Ignition switch ground

[blueldr]

I looks to me like your problem is your A&P and/or AI. If they don't understand the grouding of the primary in the Magnetos they ought to turn in their tickets.

[Bruce Fenstermacher]

I think, which has been pointed out, it comes down to what your trying to accomplish with the shielding of the cable in the application of wiring a mag switch. Are you trying to shield the wire within or are you trying to provide a positive direct ground between the switch and the mag. And I think this is where people are getting caught. It is highly unusual to find a person extremely knowledgable of all aspects of maintaining an aircraft. There are just to many disciplines to know. Electricity is one often not fully understood and then you get into the highly technical field of radio transmission and reception. I think it safe to say an IA would know what it takes to ground. How to do it can always be argued.

Here are my thoughts in the subject. My Cub, which at one time didn't even have shielded ignition, does not use shielded cable between the mags and mag switch. Instead there is a ground wire and a wire for each of the left and right P leads. No one was concerned with shielding anything because there was not radio to shield anything from. My handheld radio and GPS work fine.

Moving forward shielding has come into play. And shielded cables use more prevalent because there are things than need to be shielded in order for signal transmission. But I'm not sure the mag wires are one of those things that need to be shielded. When a mag switch is ON remember there is no continuity. There are no electrons moving in the P lead. It is just a wire connected to the mag points to ground them when necessary. When it is grounded electrons may flow but do we really care if it is shielded then?

I personally would use the shield for a ground between the mag and the switch if I had shielded wire installed between them. And as Fred has pointed out I might actually be shielding the wire within though not my first intention. If I didn't have shielded wire I'd run a ground wire directly between the mags and the switch to insure a positive ground.

[bagarre]

Sorta off topic from the original question but...
Radio Noise can happen in a lot of ways and doesn't really need electrons 'flowing' thru a circuit.
Unshielded PLeads act more like antennas for the noise generated by the mags charging and discharging at a given frequency (rpm) and that radio transmission (noise) gets picked up by other systems in the airplane. In radios, signal and noise are like grass and weeds. They are the same thing except one is desirable

That's a really watered down explanation and not meant to be used as a guide to electrical engineering or radio theory.
It only helps illustrate the basic reason why PLeads and other systems need to be shielded.

Connecting the shielding to the mag and to the switch gives you that radio noise shielding as well as a direct ground back to the mag.

Either way, the mags stop making spark because the PLead is connected back to the Mag's case and thus stops the coil from charging. Connecting your mags to the negative terminal on your battery wont do squat unless your negative terminal is also connected to your motor which is connected to the mag which stops the coil from charging.

[interstellardust]

I took a multimeter out to the hangar this morning. The new P-leads are fine. The mags are definitely grounded to the airframe. The ignition switch case is also grounded, so I removed the switch. It also tested perfectly using the ground contact on one end against the L and R contacts. What could it be?

I disassembled the switch and guess what? There is nothing inside that connects the ground contact attachment to the case. All of the internal contacts are insulated from the case by the Bakelite piece they are attached to. In other words, the internal mechanism of the switch itself is not grounded even though the case is bolted to the instrument panel. A new wire from the ground contact point to the back of the instrument panel fixed that. The mags work perfectly now.

The switch must have only been grounded before where the woven metal shielding was zip tied to the instrument panel bracing. There's a scary thought. I imagine it's been that way since my dad switched from Bendix to Slick mags in the 70s.

Looking at pictures of Bendix mags, it looks like the woven metal shielding would have been grounded through the original hex fitting itself at the mag end and to the G contact at the switch end. This obviously lends credence to the opinions that the shilding itself can be used as the ground by attaching it to both the switch and the mag ground scrwes.

When the Slick mags were installed, it looks like the mechanic just clipped off the Bendix hex fittings, pulled the woven shield back a ways and attached ring fittings to the ends of the P-lead wire.

Thanks all for pointing me in the right direction.

[DWood]

Bill:

Since the shielding at the mag end of the P lead has been cut or modified, make sure that it can't inadvertently make contact with the P Lead connection on the mag. You might want to tape the shielding back. My airplane was like that when I bought it and I lost one mag in flight.

You are now relying (as David has stated) on the ground going thru the airframe to the engine/mag. It isn't the most reliable ground but will probably work. Or you might spend another $100 and replace the P Leads and connect it as designed.

Dan

[Bruce Fenstermacher]

Bill I'm hoping you where helping the mechanic trouble shoot this and not him actively engaged in the process. As you describe the story the first clue to the problem should have been he didn't connect a ground wire to the Slick mags that provide a lug specifically for this. You are correct in that the old Bendix plug grounds the shielding to the mag. If your mechanic was really at a loss as to the problem well that would make me wonder.

