Mag leads

[dfcpac]

On my a 65 I have slick mags. The right mag has 2 wires on it comming from the switch but the left mag on has one wire. How does that work? Is that correct?

Thank you
Dan carley

[MikeB]

One is ground if I remember correctly, been awhile. You should be able to fiqure the switich squence out with a ohm meter.
Mike

[dfcpac]

so this is normal, using the second wire for the ground. i just found it odd

danny

[MikeB]

Looking at the switch wiring diagram it appears the yellow (if they're colored) goes from the switch to the left mag. The red wire is the switch ground terminal but goes to the right mag (ground terminal) and the blue wire goes from the switch to the right mag. You should be able to identify the wire connections looking at the back of the switch. Using an ohm meter and hooking one connection to the ground and the other to either the left or right terminal and moving the switch position you should be able to check the operation of the switch. This is assuming you have the old style switch and not a newer ignition (automotive type) switch. All the ground wire is doing is grounding the magnetos to shut them off.



Hope this helps.

MikeB

[N86250]

so this is normal, using the second wire for the ground. i just found it odd

danny

Many mechanics also as a good practice add a jumper wire from the ground post on the left mag to the ground post on the right mag to insure the magneto cases are electrically connected. I must be missing something. Please explain why you find it odd. Do you find the ground wire from the switch to the right mag odd or do you find the absence of a ground wire to the left mag odd? Please explain. jrh

[rkittine]

JRH,

I think I have the same set up, but with Bednix Mags. My 7CCM is the first aircraft that I have owned without a starter and twice I have had the engine try to fire with the mags off, which I assume is a faulty ground, though I can not see to find the ground faulty when shooting continuity when the engine is off and I have never had the engine keep running when I turn the mags off, which is how I shut down with no Mixture control on my Champ. Should I try putting a ground strap on one or the other mags in addition to the shunt?

Thanks - Robert

[N86250]

JRH,

I think I have the same set up, but with Bednix Mags. My 7CCM is the first aircraft that I have owned without a starter and twice I have had the engine try to fire with the mags off, which I assume is a faulty ground, though I can not see to find the ground faulty when shooting continuity when the engine is off and I have never had the engine keep running when I turn the mags off, which is how I shut down with no Mixture control on my Champ. Should I try putting a ground strap on one or the other mags in addition to the shunt?

Thanks - Robert

From afar I think one of your mags has lost its electrical connection to the the accessory case. With the mag sitting on a paper gasket insulator the electrical path for the ground is through the mounting studs and nuts. If there is a little corrosion in the stud threads in the aluminum or magnesium case, then the electrical path depends on the bearings, gears, and shafts of the magneto and the accessory case for an electrical path. I can see this as an intermittent open and intermittent hot mag that would be really hard to diagnose. Good wires and good connections beteen both mag grounds, both P leads, and the mag switch eliminates any corrosion faults in the engine or airframe and divides the problem between the switch and an internal problem in the mag itself. Don't forget the switch and the mags have brass parts in them and 65+ year old tarnished brass might not be making good electrical contact every time. If you can reproduce the hot mag somewhat regularly, you might put jumper wires directly from the magneto ground studs to their individual P lead studs. Short cords with alligator clips on both ends should work. If the mag is still intermittent hot with its P lead shorted to the case ground, the problem is in the mag itself as you have eliminated the switch, the wiring, and any mounting corrosion. jrh P.S. Find a young guy that can spin the prop for however long it takes.

[MikeB]

Robert,
If you have the old style (between the seats) switch most likely the contacts are getting bad. It's possible to (very carefully) take the back off the switch and clean the contacts. After 60 years or so they seem to get corroded and a good clean up with a scotch brite pad does wonders. It's a fairly simple setup when it comes right down to it. The failure to make the ground inside the switch is the usual reason for firing when you're pulling through for priming. Of course you always treat the prop like a loaded gun anyway.
Mike

[rkittine]

Thanks Mike - Yep I have the old Between The Seats Switch. I was thinking about replacing it with a Key Switch anyway, so maybe I will just go ahead to do that. I agree about the Treating Like A Loaded Gun. I wanted to get into the side panel anyway and do some work.

[MikeB]

I think the switch is called "A-7" and fit some kind of an aircraft during WW2 which lets you know how long it's been around. Once in awhile you see them floating around on EBAY but I suspect they're on there because they were removed for some reason (probably not a good reason). While it doesn't meet the spirit of 'certification' I often thought two toggle switchs mounted on the dash with a wire from the P lead to the switch and a ground going to each magneto would accomplish the same thing. One of my friends has a key switch (like sold by ACS).

MikeB

[rkittine]

I agree Mike. I have owned a number of aircraft with Toggle Switches for the mags including 2 C-310s. I probably will just buy a new switch. - Cheers - Robert

[james gevay]

Robert, you probably have already seen this post, but if you haven't, it has some pictures showing the Briggs and Stratton A-7 Mag switch. There is an excellent article in a past issue of the EAA's Vintage Airplane magazine which shows how to rebuild the switches. It's the April 2003 issue, the one with Bill Pancake's blue and white 7AC on the cover.
Yes they do come up on E-bay from time to time, but, they're very sought after, meaning expensive. If you decide not to use it, please DO NOT throw it away, someone would be happy to have it.
here's the link,

http://www.joea.com/phpBB2/viewtopic.php?f=22&t=3054

[rkittine]

Thanks James, I may put in a new Key Switch, but if I do, I will have the one I have rebuilt and hold onto it. I wonder if this is one of the things that Bill Pancake rebuilds? I have talked to him about rebuilding my fuel valves, which apparently he does on an on-going basis.

Cheers - Robert

[Paul Agaliotis]

Robert,
If you plan to put it in the same location as the A-7, make sure and measure the depth. There's not a lot of room for a longer switch.
Paul

[dfcpac]

i checked all the wiring, everything is correct. my left mag drops 75 rpms but no drop on the right mag? timming is good. can this just be normal

dan carley