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Wiring Mag Switch

alpinelakespilot2000

Well Known Member
I have the Van's A-510-2 keyed ignition switch. I'm using 18AWG shielded wire. Van's drawing shows that the shield is separated from the P-lead at the magneto end but it shows it unseparted at the switch end. (There is only one connection to make at the switch per Van's drawing.)

To put a ring terminal on the switch end, should I just strip the insulation and shielding off and crimp the terminal just as I would with an unshielded wire, or do I need to do something special with the shield at the switch end? (Specifically, I'm wondering if I somehow need to make sure that the shield can't come in contact with the main wire or terminal.)

Thanks.
 
I have the Van's A-510-2 keyed ignition switch. I'm using 18AWG shielded wire. Van's drawing shows that the shield is separated from the P-lead at the magneto end but it shows it unseparted at the switch end. (There is only one connection to make at the switch per Van's drawing.)

To put a ring terminal on the switch end, should I just strip the insulation and shielding off and crimp the terminal just as I would with an unshielded wire, or do I need to do something special with the shield at the switch end? (Specifically, I'm wondering if I somehow need to make sure that the shield can't come in contact with the main wire or terminal.)

Thanks.

Definitely, the shield must not contact the wire it is shielding. If it does the mag will be grounded and not operate.

The purpose of the shielding is to suppress electrical noise that may interfere with radio reception. Some sources recommend that both ends of the shield be grounded, some say one end grounded will do the job.
 
Shield use ?

On the 510 switch, the shield on the P-Lead wire attaches at the mag(ground) and to the GRD(center terminal) on the switch end. In the Switch Off position, the shields are used as a ground return for the mags. In the Switch ON position, the shields are protection from electrostatic coupling of the mag noise.

On the switch end, the shield goes to the center GRD terminal on the switch only and not actually to ground at the panel.

See Diagrams Z26/27 and Note 3 in the Aeroelectric Connection for more detail.

Bill S
7a Finishing
 
See Diagrams Z26/27 and Note 3 in the Aeroelectric Connection for more detail.
Thanks Bill, but I'm going according to Van's drawings. I saw that Bob Nuckholl's grounds his at both ends, but he seems to be using a different set of switches than Van's A-510-2's. Also, maybe Van's drawings should show the shield grounded at the switch (?) but it definitely does not (at least as I'm reading it).
 
The Bob N. way is safer....

Thanks Bill, but I'm going according to Van's drawings. I saw that Bob Nuckholl's grounds his at both ends, but he seems to be using a different set of switches than Van's A-510-2's. Also, maybe Van's drawings should show the shield grounded at the switch (?) but it definitely does not (at least as I'm reading it).

This document from AeroElectric Bob does seem to cover exactly the same type of switch that you mention from Vans...
In fact the upper schematic on page 3 uses a Gerdes switch with the same part number you quote....

http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf

I believe it is a little safer if you follow Bob's wiring - also suggested by Bill S. above.

A detailed thread is here...

http://www.vansairforce.com/community/showthread.php?t=24950&highlight=magneto+switch

Check my post #5 in the linked thread for my reasoning.
 
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P-Lead

Steve,
I had the same confusion. After countless inquiries, talks, reads, etc., the 510-2 switch the shielding should be grounded on the mags and grounded on the central ground terminal of the switch. Bob at Aero Electric shows it and you should really read this:
elippse Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 309



--------------------------------------------------------------------------------

The "P", "primary" lead, is connected to the primary of the spark coil. Under high power conditions, the voltage pulses on that lead may be in excess of 100V. Any current which flows from a source must return to that source, by whatever path is available. The voltage pulses in the shielded wire will couple to the shield, electrostatically and electromagnetically, and so the current in the shield must get back to the return side of the spark coil primary. The shield should be connected at the magneto case and at the ground terminal of the switch, AND NO WHERE ELSE! If you connect the shield to the aircraft "ground", these currents will then use the aircraft skin to go to the engine and then back to the magneto, radiating electromagnetically all along the way. This is where you get spark noise into your radios. When the current only flows back through the shield, the current flowing in both directions will effectively cancel as far as radiation is concerned. And whatever you do, please don't install a "filter" in the "P" lead circuit. The reason you had ignition noise is because you didn't have a properly shielded and grounded circuit, and the filter just adds another alternate current path
Hope that this solves the confusion,
Mike H 9A/8A
 
Thanks Mike and Gil-
I'll look into it some more. Sounds like good reasoning to ground at both ends. I wonder why Van's doesn't suggest this in their drawings?
 
Some certified planes...

Thanks Mike and Gil-
I'll look into it some more. Sounds like good reasoning to ground at both ends. I wonder why Van's doesn't suggest this in their drawings?

...use Vans wiring, but while it works, it's not optimum.

My Tiger uses Vans approach, but many more joints are involved, and one wire then affects both mags. One hot mag is bad, but two might be worse.