Good grounding is often overlooked. All to often ground wires that jump the engine to motor mount and motor mount to firewall, are left off after rebuild. Your story of scary grounding is unfortunately all to common. A friends 170 with Lycoming conversion always seemed to have a weak battery and would not turn over. It was discovered after years the only ground to the engine was the throttle cable.

[170C]

A learned friend of mine once told me that many, many electrical problems turn out to be for lack of a proper/good ground. I have seen it be the issue on one of my radios several years ago.

[bagarre]

FWIW, you can make your own PLeads for about $5.00 in material from Aircraft Spruce.
That's what I ended up doing.

It's just 18 gauge shielded wire, some crimped terminals and shrink tubing.

[interstellardust]

Yes, I did all my own testing before "my" A&I arrived including figuring out what the real problem was. When he arrived I had him double check my work and approve the fix.

Yes, the shield was clipped off just inside the shrink tube and is well away from the P-lead terminal.

I understand the ground path is now from the Switch to the instrument panel to airframe, through the jumper band from the firewall around the Lord bushing to the engine mount, through the jumper ground wire around the forward starboard engine isolation mount to the engine block, then the mag housing. I agree, that's a lot of contact points. Although it is working fine now, I will probably replace the modified P-leads again so I can attach the P-leads and shields to their own terminals at both the switch and the mag. I assume I should then remove the jumper I just added from the ground terminal on the switch to the instrument panel to avoid providing a ground through the shields if the other ground path fails. Yes?

Of course this means I'll need an new A&I since "my" current one won't sign off on that configuration. That may be all for the good.

[Bruce Fenstermacher]

What happens between you and the AI is one matter but you might decide to compromise. Just run a standard wire from the ground jumper wire you attached all the way out and directly ground the mags. Your IA can't have a problem with that as you are still "shielding" the P lead as he desires. And all your doing is duplicating through a more direct and positive route, the ground path he believes is sufficient. And no I would not remove the ground jumper you installed no matter what additional ground path you provide. It just becomes a redundant ground.

BTW you forgot the mag case is shielded from the engine by the gaskets. It is only grounded by its mounting studs and then only where the nuts scrape through the paint to the mag case when tight.

[bagarre]

You don't need an IA to sign off on PLead installation. Any A&P can do that.

and if he tries to fail your annual for having PLeads attached at both ends, I'd ask him to surrender his airplane inspection permissions cuz he's just not smart.

But yes, if you were to connect both ends of the PLeads, I'd remove the jumper from your switch ground to the arframe. IF your engine ground strap was to ever fail, your starter amperage would come back thru your shields across the little wire and possibly cause a fire hazard in your panel.

best of luck!

[interstellardust]

And the plot thickens...

Since I've gotten opinions on both sides of the argument, I decided to email Champion Aerospace, who now own the Slick mag concession.

I actually got a phone call from their head of technical support who is at Sun n Fun. He sides with the A&I. He says shields should only be grounded at one end to avoid a ground loop in the shield which would cause radio interference. He says that grounding the switch to the instrument panel is correct and was not bothered that there was nothing attached to the ground screws on the mags themselves. He promised to respond via email when he returns form Sun N Fun. I'll be happy to post what he sends me here.

[n2582d]

From the 1977 C-172 Service Manual:

[Bruce Fenstermacher]

With the Slick Reps response and then Gary's Cessna 172 schematic this plot has gotten so thick it doubt it can be stirred any longer.

Looking at the Slick 6300 series installation manual, it says -

• "C. Attach P-lead ground shield, if applicable, to the ground screw on the side of the magneto below the magneto data plate. Torque the P-lead ground shield screw to 18 - 20 in/lbs."

There is nothing said about the other end of the shielding.

[bagarre]

I wouldn't expect the Slick installation manual to say anything about the switch side of the PLead. That would be in the switch installation manual

Regarding the ground loop issue, the Shield actually IS only grounded at one end. The Mag end. The other side is connected to the switch ground which is connected to ....nothing. Its only connected to the PLead wires when the key is off...which grounds the coil. QED

So, I read up up on ground loops and learned they are caused by using two different grounds of different potentials. So, 'looping' a ground wire to two places isn't what does it.
For the electrical system, there is only ONE ground; the battery. Having multiple ground POINTS isn't multiple grounds as all the electrical grounding still goes back to the battery. So, saying that shielding grounded to two places can cause a ground loop is like saying your landing light will cause a ground loop because it's 'grounded' to a different part of the airframe and thus creating an electrical loop. (which isnt true) Everything grounds back to the battery via the airframe.
I think the ground loop thing seems believable because we think about it as a feedback loop between an mic and a speaker but that has nothing to do with DC current. (learn something new)

Also if the reliability of magneto ignition is that it is completely independent of your electrical system, why make them rely on that electrical system to shut them off? Would your IA or Slick have any issues with your running a third wire from your switch back to the mag? Cuz that's all your doing with the shielding.