Possible Magneto Failure?

JLowe

JLowe

Pre-takeoff checklist
Joined
Jun 7, 2012
Messages
143
Location
Addison, TX
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Display name:
Jon
Hey guys, kinda new here hoping you could help me out. Recently purchased a 172P and ran into my first maintenance issue the other day. I want to make sure I'm not being stupid and missing something.

Symptoms: The issue first presented itself by failed mag check. I was able to run it lean and clear it out, and after a good test took to flight.

The next day I went up again and the plane failed the OTHER mag check. This time I was not able to clear it. I took a couple laps around the pattern and noticed it wasn't running perfectly, but not too much of a loss of RPM.

Landed and took it to the A&P. We tried to start it up, but it wouldn't start. First thing we did was take the plugs out and clean/gap them all. After a reinstall it would start but was severely failing R mag check... the engine wouldn't stop, but very strong vibration and loss of rpm. I overnighted some new plugs, and we replaced them with the same results... although it was running a bit smoother there were still some misses.

PLeads checked, ignition checked, timing checked, harness checked, leads us to the mags -- which by my look through the MX records are original (1982, 3000ttaf) It's possible they were replaced when the engine was replaced about 1000 hours ago, but that wasn't noted anywhere so I'm going with them being original. Oh yea, and they are Slick (4370/4371) mags.

There was a spare mag on field we took the 'flywheel' out of (Maybe thats not the name) and replace it into mine, but it seemed just as or more corroded as mine was. The new part didn't help any, so I decided not to fly it to my home base this weekend (KADS) and rather left it in Houston until she's better.

So, I've bought two new mags (expensive) and they are getting shipped to Houston for install on Wednesday morning. I was ok with the expense because after some investigation I've heard this should really help with fast starts, better power, reliability, and of course my piece of mind.

A&P seems pretty sure this will resolve the problem and we'll be good to go.

Could I be missing anything? Could there be another problem?

Thanks!
 
Just replaced both mags on mine and it runs fine now. I had failed mag checks as well. My situation was a little different though. I had a homebuilt electronic ignition on the right with a mag on the left. The electronic ignition failed and the left mag (800 hrs) just could make up for the failed side. I ordered 2 new mags for an outrageous price and had them installed. Runs like a charm now.:wink2:
 
Ignition switch, chafed P lead wires...

Swap the p-leads and see if the problem "moves" witht the wire switch. Also be sure the P leads are not already hooked up backwards. It would suck if you were working on the good mag because the wires are crossed.
 
Even if the 2 new mags don't fix it, it was time.
 
Tom-D said:
Even if the 2 new mags don't fix it, it was time.
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true-story-neil-patrick-harris.png
 
Tom-D said:
Even if the 2 new mags don't fix it, it was time.
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Yep that is kind of my thinking, but I would be very unhappy if this didn't fix the issue. Glad to hear you guys don't think I've made a totally stupid mistake.
 
JLowe said:
Yep that is kind of my thinking, but I would be very unhappy if this didn't fix the issue. Glad to hear you guys don't think I've made a totally stupid mistake.
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Mags can be changed any time, that is why many owners do not change them out at overhaul. thus you may have the OEM set.

which is why I do not recommend they 500 hour SB be complied with. they normally go TBO to TBO with no problems.

yours certainly did.
 
I can't add anything to what the A&P IA's have already posted on this thread. What I can say is that there is a shop in Mexia, Texas that can and will fix the problems you describe.

Buddy (shop owner) rebuilt my Bendix mag on a Saturday afternoon when it failed on a trip from Houston to Denver. You will not find a finer shop. They normally do heavy maintenance & inspections on turboprops, but also work on the single engine stuff when it presents itself. They are knowledgeable and have the parts to get the job done.
 
Clark1961 said:
I can't add anything to what the A&P IA's have already posted on this thread. What I can say is that there is a shop in Mexia, Texas that can and will fix the problems you describe.

Buddy (shop owner) rebuilt my Bendix mag on a Saturday afternoon when it failed on a trip from Houston to Denver. You will not find a finer shop. They normally do heavy maintenance & inspections on turboprops, but also work on the single engine stuff when it presents itself. They are knowledgeable and have the parts to get the job done.
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This would have been great to know Friday. :). Oh well, bright side is the A&P I found seems reasonable and hasn't minded my thousand questions. Down side is no parts on field, and I probably didn't win any there since I paid expedited shipping.

I appreciate all the help and advice. We'll see Wednesday afternoon.
 
JLowe said:
PLeads checked, ignition checked, timing checked, harness checked, leads us to the mags -- which by my look through the MX records are original (1982, 3000ttaf) It's possible they were replaced when the engine was replaced about 1000 hours ago, but that wasn't noted anywhere so I'm going with them being original. Oh yea, and they are Slick (4370/4371) mags.
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3000 hours is a dangerously long time for a magneto to run without inspecting. Even 1000 hours on a Slick will have the points pretty much burned out and the E-gap way off. The spark would be weak and maybe intermittent. A 3000-hour Slick would have worn-out distributor bearings and worn plastic gears, and probably a worn-out carbon brush.

