Slick mag problem

I agree with Pete, sounds like the spring tension in your A7 switch is weak. I would try and locate another switch and install the toggles temporarily and see how that works.

Twice in five years working in a large shop I saw mags fire. Thankfully both times no one was in line with the blades.it was hurrying and careless but happened. After the second we had a meeting and policy in place to check for keys on the glare shield before installing a spark plug. The engines didn’t start but one cyl. Firing is lots of power.

algonquin said:

Twice in five years working in a large shop I saw mags fire. Thankfully both times no one was in line with the blades.it was hurrying and careless but happened. After the second we had a meeting and policy in place to check for keys on the glare shield before installing a spark plug. The engines didn’t start but one cyl. Firing is lots of power.

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I was between prop and hanger door right behind me when I managed to start the 185 with the key switch in OFF position. Broken p lead. Mixture lean, just some residual fuel in system That got my attention.


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Temporarily install the toggles and see if the symptoms change. If so, the switch was the problem. if not it's p-leads or mags.

Web

Thanks all. I’m going to the airport today and will probably put the toggles back on and see if that solves the issue. I can take the A7 apart again and check that the rotating contacts (two copper plates bent toward the stationary contacts) have enough “bend” in them to make good contact.

One thing is still bugging me. I looked at three other planes, all w/o electrical systems and that have various mag switches, on the field yesterday and not one had both ground wires attached to the ground screws on the mags. Two planes had a single ground wire connected to the L mag ground screw (nothing connected to the R mag except the P-lead, of course) and the third plane (with an A7 mag like mine) had its ignition ground wire connected to a bolt on the engine case—no wires connected to the mag ground screws at all. Wonder why?

Some OEM drawings show the ground tab, from the switch, connected to the airframe. This works, assuming the mags are properly grounded to the engine and that the engine is grounded to the airframe, through the engine mount. That means trusting several sheet metal seams and painted/powder coated connections. Providing a ground path from the mag body, through the p-lead, eliminates any grounding issues through the airframe. It also limits any mag noise issues as no current path from the mag is in contact with the airframe.

Web

The mag "ground screw" is just a connection to the mag case, grounding to the engine which has continuity to the mag case has the same effect (as long as electrons have a path to follow). In planes that have been around for 60 or more years the how and why of current wiring would most likely be do to THIWT (that is how I was trained) then anything else. Take someone that trained around all wood airframes, You need a hard wire pass back to the mag so they always wire that way (They attach shield wire at both switch and mag). Metal guys might just ground to nearest airframe part and let that be the path. Different manufacturers also have different wiring methods, most likely due to THIWT!!! We also have to add in what any stray electrons are doing to the avionics because planes with no electrical system/radio really don't care about stray electrons. OR I have a friend that says the IA/A&P guys/gals all get together on Thursday nights get drunk and go out to randomly rewire stuff just to screw with the plane owners. Don't know which is true but mag wiring seems to be a black art, kind of like compass placement.
DENNY

Edit: Web beat me too it, I type too slow.

Talking about mag switches, has anyone checked their bendix switches for the 100 hr AD or the ACS for the 2000 hr AD?

astjp2 said:

Talking about mag switches, has anyone checked their bendix switches for the 100 hr AD or the ACS for the 2000 hr AD?

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I do it every year, shortly before I do my (owner-assisted) annual.
Takes about 5 seconds.
AD76-07-12

astjp2 said:

Talking about mag switches, has anyone checked their bendix switches for the 100 hr AD or the ACS for the 2000 hr AD?

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Of course. If you read the A.D. or the service bulletin, the procedure called out for checking switch operation, is something that should be done at each annual anyways.

My issue, when finding a questionable switch, is whether to replace it or try to repair it. For a switch that may be 30 or 40 years old, I'd rather replace with new instead of trying to repair the old.

Web

wireweinie said:

...<snip>...For a switch that may be 30 or 40 years old, I'd rather replace with new instead of trying to repair the old.

Web

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That’s just the conundrum: Trying to return the Cub to original means using a 40+ year old mag switch. Thus, repair is the option. Thus, these problems and my questions.... Appreciate all the advice and ideas, guys.

A Bendix replacement switch isn't original enough? If not, find a used switch that passes inspection.

Web

This is an AAF A7 mag switch. Rarer than hen’s teeth. This one is in very good shape and repeatedly passes continuity checks (by different folks using different meters), has firm and definite indents, etc., etc. So I’m not 100% convinced it’s the mag switch itself. I’m waiting to hear back from an old timer at Slick on the wiring. Spoke with him once when I was ordering new P-leads and he mentioned something “unique” about how to hook them up. That idea was overruled by the local A&P so I want to talk with him (the Slick guy) again to be sure I am clear about his method so we can give that a try.

if I remember correctly, he said to hook up both P-leads to the mags and only one ground wire to one of the mags and to leave the other one disconnected. Interesting. But after seeing several other planes on the field with only one mag ground wire connected maybe that’s the secret sauce.

If the switch passes checks then it's good.

