Mag Problem?

danhagan

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danhagan
I have an intermittent problem that is getting annoying. I thought I had a fouled plug issue as my right mag is horrible in the run up area especially after the mechanics do run up testing (was theorizing they were full rich and fouling the plugs). After several minutes of running lean attempting to burn it off, it has cleared twice recently.

This past weekend I gave up, was taxiing back, and tested it again and miracle of miracles, it was smooth. Mag drop was HUGE this weekend, like it wasn't there HUGE, then a backfire. A buddy indicated check mag as well as ignition switch (which they are).

My question: How do you test a mag as a mechanic? Is there some type of equipment or bench test? I don't want to just haul off and change it, only to find I still have the same issue after new install.
 
Do you have a engine analyzer installed?


That makes it much easier and faster to figure out.


You running slicks?
 
I have an intermittent problem that is getting annoying. I thought I had a fouled plug issue as my right mag is horrible in the run up area especially after the mechanics do run up testing (was theorizing they were full rich and fouling the plugs). After several minutes of running lean attempting to burn it off, it has cleared twice recently.

This past weekend I gave up, was taxiing back, and tested it again and miracle of miracles, it was smooth. Mag drop was HUGE this weekend, like it wasn't there HUGE, then a backfire. A buddy indicated check mag as well as ignition switch (which they are).

My question: How do you test a mag as a mechanic? Is there some type of equipment or bench test? I don't want to just haul off and change it, only to find I still have the same issue after new install.

Example of a bendix

The magneto manual gives you the dimensions of two points separated by air. They call it a spark gap. The mag is hooked to a harness and a some sort of speed control / motor to turn the drive end. The manual specifies something like "at a 150 RPM the mag should fire with intermittent skips and at 200 RPM there should be no skips" That's the bench test for an assembled magneto.

There is a resistance check for both the secondary and primary coil.

There is a capacitance check of the capacitor (otherwise known as condenser")

There is a check ensure the points open at the right time called "e-gap"

That's about all there is to a msgneto.

Quick easy stuff you can do is:

If you have EGT gauge that covers all cylinders, let it run at idle on the suspected bad magneto. Watch for EGT drops or a very low one. If its only one cylinder greatly affected its likely a spark plug in that cylinder. Then note what position the mag switch is in.

Assuming your engine monitor readout matches the cylinder # on the crankcase, you can easy figure out which cylinder is having issues. Assuming the ignition switch wiring is correct you should be able to trace the spark plug wire from the mag that was running rough to the cylinder that had the EGT drop. Pull that spark plug, clean check the resistance and set the gap. If that solves your problems its just plug fouling.

I am an A&P and haven't had a fouled plug in a few years and found myself sitting on one end of the ramp trying everything to get a good mag drop check. Eventually I gave up after noting which mag was the problem and which cylinder was dying. This is what I found. From start to finish it took me about an hour to leave the airport that morning.

 
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Symptoms of a bad coil:

A bad coil usually works fine until it gets hot and then that magneto is completely dead and won't fire any cylinder. After the engine cools off on the ramp it might start and run normally again until the bad coil gets hot again.
 
Remove cowl, remove, clean and gap all the plugs, replace and do the mag check over again.

If the problem is still occurring remove the mag leads from each spark plug, allow the lead to set in the top of each plug. Start the engine, allow it to idle, while pulling each lead off the plug and see how far you can hold the lead away from the plug and still get a spark, minimum should be no less than 5/8th" and the spark should be bright blue. Any less than 5/8th" and any yellowing of the spark, simply replace the mag.
 
Swap plugs. See if problem moves to the left mag.
 
i have had a lot of problems with mag drop as well. It has been spark plugs every single time. We have to periodically clean and swap plugs. My partners on the plane do not always lean out the mixture during taxi etc and I think that has a lot to do with the fowling plugs and mag issues, in my scenario anyway per my A&P
 
How long has it been since your mags were rebuilt?
 
How long has it been since your mags were rebuilt?