I always did the Slick 500-hour checks on the school's mags and never had any failure issues beyond the failures caused by poor quality control at the factory on new mags. As noted above, the points were usually getting bad by 1000 hours, and as they wear, the E-gap (internal timing) shifts. There's no fix other than taking the mag off and doing the work.

Dan
 
Mags, we always hear about the failures, we never hear about the ones that go to TBO with out failure.
 
So all replaced and good to go. There is about a 120 rpm drop now on both of the mags, which to me seems a bit excessive since they are brand new, but it will run pretty smoothly on each mag, where it wouldn't at all before.

I was seeing some better performance again, so the new mags seem to have boosted my HP a little giving me about 10 more knots of airspeed.

Thanks for everyone's help.. Yea I agree 3000 hours is a ton of time to not have them overhauled. they didn't 'look' all crazy worn out on the inside, but I scoured the MX records for reference and there was none.

Anyways she flew home nicely yesterday, and I'm extremely happy to have her back where she belongs.

Side note, do girls think of their planes as boys?
 
Glad to hear things are fixed. Aircraft ownership is just wonderful.

Dunno 'bout how gals think of aircraft but Russians call ships "he" - they're a strange lot over there...

With the high mag drop, ya might wanna have the mixture (fuel flow) checked.
 
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Tom-D said:
Mags, we always hear about the failures, we never hear about the ones that go to TBO with out failure.
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I think I've lost one, right at TBO.

The mags on my plane probably wouldn't have made it, but they got pulled when they started to tell me it was time.
 
JLowe said:
So all replaced and good to go. There is about a 120 rpm drop now on both of the mags, which to me seems a bit excessive since they are brand new, but it will run pretty smoothly on each mag, where it wouldn't at all before.

I was seeing some better performance again, so the new mags seem to have boosted my HP a little giving me about 10 more knots of airspeed.

Thanks for everyone's help.. Yea I agree 3000 hours is a ton of time to not have them overhauled. they didn't 'look' all crazy worn out on the inside, but I scoured the MX records for reference and there was none.

Anyways she flew home nicely yesterday, and I'm extremely happy to have her back where she belongs.


Side note, do girls think of their planes as boys?
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You know I think I got a little more performance as well when I replaced mine. Maybe I'm just imagining it but she seems to have more accel on climbout than before. :dunno:
 
Starting this year I have been getting mag check failures occasionally too. Usually on hot days after a long taxi, but sometimes on not so hot days and not so long taxi. And it happens on both sides.

Leaning aggressively does seem to reduce the frequency, but we never used to have these failures even running rich during taxi (which we don't do any more). And it does happen occasionally even when we did lean it during taxi.

Just out of curiosity, what would be an approximate cost to replace a pair of mags for a Cessna 172 w 180 hp engine (if that makes a difference).
 
I have the 180hp too... the mags were about 2000$ plugs 200$ labor 450$
 
Flying101 said:
I guess to own your own plane you need to have BIGGGG bank account:eek:
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How can you have a big bank account when you spent it all on the aircraft?

You need to spend 1/2 of the account on the buy, the other half should be the operating budget for the aircraft.

IOWs if you want a 20k aircraft you should have 40k in the bank.
 
I don't want to hijack this thread, but the content of my question is still relevant. I contacted my A&P and he said my mag check failures are more likely due to plugs. I had forgotten that the mags were overhauled at the same time as the engine (about 250 hours ago). Does this conclusion sound reasonable, and if so could I expect any type of power boost by replacing the plugs?


JOhnH said:
Starting this year I have been getting mag check failures occasionally too. Usually on hot days after a long taxi, but sometimes on not so hot days and not so long taxi. And it happens on both sides.

Leaning aggressively does seem to reduce the frequency, but we never used to have these failures even running rich during taxi (which we don't do any more). And it does happen occasionally even when we did lean it during taxi.

Just out of curiosity, what would be an approximate cost to replace a pair of mags for a Cessna 172 w 180 hp engine (if that makes a difference).
Click to expand...
 
JOhnH said:
I don't want to hijack this thread, but the content of my question is still relevant. I contacted my A&P and he said my mag check failures are more likely due to plugs. I had forgotten that the mags were overhauled at the same time as the engine (about 250 hours ago). Does this conclusion sound reasonable, and if so could I expect any type of power boost by replacing the plugs?
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It's plausible. I had a bad plug (first flight after annual/cleaning/gapping last year...) that was discovered on mag-check. Recently saw some flakiness on mag-check, noted age of plugs, replaced them all. Smooth now.

