to expand...

As noted by PCHunt, your mag switch must have one of the throws connected to ground when in the Off position...just not the engine ground using the shield of the P-lead (otherwise, your mags would be always hot).

Always more than one way to skin a cat :)

(But you should find out how the switch is wired and to where, for maintenance purposes and annual inspections, as well as safety).
 
RV7A Flyer said:
....... In fact, when the mag is "hot" and the switch is open, the shield portion of the circuit is doing exactly what you describe...since it's "open" at the switch, the shield is *only* connected at the one end (the engine, via the mag).
Click to expand...

Hmmm. Need to think about that. Not sure that's true. I am thinking that the switch opens only the P-Lead itself, and not necessarily the shield. Isn't the shield permanently connected to "earth" at both ends in your setup?

If the single-wire P-lead is "open", then the mag is HOT. Grounding the P-lead shuts the mag off. But the shield isn't switched, if I recall correctly.

If grounding the P-lead shield on both ends works for you, all the better. Most wiring diagrams show grounding the shield only on one end.
 
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Come on guys. Mag wiring is beautiful in its simplicity, a simple loop circuit independent of ship's ground. Here's a clip from Nuckolls:

2ex2bl0.jpg
 
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RV7A Flyer said:
As noted by PCHunt, your mag switch must have one of the throws connected to ground when in the Off position...just not the engine ground using the shield of the P-lead (otherwise, your mags would be always hot).

Always more than one way to skin a cat :)

(But you should find out how the switch is wired and to where, for maintenance purposes and annual inspections, as well as safety).
Click to expand...

The ACS switch has that GND connection -

A-510-2-INSTALL-DIA.jpg
 
RV7A Flyer said:
Here it is...

http://media.avit.und.edu/f7_Virtual Engine/f1_Magneto/Virtual Engine.php

Under "Components", you can see animations of how each piece works, and there's a submenu "Operation" that steps you through each phase of the mag in operation, showing the flux lines, the current through the wires, etc.

Terrific videos, worth the time to study.
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This is a great video, many thanks for sharing it.

But I am still unclear about the grounding aspect of it and how is it that the p-leade always shows a closed circuit to the ground (regardless of the switch position)

Just like you, I had thought that at some point/position of the rotor when the ground connection (to the p-lead) should break but when I put my buzzer on and set the prop to the proper position (25 BTDC) I still had full continuity on the p-lead.

As mentioned, I also tested it with my spare Mag and turning the rotor by hand did not break continuity between the p-lead stud and the body at any point.
 
DanH said:
Come on guys. Mag wiring is beautiful in its simplicity, a simple loop circuit independent of ship's ground. Here's a clip from Nuckolls:

2ex2bl0.jpg
Click to expand...

Yep...that's what I have. That's why I said when the switch is "On", the P-lead is ungrounded, and the shield is only connected to ground at the engine end (via the mag), thus acting as (ta da) an RF shield.

Dirt simple, easy to check for wiring condition during inspections, etc.
 
az_gila said:
The ACS switch has that GND connection -
Click to expand...

....and dumb instructions. Connecting the shield(s) to the switch ground makes it conventional mag wiring. Because you retain individual ground paths for each mag (the shields), failure of any single connection in the ships ground path doesn't leave both mags hot. Of course, failure of the single switch ground tab, internal or external, again makes both mags hot.

You're probably not surprised to hear I prefer individual mag switches ;)
 
Bavafa said:
This is a great video, many thanks for sharing it.

But I am still unclear about the grounding aspect of it and how is it that the p-leade always shows a closed circuit to the ground (regardless of the switch position)

Just like you, I had thought that at some point/position of the rotor when the ground connection (to the p-lead) should break but when I put my buzzer on and set the prop to the proper position (25 BTDC) I still had full continuity on the p-lead.

As mentioned, I also tested it with my spare Mag and turning the rotor by hand did not break continuity between the p-lead stud and the body at any point.
Click to expand...

Honestly, I tried exactly what you did and was mystified, too...IIRC, you can't check the mags just by checking continuity or lack thereof. You need a mag timing box (it has some sort of magical circuitry inside it :)...actually, I think you have to have some sort of impedance or resistance in the circuit, or something like that, otherwise you get the indication you are seeing).

Use a mag timing box.
 
With a bit of search, I believe I understand the operation now.

The p-lead stud (which I was using with my ohm meter) is connected to a capacitor internally and it has a slow discharge, give it a reading as if we are connected directly to the ground when tested with an ohm meter which I was doing.

The proper test is to test the p-lead wire from switch disconnected from the magneto and then check to see if we are getting grounded properly or not.

Hope this helps.
 
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