One item that I found when researching this on other brands of certified planes was finding the actual wiring schematic for the key switch.

The magnetos "belong" to the engine in the maintenance manual, and seem to be ignored in the "electrical" section with the actual 12 volt wiring diagrams - I checked 2 Piper and one Cessna Maintenance manuals, and could not find a specific wiring diagram that showed shields for the magnetos.

Hence a lot of A&Ps wire it up their own way when the P-leads break (I've seen lots of poor connections to the shields here) which is a relatively frequent repair.

Heatshrink for support is good here - again Bob N. shows how. Twisting an exposed length of shield and crimping a terminal on the end almost guarantees a future break at the point the shield exits the cable - but this is the way I've seen many done.
 
Just as another data point...

ACS has a picture / drawing of this switch in their catalog, as well as on their web site. Bob Nuckolls also provides a picture and the switch and position / function chart in his Z-27 drawing. And, if you order the switch from ACS, you will get a small slip of paper packed with the switch which "identifies" the terminals on the switch.

I found that none of the drawing of the switch agreed with each other. There is a subtle difference between them. I think it was the location of the BAT and S terminals were reversed. For my implementation, it did not matter, but it did cause me to double check everything to be sure there were no other issues.

Just check your switch against your implemented wiring scheme.
 
...Hence a lot of A&Ps wire it up their own way when the P-leads break (I've seen lots of poor connections to the shields here) which is a relatively frequent repair.

Heatshrink for support is good here - again Bob N. shows how. Twisting an exposed length of shield and crimping a terminal on the end almost guarantees a future break at the point the shield exits the cable - but this is the way I've seen many done.
Is this the easiest, best way, then Vern?
http://www.aeroelectric.com/articles/pigtail/pigtail.html
I wonder if trying to solder the sheild to a wire will tend to melt the insulation on the main conductor??
 
There are many ways...

Is this the easiest, best way, then Vern?
http://www.aeroelectric.com/articles/pigtail/pigtail.html
I wonder if trying to solder the sheild to a wire will tend to melt the insulation on the main conductor??

...but that one is effective. The heat shrink is the key.

If you have real aircraft shielded wire with a tefzel inner core, then no melting should occur.
If you use the cheap wire with the PVC inner stuff, then the inner insulation may melt...:(

If it's real mil-spec wire, then is is marked on the outer layer.

Generically,

MIL-W-27500-A-xx-TE-*-zz
xx = wire gauge
TE = standard tefzel 22759/16 wire (the stuff we usually use)
* = Shield type
zz = Jacket type (14, 15, 64, 65 are good here...)

Full details of interpreting the long number is here -

http://www.worldwidewire.com/tefzel-shielded-cable/mil-c-m27500-build-specs.htm

If it's a real Mil-Spec shielded cable, the above link will tell you what exactly you have....

Spruce sells it with a 22759/18 innner wire, which is the same as the 22759/16 with a slightly thinner tefzel for protected areas.

Not sure which version Stein sells, but I'm sure it's a good one...:)
 
Is this the easiest, best way, then Vern?
http://www.aeroelectric.com/articles/pigtail/pigtail.html
I wonder if trying to solder the sheild to a wire will tend to melt the insulation on the main conductor??

No need to go through all that work when there is devices already made for that purpose. They are called solder sleeves or solder splices and are stocked in a number of different sizes...some with pigtails already in them. You simply put them over the shield (with your pigtail), heat them and you're done. If you look at the wiring that most avionics shop do, it's the way we all terminate shields...and P Leads.

Here's a pic of a P-Lead done with a solder sleeve:

http://www.steinair.com/images/store/SS1.jpg

It's a $.75 that will save you a PILE of time on P-Leads and such (Mkr beacon pigtails, etc..).

Cheers,
Stein
 
Thanks Stein...

I forgot about the solder sleeves you sell - the built-in pigtail version solves the P-lead termination issue.

Much quicker and easier....:)

You didn't mention the spec of your sheilded wire...

Which MIL-W-27500 part number do you sell?

gil A


No need to go through all that work when there is devices already made for that purpose. They are called solder sleeves or solder splices and are stocked in a number of different sizes...some with pigtails already in them. You simply put them over the shield (with your pigtail), heat them and you're done. If you look at the wiring that most avionics shop do, it's the way we all terminate shields...and P Leads.

Here's a pic of a P-Lead done with a solder sleeve:

http://www.steinair.com/images/store/SS1.jpg

It's a $.75 that will save you a PILE of time on P-Leads and such (Mkr beacon pigtails, etc..).

Cheers,
Stein
 
Here's a pic of a P-Lead done with a solder sleeve:
http://www.steinair.com/images/store/SS1.jpg
It's a $.75 that will save you a PILE of time on P-Leads and such (Mkr beacon pigtails, etc..).
Cheers,
Stein
Good idea, but does anyone have any link to more thorough instructions for exactly how to use these? Specifically, how much shielding should you unravel and how much should be inserted into the solder sleeve? Also, will these sleeves work for the 18AWG shielded wire of the p-leads? (I notice that the pigtail on the sleeve is only 22AWG.) Thanks.
 
