Ignition switch ground

[russfarris]

What's the normal ground path for the ignition switch? And on a related note, is the shielding on the P-lead wires grounded at both the ignition switch end and the magneto? Russ Farris

[GAHorn]

According to AeroElectric the mag P-leads are grounded at the mag/engine end and also connected to the Ground terminal at the back of the mag switch. AVOID grounding the P-leads at the firewall/instrument-panel AND at the engine or you'll be using the P-lead coax as an airframe-to-engine ground-strap. You should have a heavier-duty, dedicated strap for that purpose already installed somewhere, preferably at the Battery Ground Cable-to-EngineMount-to-Engine.

Here's an article on magneto switch wiring, but beware that he spends more time addressing single-impulse coupling installations (where the non-impulse-coupled mag is grounded/off during start) than he does to our dual-impulse-coupled Continentals. (But he did include a wiring diagram at least.)
http://www.aeroelectric.com/articles/Ma ... ptions.pdf

[russfarris]

Thanks, Ghorn that's just the info I needed.

On the '57 172, the original shielding was still on the P-leads - grounded at the ignition switch, but not at the mag end. There was no ground wire
at all from the ignition switch to the airframe. It's worked fine like this for the 30 plus hours I've flown it, but then failed the mags off test, which I do after every flight. Checked the P-leads, then decided to replace them - shielding grounded at the switch but not at the mags. Same hot mag problem. My IA then grounded the shielding at the mag end. That worked. The diagram you linked to shows a ground wire from the switch to the airframe so i'll be adding that. I guess it worked before because the switch was grounded by it's mounting in the instrument panel. It's fixed now, just confusing to me because of the missing ignition switch ground wire. Russ Farris

[interstellardust]

OK I'm confused. I'm not sure why it would matter if the P-lead SHIELD is grounded at one or both ends as both the ignition switch AND the mag are grounded to the airframe aren't they?

In either case, the shield shouldn't have anything to do with the P-lead itself and its function of grounding the primary mag coil to keep it from charging the secondary coil and firing the spark plugs.

Am I mistaken?

[interstellardust]

The reason I ask is that I'm having a similar hot mag problem.

During a run-up during annual inspection, the engine would not shut off via the ignition switch. The right mag showed no drop.

After replacing both P-leads, the problem persists. Now neither mag shows any drop.

In addition to replacing both P-leads, I took apart the switch itself which showed inconsistent test with a multi-meter for the right mag. The contacts showed some minor wear so I sanded with 1500 grit sandpaper on a flat surface, polished the contacts to a mirror shine and reassembled the switch. The switch now tests fine on the multi-meter. Unfortunately the engine runs fine with the switch off.

The mags are slick 6364 which were installed new at major overhaul 266 hours ago. The P-lead shields are attached only at the switch end which my A&I (new to me this year) insists is correct. In fact he cut the ring connector off the shield at the mag end so they cannot be connected. so why do new leads come with shield connections at both ends?

Bill

[bagarre]

My $0.02

The mags will ground just fine if the PLeads are grounded at both, one or neither ends; so long as the motor is grounded to the airframe along with the switch.

But, I'd rather not have my Mags rely on the ground straps from the motor to airframe. So, I grounded the mag side to the mag and the switch side goes to the ground on the switch (and NOT to the airframe ground). This way, the ignition system is isolated to JUST the mags, PLeads and switch...easy to troubleshoot.

Also if the PLeads are grounded on both (To the mag and to the airframe at the switch) AND your engine ground strap fails, you'll run the full current of your starter thru your PLead shields and I don't think their rated for that. (something burning at startup?)

So, yes it will work but grounding the shielding to the mag and switch (and not grounding the switch to anything else) keeps the circuits separate and much easier to trouble shoot later on.

[bagarre]

The mags are slick 6364 which were installed new at major overhaul 266 hours ago. The P-lead shields are attached only at the switch end which my A&I (new to me this year) insists is correct. In fact he cut the ring connector off the shield at the mag end so they cannot be connected. so why do new leads come with shield connections at both ends?
Bill

Fascinating...but for off topic reasons.

They come with leads on both ends for a good reason. So you can ground the lead via the switch directly back to the mag...
Is your switch grounded to the airframe? If you can't turn off the motor with the key, it sounds like it's not.
OR your motor isn't grounded to the airframe but, that'd mean your starter woudln't work.

Others will chime in soon and help with this but, an IA should know that wires have terminals for a reason.

[interstellardust]

Yeah this is weird. I actually have two A&Is telling be the Shield should not be attached at both ends. So why do they come with fittings at both ends?

My switch only had three contacts one G where the shields are attached. a L and a R where the P-lead itself is attached for each mag. there is no wire from G to the airframe, but isn't the switch grounded by virtue of the metal housing being bolted to the instrument panel?

[bagarre]

Grab a multi-meter and test that theory as it may not be grounded thru the body of the switch.

[DWood]

I actually have two A&Is telling be the Shield should not be attached at both ends. So why do they come with fittings at both ends?

You are asking the right question:
There is a screw hole near the P Lead connector on the slick mags so you can connect the ground. Otherwise the mags can remain hot.

http://www.qualityaircraftaccessories.c ... C-0060.jpg

[interstellardust]

I guess I should mention that the original P-leads only had the shielding attached at the switch end like the new ones are now. There was no connector at the mag end of the shielding.

[interstellardust]

Yes I see the ground screw on the mag housing but they are telling me to connect the SHIELD to one end or the other, NOT BOTH. The P-lead itself, not the shield, is what's supposed to kill the mag. that does assume that the switch itself is grounded.

Bill Garnett
1955 170B

[bagarre]

They are right, if you are using the shielding on the PLeads as a radio shield. Radio shields are normally connected to one side to avoid electrical ground loops but I've heard debates on this as well. (I don't know)

But in this case, you're using the PLead shielding as a ground wire so you can ground the PLead back to the mag as directly as possible.

Else, you need to verify the ground path from the switch ground terminal to the airframe to the motor, to the mag.

[FredMa]

Radio shielding is not normally only grounded on one end. Who is saying this? There is no such thing as a ground loop on DC systems with a single ground. To have a ground loop you have to have multiple grounds with a difference of potential between two or more grounds. We only have ONE ground and NOT an AC system. If you are confusing multiple circuits that are grounded with having multiple grounds, here is a little test for you. Remove the ground cable from your battery and see how many circuits still have a path to ground. None because that is the only ground. That is like confusing multiple circuits powered by the same battery as having multiple sources of power. The shielding may work fine if only grounded at one end but would work even better if grounded at both ends. Take a look at any coax cable on your aircraft. Grounded at both ends through the connector isn't it?

[bagarre]

Radio shields are normally connected to one side to avoid electrical ground loops but I've heard debates on this as well. (I don't know)

Either way, grounding mags to the negative side of the battery isn't the right way to ground mags.
You ground a mag back to the mag. Else, how could you ground a mag in an airplane without an electrical system?

So, that's why PLeads are 'grounded' on both ends. to ground the lead back to the mag.