A little mag trouble

iflyvfr

iflyvfr

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Columbus, OH
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Greg
This started last weekend with what I thought was a fouled plug. It cleared up and off we went for an entire day of flying. Today we were headed NE to a new field with a restaurant we'd never been to with CAVU wx the whole way.

As I did my runup, the left mag stumbled. Leaning out did no good so I called ground and asked to taxi back. In 11 years of ownership, I've never had a fouled plug.

My mechanic pulled the lower plugs fed by the left mag and noted the bottom rear pilot's side plug looked different than the rest, like the cylinder wasn't getting spark. We replaced the plug with new, cleaned all the other bottom plugs - no joy, still a stumble. The more we tested, the more we realized that jug was cold.

After removing the mag cover and pulling the leads he found our culprit: IMG_6885.jpg There was a tell-tale streak along the bottom of the housing for this lead and the lead itself had this same green corrosion on it which was cleaned up. Using several techniques we got it to look like this: IMG_6895.jpg. However, when we did a runup - confident we were finally going flying - I got the same stumble on the left mag.

So now we're looking at rebuilding a mag or replacing a harness. Not sure which yet, but that streak tells me we might be looking at a micro-crack in the housing that would warrant a new mag. Ah well, best to get it figured out before flying season begins in earnest.

Any thoughts about what caused that corrosion? My thought is it's gotta be a crack allowing moisture in??
 
you could ohm out the lead too, prob not the problem -but easy to check before proceeding
 
What kind of mags?

How many hours on them?

I had a similar issue after I got my plane with my slicks, but we knew they had a few hours during the prebuy, ended up just swapping over to bendix with a new harness.
 
You can test the harness leads to see if any are faulty, to confirm or eliminate one possibility. Your mechanic likely has one of these:
https://www.aircraft-tool.com/shop/detail.aspx?id=E5

The corrosion looks like copper carbonate. If the cap was cracked one might expect to see black carbon tracking, not green corrosion I believe.
 
Let'sgoflying! said:
you could ohm out the lead too, prob not the problem -but easy to check before proceeding
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You want to apply a high voltage test to the ignition leads to see if there's a breakdown of the insulating layer between the center wire and the shield. An ohmmeter test may not identify that problem. The insulation can break down from heat, vibration and flexing over time.
 
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GRG55 said:
You want to apply a high voltage test to the ignition leads to see if there's a breakdown of the insulating layer between the center wire and the shield. An ohmmeter test may not identify that problem. The insulation can break down from heat, vibration and flexing over time.
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And even then the fault can be intermittent. If the harness has 1,000 hours just replace it. Sorta drastic but it costs less than a mag overhaul. On the other hand a mag overhaul by a competent mechanic isn't a bad idea either. It is getting tougher to find mechanics who will overhaul a mag even though they are simple.
 
The black object that we see is the distributor block of the mag. when cracked the electrons will escape thru the crack to ground, just takes a little moisture for a conductor. You'll not see any carbon tracking when this is the case.
I'd bet you'll need a distributor finger too.
 
Clark1961 said:
And even then the fault can be intermittent. If the harness has 1,000 hours just replace it.
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Why replace the whole harness? you can simply replace the 1 lead that's bad.
 
Clark1961 said:
And even then the fault can be intermittent. If the harness has 1,000 hours just replace it. Sorta drastic but it costs less than a mag overhaul. On the other hand a mag overhaul by a competent mechanic isn't a bad idea either. It is getting tougher to find mechanics who will overhaul a mag even though they are simple.
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How many A&Ps even have the tools anymore?
 
Clark1961 said:
Three and you aren't one of them.

Regret asking the question now?
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I asked because I haven't seen the little protractor in anyones box in a very long time.
If you would like pictures of mine I'll play show and tell with ya.
 
Let'sgoflying! said:
Ha, realized my mistake the moment I hit Enter. Decided it's fun to see you guys jump at the chance to correct someone.
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There is a delete button :)
There is a formula, R X length but I've forgotten what R =
When a lead is proven bad You just order a new one by length anyway .
 
Can you just switch leads and see if the cold cylinder follows that lead or stays with that mag position?
 