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The pic I showed is of a P-Lead we whipped up in the shop. Other than some extreme circumstances, I can't see needing more than an AWG22 pigtail.

I don't have specific instructions, but they are pretty self explanatory. It's a high temp heatshrink sleeve with a bank of low temp solder in the center. Strip off enough shield to use as a contact point for the pigtail/solder, but not enough to extend beyond the ends of the of the sleeve itself. Each end of the sleeve has some adhesive in it that will seal up the entire connection. Just look at some pics (or the pic that I posted) and make yours look the same.

You might get by without shielded wires, but I would strongly encourage you to make them shielded.

BTW, the mil spec we have is either a 18TE or 18TG center insulation, depending on who has stock when from the mfgrs and pricing. There is little difference between them.

Cheers,
Stein
 
Thanks for the reply...

...
BTW, the mil spec we have is either a 18TE or 18TG center insulation, depending on who has stock when from the mfgrs and pricing. There is little difference between them.

Cheers,
Stein

Stein....

The TE or TG specs are both Tefzel, so the builder with the earlier post about melting the inner core during soldering can be at ease...:)

The TG is just slightly thinner on it's insulation, and designed for protected areas, but being inside a shield, it's certainly protected....:D

Those builders buying surplus wire really need to check the full part number on any shielded wire - there is a lot of variation here....

Just buy it from Stein!
 
Just for clarity ?

Thanks Mike and Gil-
I'll look into it some more. Sounds like good reasoning to ground at both ends. I wonder why Van's doesn't suggest this in their drawings?

Steve, this cat is probably flat and I know it's a fine point but understand that while you are attaching the P-Lead shield to the center GRD terminal, that is not actually gounded at both ends. All the center GRD connection on the 510 switch does is provide a path for the center P-Lead wire to get to ground which is actually through the center GRD teminal then through the shield and back to the mag itself. The switch just makes a connection from the center P-Lead to the shield wire at the center GRD terminal which takes it back to ground at the mag.

Highly recommend Aeroelectric Connection for general wiring info. Here is a link to a great articles page. You can read for hours here.

http://www.aeroelectric.com/articles.html

Hope that's clear. It confused the **** out of me for a long time.

Bill S
7a finishing
 
Steve, this cat is probably flat and I know it's a fine point but understand that while you are attaching the P-Lead shield to the center GRD terminal, that is not actually gounded at both ends. All the center GRD connection on the 510 switch does is provide a path for the center P-Lead wire to get to ground which is actually through the center GRD teminal then through the shield and back to the mag itself. The switch just makes a connection from the center P-Lead to the shield wire at the center GRD terminal which takes it back to ground at the mag.

Highly recommend Aeroelectric Connection for general wiring info. Here is a link to a great articles page. You can read for hours here.

http://www.aeroelectric.com/articles.html

Hope that's clear. It confused the **** out of me for a long time.

Bill S
7a finishing
Thanks for taking the time Bill. I have read a bunch in Aeroelectric. I just don't have the electrical knowledge to tell when Bob's ideas are better than Van's ideas.:eek: Moreover, I do know enough to know that mixing and matching wiring ideas from different sources may not always be a good thing!

So if I wire my switch as per Bob's instructions, does that mean the switch itself is not really grounded to anything? I originally wired that center GRD terminal on the switch to my main panel ground block as per Van's drawing, but I'm gathering now from the above comments that if I wire per Bob's instructions, this ground wire off the center GRD terminal would be serving no purpose and, in fact, would be undesirable. Therefore leave it off entirely and only use that center GRD terminal for the shields of both p-leads. Sound about right?

Thanks again for the help guys.
 
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That is correct....

.....So if I wire my switch as per Bob's instructions, does that mean the switch itself is not really grounded to anything? I originally wired that center GRD terminal on the switch to my main panel ground block as per Van's drawing, but I'm gathering now from the above comments that if I wire per Bob's instructions, this ground wire off the center GRD terminal would be serving no purpose and, in fact, would be undesirable. Therefore leave it off entirely and only use that center GRD terminal for the shields of both p-leads. Sound about right?

Thanks again for the help guys.


...for the Center terminal.

The other terminal labelled GND needs to be wired appropriately for your starter relay circuitry.

The two terminals on the switch labelled GND are not connected to each other.
 
...for the Center terminal.

The other terminal labelled GND needs to be wired appropriately for your starter relay circuitry.

The two terminals on the switch labelled GND are not connected to each other.
Yes, only the p-lead shields use the center GRD terminal. I will use the other GRD terminal only to ground my right p-lead with the jumper provided with the switch. (As I interpret both Bob's and Van's drawings, and given that my impulse coupler is on the left mag, I therefore only GRD the right mag p-lead). All this per View A on page three of this link:
http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf
 
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