Just checked and they have 1400 on them. mag drop isn't "mild" when it occurs, but it is intermittent. It improves/corrects after the motor has been running and has NEVER occurred while airborne
 
Just checked and they have 1400 on them. mag drop isn't "mild" when it occurs, but it is intermittent. It improves/corrects after the motor has been running and has NEVER occurred while airborne

There is your problem!
 
Just checked and they have 1400 on them.

1400 hours on overhaul or 1400 hours since any service was performed on them?

If it is the latter, I'd suggest having them looked at. If they're Slicks you're probably best off just buying some new magnetos at this point.
 
How 'bout the harness? If that's high hours too, check the cigarettes for little black spots or other signs of arcing to ground. Find somebody with a lead tester. Could also be carbon arcing on the distributor block if it's been that long since the mags have been serviced.

What you're describing sure sounds like a bad plug though. These are the times an egt/cht pays for itself.
 
1400 hours on overhaul or 1400 hours since any service was performed on them?

If it is the latter, I'd suggest having them looked at. If they're Slicks you're probably best off just buying some new magnetos at this point.

Slicks will easily make TBO if they're checked at the recommended 500-hour intervals. If they're not, they get dirty and worn inside to the point that they might not be salvageable at 1400 hours. Carbon dust can build to the point that flashover happens, which burns distributor parts, which are horribly expensive. 500-hour checks get that cleaned up before it makes trouble.

It's false economy to let mags run until they act up. It's like never changing the oil in the engine until it rattles apart.

There are two service bulletins dating from 2008 that might have a bearing in this case. Slick had some QA problems that made for short mag life, but if those SBs were done they were just fine.

Dan
 
1400 hours on overhaul or 1400 hours since any service was performed on them?

If it is the latter, I'd suggest having them looked at. If they're Slicks you're probably best off just buying some new magnetos at this point.

They're Slicks ... I can't find anything in the logs other than timing adjustments that anything was performed on them. If there's an SB, I guess it wasn't addressed as far as I can see. I don't see anything about inspecting or cleaning whatsoever.

To replace with core charge is about $900 each.

Pretty stupid of me, but I was assuming they were checked at each annual as part of the inspection (i.e. mags are not at 500 yet, so check again next year).
 
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You're lucky if you got 1400 out of them. Most don't make it through their second 500 hour inspection without repairs that cost more than the mag is worth. Heck, I've seen lots that failed their first inspection and a couple that grenaded within a few hours. The two SB's that the most relevant to you would be for the cam and the brush. Both are issues with hardness of the part. Cheap fixes but I'd be surprised if the rest of the mag isn't scrap already. You can't expect something to last when it's all cheap plastic assembled by a bunch of dyslexic monkeys on a bender.

Regardless, it's very rare for a mag to run better when hot or at altitude.
 
I stand by cheap first then move on.

#1 clean gap and measure resistance of plugs. Any over 5000 ohms get replaced.

#2 visual inspection of the plug wires. Age? Chafing? Burned areas?

#3 pull the harness caps off the mags and look for black sooty residue around the plastic distributor. If very dirty the mags need to come off the engine and get opened up for further eval.

#4 with mags on the bench it only takes a few minutes to take them apart and see the expensive bits. It bad, then you're looking for a different set.

I like TCM over champion/slick. I converted my engine to TCM mags. It's pretty simple but requires a set of spark plug wires too.
 
...not yet, anyway...

To me the carbon brush deal on slick should have been an AD.

Also, with them quietly changing the resistor installation on all thier spark plugs after doing nothing but denying denying denying that high resistance causes any problems, it seems to me they are more worried about profits than quality service.

Slick/champion parts prices are beyond ridiculous.

It's all just an opinion.
 
Here is what he will find, these pictures are from a 350 hour mag, what will he see at 1400 hours?
 

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Meh....have the mechanic IRAN the mags. Probably all wear items will need replacing. It really isn't a big deal for either Slick or Bendix mags. If you're not into having the repairs done....then exchange them for OH'd mags.