Don't get wrapped around the axle over p-leads. It's as simple as it gets whether it's Slicks or Bendix. Each p-lead is a shielded, single conductor wire. Center conductor connects to p-lead terminal on mag and 'L' or 'R' tab on switch, as appropriate. Mag end of shielding connects to mag body. Switch end of shielding connects to ground tab on switch. Repeat for opposite p-lead.

That's all there is to it. No magic an no 'unique' connections due to make or model of magneto or switch.

Web

did someone professionally overhaul your switch and put it back together wrong? go to pierces post number15.https://www.supercub.org/forum/showthread.php?35246-Lever-Type-Ignition-Switch

Checking with an ohmmeter is the same for any mag switch. Open contacts = mag is 'on'.

'OFF' - continuity between 'Left' and 'ground' and 'Right' and 'ground'.
'Left' - No continuity between 'Left' and 'ground'. Continuity between 'Right' and 'ground'.
'Right' - No continuity between 'Right' and 'ground'. Continuity between 'Left' and 'ground'.
'Both' - No continuity between 'Left' and 'ground' or 'Right' and 'ground'.

When you do these checks, make sure to wiggle the key or toggle to see if the reading is intermittent.

Web

YellowFeverAL said:

This is an AAF A7 mag switch. Rarer than hen’s teeth. This one is in very good shape and repeatedly passes continuity checks (by different folks using different meters), has firm and definite indents, etc., etc. So I’m not 100% convinced it’s the mag switch itself. I’m waiting to hear back from an old timer at Slick on the wiring. Spoke with him once when I was ordering new P-leads and he mentioned something “unique” about how to hook them up. That idea was overruled by the local A&P so I want to talk with him (the Slick guy) again to be sure I am clear about his method so we can give that a try.

if I remember correctly, he said to hook up both P-leads to the mags and only one ground wire to one of the mags and to leave the other one disconnected. Interesting. But after seeing several other planes on the field with only one mag ground wire connected maybe that’s the secret sauce.

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Yep, that is normal, it just provides noise suppression for the radio from the P-leads. You also dont want any capacitors on a P-lead for a slick, only bendix modern mags.

Disconnected a ground wire from a mag, wrapped the connector in electrical tape and zip-tied it out of the way. Started (and stopped! by turning off the mag switch) five times yesterday. Still not ready to claim success but it’s looking better, for sure.

astjp2 said:

Yep, that is normal, it just provides noise suppression for the radio from the P-leads. You also dont want any capacitors on a P-lead for a slick, only bendix modern mags.

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This is very interesting. I’d love to learn more about the “capacitor” thing. If you wouldn’t mind expanding on your post I’d appreciate it. Are you suggesting (going out on an electrical limb here) that how mags are connected through a switch somehow creates a capacitor? Perhaps after the engine runs for a while? I noticed before that I could start and stop it after a minute or so and it’d shut down Ok. Then, after a flight of a half-hour or longer it wouldn’t shut down. Perhaps that gives the “capacitor” time to charge? Or am I imagining all this (I have only vague knowledge of electricity/electronics). Yet, given this project’s history, this is why I am reluctant to declare victory (even after a .4 flight with successful shut-down).

astjp2 said:

Yep, that is normal, it just provides noise suppression for the radio from the P-leads.

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This is NOT correct.

IF the ground tab is connected to airframe ground AND the mags/engine/engine mount are all grounded properly, this may be true. But, as some of us detailed above, the shield is used as part of the p-lead circuit.

With the center conductor on the p-lead stud and the shield on the mag body The current path is from the p-lead stud, through the switch, and back to the mag through the shield. As stated previously, this provides the shortest path for current and isolates the p-lead circuit from the airframe. There are two reasons for wiring it like this. First is noise suppression. As I've said, having a path for the p-lead circuit to not use the airframe, means that any noise generated from the mags will only be present on the p-lead circuit and not routed through the airframe. This minimizes the chances of that noise being 'heard' by your radios or intercom. Second, is reliability. Using the shielded wire as a p-lead means that the p-lead circuit has a minimum of connections. The current path routes directly from the mag, to the switch, and back to the mag. No riveted seams or bolted joints to pass through.

If you disconnected a ground wire from the mag and the mag was still controlled by the switch, it only means that the ground tab on the switch has been connected to airframe ground. Do yourself a favor and connect your p-leads correctly.

Web

Also, you stated that you purchased Bogert p-leads. Their instructions direct you to connect center conductor to p-lead/switch and shield to mag body/ground tab.

Web

Web said it better than I could, also in the slick instructions it says to remove any inline capacitors that are in the P-lead.

When my P-leads arrived from Bogert’s there were no directions. Just two sealable bags with the two P-leads in them. Nothing suggested teasing out shielding and affixing it to a ground. This photos below are as close to instructions as I got.

It was a fellow at Slick that (if I recall correctly) said to attach one ground and not the other.

I see one of your problems. You think the Bogart p-lead "come with the P-lead shielded wire and a ground wire for each mag". It is a single shielded wire. The ground is the shield wire. They attach eye terminals to the end of the shield wire (looks like the blue wire to me) and cover it all up with heat shrink. A few years back I spent about 3 weeks trying to figure it all out, between internet time, books, and phone calls to manufactures. I ended up doing it just like Web advised. Also agree no external capacitor on a slick.
DENNY