455 Bravo Uniform said:
Can you just switch leads and see if the cold cylinder follows that lead or stays with that mag position?
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Some times that's not possible, they will only reach so far. testing them is the easiest method.
 
When was he mag last IRAN'd? If it's more than 500 hours ago, just bite the bullet. Then wait a few hours, and do the other.
 
It would be interesting to know what the corresponding plug gap was that initiated the high tension failure.
 
Lots of good info here - I appreciate the responses. We did test the leads for resistance and all were similar. There was carbon tracing on the bottom of the distributor finger in question. It looks like the leads have considerable extra length in them as they are tie-wrapped in the accessory compartment, so we were thinking of swapping the leads to verify the cold cylinder does not change - thereby ruling out the wire as the culprit.

They are Bendix model 6364 - right mag was replaced when I bought the plane back in '07, this is the left. I'm not sure how many hours are on the harness, but I've not replaced them. We had some record hot/cold days recently and my concrete hangar floor was very wet. I was telling myself this cannot be good for the plane to be sitting in this dampness.

The consensus is send it out for inspection & repair/replacement, then I'll do the other one at annual which is fast approaching. A new block runs @ 1/3 AMU.
 
nrpetersen said:
It would be interesting to know what the corresponding plug gap was that initiated the high tension failure.
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Are you saying the plug was the failure mechanism? Not familiar with the term high tensions failure, could you explain?

I don't have a gap feeler gauge in my hangar, but probably not a bad tool to have. It has been 9 months since they were last pulled, cleaned and gapped at annual. That plug looked very different from the others: it was black and grungy looking whereas the others were gray with some minor lead deposits.
 
High tension Ignition is what most aircraft have, this type of system sends a very high voltage from the mag to the spark plug via a shielded cable.
low tension system has a coil on each spark plug and has no ignition cables. I've only seen it used on large radial engines.
 
Thanks Tom & Jaws! As you know, I'm not overly mechanical but I swear, every failure I have (last time it was my voltage regulator) I learn so much from the experience. So here is a couple more snaps of the bottom of the distributor cap. These were taken when we first found the problem and wanted to get a better look. Not having a scope handy, we used our phones to grab some shots from below pointed skywardIMG_2612.JPG IMG_2604.JPG . Sorry about the (lack of) focus - they look better zoomed out.
 
iflyvfr said:
View attachment 60863
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That just gives me the impression that there was water in there at one time, the green gook was copper corrosion, is a good sign too, but the carbon arcing is pretty good proof of it.

Mag maintenance is a prime opportunity to be penny smart, and pound foolish. can spend quite a lump of cash every 500 hours, getting the SB complied with. or you can buy a new TCM electronic mag kit, and not have to worry about them the life of the engine.
 
Tom-D said:
That just gives me the impression that there was water in there at one time, the green gook was copper corrosion, is a good sign too, but the carbon arcing is pretty good proof of it.

Mag maintenance is a prime opportunity to be penny smart, and pound foolish. can spend quite a lump of cash every 500 hours, getting the SB complied with. or you can buy a new TCM electronic mag kit, and not have to worry about them the life of the engine.
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Tom makes a good point.
The mag's condition should dictate whether it's worth servicing or just replacing. Over the past 3 years I have replaced one LH starting magneto with a new one, and had 500 hr services done at a reasonable cost on two others by a grizzled old mechanic at my airport that has the tools and knows how to do it. The fourth mag on my Aztec will be done sometime this year - we will see if it's a replace or repair when we have to pull it.

There is also a shop in Missoula, Montana that does nothing but magneto work, and a few PoA members have posted they had good service from them. I used them on a previous airplane with good success (they used to be out not far from your base in coastal Washington back then Tom).
http://www.aircraftmagnetoservice.net/
 
Tom-D said:
That just gives me the impression that there was water in there at one time, the green gook was copper corrosion, is a good sign too, but the carbon arcing is pretty good proof of it.

Mag maintenance is a prime opportunity to be penny smart, and pound foolish. can spend quite a lump of cash every 500 hours, getting the SB complied with. or you can buy a new TCM electronic mag kit, and not have to worry about them the life of the engine.
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They make those? Were it I, that would be a sold, along with new wires and maybe plugs.
 