Either are very similar in construction IMHO.
 
Here is what he will find, these pictures are from a 350 hour mag, what will he see at 1400 hours?


Nothing unusual in any of those, for a Slick. I always did the 500-hour checks and those Slicks--every one of dozens of them--made TBO. I replaced exactly one distributor block out of all of those mags. I also did the SBs as soon as they came out. In most cases they needed new points at the 1000-hour mark, and nothing else at all beside cleaning out.

Dan
 
Nothing unusual in any of those, for a Slick. I always did the 500-hour checks and those Slicks--every one of dozens of them--made TBO. I replaced exactly one distributor block out of all of those mags. I also did the SBs as soon as they came out. In most cases they needed new points at the 1000-hour mark, and nothing else at all beside cleaning out.

Dan

I've seen several that were ignored with melted parts and it was pretty obvious the lack of cleaning them leads to an early death.

Who really wants to let a conductive powder collect in them until it causes premature failure?
 
A few typical Slick pics.

Pic #1 - Coil tab worn down to around 50% thickness. First inspection at 450 hours since new. Very common.

Pic #2 - New mag locked up within minutes, stripping acc. case gears.

Pic #3 - Small gear not installed correctly. Damage to both gears, block and contaminated contacts. First inspection at 500 hours since new. Common.

Pic #4 - Brush failed. Damage to gear and coil. Failure at 750 hours since new with no inspections.

Pic #5 - Contacts burned and not aligned. High mag drop at 200 hours since new. Common.
 

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A few typical Slick pics.

Pic #1 - Coil tab worn down to around 50% thickness. First inspection at 450 hours since new. Very common.

Pic #2 - New mag locked up within minutes, stripping acc. case gears.

Pic #3 - Small gear not installed correctly. Damage to both gears, block and contaminated contacts. First inspection at 500 hours since new. Common.

Pic #4 - Brush failed. Damage to gear and coil. Failure at 750 hours since new with no inspections.

Pic #5 - Contacts burned and not aligned. High mag drop at 200 hours since new. Common.

I never saw worn tabs like that first photo unless the tab was misaligned as per SB2-08A. Must have had a really abrasive carbon brush in it. Slick's QA, probably. I have seen the worn nylon gears, due to the small gear not being pressed on far enough. The plastic dust got into the points and made the mag rough, which is why the whole issue was discovered. The failed brush in #4 is also a SB 2-08A issue, and misaligned points were again a QA problem.

Slicks would be fine if they were assembled by competent people, but the typical North American factory these days tries to control payroll load by hiring cheap labor, and this is what we get.

Dan
 
I've seen coil tabs like that also on Slicks. The TCM magneto service manual allows you to repair coil tabs, saving money, but no such thing in the Slick/Champion manuals.
 
is there a reason those gears are plastic ?

First, so that they will run a long time with no lubrication. Take a photocopier or printer apart sometime. Or an electric clock. Or most electric drills these days. Plastic gears in them. Nylon has a self-lubricating quality to it, and in the old days they used Micarta, which was laminated layers of linen fabric mpregnated with phenolic resin (also know as urea-formaldehyde). It, too, needed little or no lubrication. Grease on magneto gears is a bad thing, since it ends up in the contact points and causes mag failure. The contact points cam uses a tiny bit of grease out of necessity, and a little too much also causes trouble.

Secondly, the distributor rotor, which is the larger of the two gears, is handing vast voltages that need insulating from the rest of the magneto. A metal gear would defeat that and short the spark to ground.

The loads involved in turning the distributor gear are so low as to be negligible. The plastic is plenty strong unless the mag isn't put together properly.

Dan
 
Meh....have the mechanic IRAN the mags. Probably all wear items will need replacing.

I never overhaul either, most A&P's in the field don't have the test bench to comply with the testing required by the overhaul manual.

Simply replace every thing worn or eroded, reassemble and test on the engine, you'll know soon enough you screwed the mag. :)
 
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