Bendix mags are bone simple and procedures are well documented. The internal timing tool is like $80. Any reasonably experienced A&P should be able to do it. About the most sophisticated tools you need are an arbor press and internal bearing puller to change the bearings.
 
My high tension note above was based on the voltage stress on the ignition leads is roughly proportional to the plug gap. If you have a plug gap that has eroded (or been set) too wide, the spark may jump elsewhere. Magneto type aircraft engines normally have a very small operating plug gaps (.016"?). As the plug gap opens up with use, the electrical stress on the rest of the system (including the magneto and leads) will increase.

That's why I tried to set the plug gaps near the minimum setting.
 
nrpetersen said:
My high tension note above was based on the voltage stress on the ignition leads is roughly proportional to the plug gap. If you have a plug gap that has eroded (or been set) too wide, the spark may jump elsewhere. Magneto type aircraft engines normally have a very small operating plug gaps (.016"?). As the plug gap opens up with use, the electrical stress on the rest of the system (including the magneto and leads) will increase.

That's why I tried to set the plug gaps near the minimum setting.
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Any mag that is operating properly will jump a nice bright blue arc half an inch. I'm having a difficult time believing that the difference between .018" and .021" will make any difference.
 
PaulS said:
They make those?
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TCM parts desk had a sale going a while back. the kit I bought for Gregs 34V was $2900.00 it included 2 new mags,2 harnesses, 12 new Champion plugs, and install hardware. The only thing I didn't like about it, was the leads were way too long, It irked me to set the ends back on a brand new harness.

What really gets expensive is when you must buy new gears and impulse couplings, those are near a grand per side.
 
you already have bendix so just OH the thing, I'd also debate just putting a new harness in if its old, I mean if you're in there anyways, also it's kinda like tires for me, cost to how they age, even if they seem fine, if they are pretty old it's really not that much and with how these things age, I'd just swap it over unless you're planning on selling the plane soon or something.
 
But under compression the increased voltage to fire with the greater clearance becomes more significant. The voltage needed to fire will be about 1/5 greater with the increased clearance.
 
nrpetersen said:
But under compression the increased voltage to fire with the greater clearance becomes more significant. The voltage needed to fire will be about 1/5 greater with the increased clearance.
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Probably a good theory, but in practice won't mean anything.
 
Thinking about it some more - A mag may generate a 1/2 inch spark (never seen quite that much tho) at atmospheric pressures. but probably won't tolerate more than 1/8 inch under compression. In that case, the harness etc breakdown margin would be much more critical.

We need a test! :rolleyes: Increase a plug gap to about 1/8 inch & see if the engine would still run or if the system would break down elsewhere. Better have a replacement mag and wire handy tho........!
 
GRG55 said:
...There is also a shop in Missoula, Montana that does nothing but magneto work, and a few PoA members have posted they had good service from them. I used them on a previous airplane with good success (they used to be out not far from your base in coastal Washington back then Tom).
http://www.aircraftmagnetoservice.net/
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I called them today and the gentleman who answered the phone talked my ear off with excellent information about my issue. He claimed to know exactly what my problem was (high resistance at the plug) and explained my options very well. The mag comes off later this week and will be shipped out for eval and repair/replacement as necessary. I'll also measure the plug to verify that it is out of spec.
 
Let'sgoflying! said:
Decided it's fun to see you guys jump at the chance to correct someone.
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Cool! Thanks for the chance, I'm jumping on it!! :)


Tom-D said:
when cracked the electrons will escape thru the crack to ground
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Negative. (pun surely intended :D)
Electrons travel from negative charge to positive so from ground into the crack.
Oh boy, now that I typed that, I might catch some hell for improper language. :)

Sorry, Tom, I just had to. :D
 
I have to wonder how many mags are actually run on a test stand before they are released?
 
Turns out there was a maintenance seminar in Columbus yesterday and my A&P attended. A couple of vendors were there including a Slick rep (I have Slick 6364s, not Bendix) who was confident my issue was moisture-based corrosion and advised to clean it up with alcohol and swabs, wipe everything down and test run it. We did and that fixed the issue. I tested all the plugs with a multi-meter (Tempest URM-37BY) and they passed with flying colors. By comparison, my spare Champion tested at 16,300 so it goes in the trash.

Sure glad it failed at home, on the ground.
 
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