Don't expect a power boost. If you get one, great, but don't expect it.
 
wsuffa said:
I
Don't expect a power boost. If you get one, great, but don't expect it.
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Yeah, that is in line with my expectations. But even without the boost, it still bothers me to have to frequently clear a failed mag check, then hold short waiting for clearance hoping it is not going to foul again. And a set of plugs is a lot cheaper than a set of mags.
 
JOhnH said:
Yeah, that is in line with my expectations. But even without the boost, it still bothers me to have to frequently clear a failed mag check, then hold short waiting for clearance hoping it is not going to foul again. And a set of plugs is a lot cheaper than a set of mags.
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Perhaps a step up in plug heat if possible on your engine will help too.
 
JOhnH said:
Yeah, that is in line with my expectations. But even without the boost, it still bothers me to have to frequently clear a failed mag check, then hold short waiting for clearance hoping it is not going to foul again. And a set of plugs is a lot cheaper than a set of mags.
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older plugs, (300 hours or more) will foul quicker than new plugs.
 
JOhnH said:
Yeah, that is in line with my expectations. But even without the boost, it still bothers me to have to frequently clear a failed mag check, then hold short waiting for clearance hoping it is not going to foul again. And a set of plugs is a lot cheaper than a set of mags.
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Lean heavily.
 
While bad plugs can cause roughness, is there any way they can cause damage to the magneto itself?
 
An open plug might cause carbon tracking in the distributor cap, but a shorted plug probably won't hurt the mag.
 
Quiet a few people run fine wire plugs in the bottom of the cylinders to help combat oil fouling.

Seems to help.

I'm running REM37BY and still getting fouling so I'm thinking of switching too. Also there are a few reports on various forums of people getting bad plugs right out of the box, I've never experienced it but there seems to be a fair bit of anecdotal evidence of it.

As someone alluded to earlier, a rich mixture will also give you larger mag drops, I guess that's why Lycoming stipulate the mag drop can be fairly high as long as it's smooth and both mags are within a certain rpm of each others drop.
 
Tom-D said:
please ! open plug? how.
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Bad resistor. Have had that happen in Champions too many times. It shows up on the sparkplug tester when the spark blows out at higher compression pressures.

Dan
 
Tom-D said:
Mags can be changed any time, that is why many owners do not change them out at overhaul. thus you may have the OEM set.

which is why I do not recommend they 500 hour SB be complied with. they normally go TBO to TBO with no problems.

yours certainly did.
Click to expand...

Am I reading this right? Don't comply with the recommended 500 hour inspection because so what if it fails, there are two?

I just had my annual done on my plane at a little under 500 hours. I had the 500 hour inspection done on both magnetos. Both looked good, with one minor part replaced. Sure it is annoying and expensive, but what price do you put on flying your family and friends safely? I will always follow recommended inspections. Anyway sorry to hijack the thread, just thought this comment was poor advice.
 
Skylane81E said:
Toms point was that mags typically make it to TBO...
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As someone who has "mag overhauls" near the top of their job description, I'll disagree with that point.

At least 25% of new Slick mags I look at won't pass their first 500 hour inspection. Either the brush has worn halfway through the coil tab or the gear is seizing in the block. This ignores the points being burned, although they could probably make 1000 hours if the rest of the mag held together.

You very seldom see new Bendix mags in the field and overhauled units reuse serviceable parts. I've seen a metric crapload of failed block bushings that take out the gear then no sparky. If you make it 500 hours on a reused block, you're doing exceptionally well. If you make it over 500 hours, you probably missed an AD that would have required removing the mag anyway.

We're also at a point where it is impossible to legally overhaul a Bendix dual mag. Some parts listed as mandatory replacement are NLA and the aftermarket companies have no intent on manufacturing them. What parts they do make are complete crap that typically fail on install. So your overhauled mag is really just an IRAN with unknown hours.

There is always a risk of failure after an inspection is done. But ignoring inspections and crossing our fingers isn't the solution.
 
Flying Ant said:
Quiet a few people run fine wire plugs in the bottom of the cylinders to help combat oil fouling.

Seems to help.

I'm running REM37BY and still getting fouling so I'm thinking of switching too. Also there are a few reports on various forums of people getting bad plugs right out of the box, I've never experienced it but there seems to be a fair bit of anecdotal evidence of it.

As someone alluded to earlier, a rich mixture will also give you larger mag drops, I guess that's why Lycoming stipulate the mag drop can be fairly high as long as it's smooth and both mags are within a certain rpm of each others drop.
Click to expand...

I have seen lots of junk Champion plugs out of the box. I wish I had stockpiled Auburns before they went away...:(
 
Gubbins said:
Am I reading this right?no Don't comply with the recommended 500 hour inspection because so what if it fails, there are two?
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We seldom see failures with out high mag drops and other signs of impending troubles.

Your engine will talk to you, you simply must listen.
 
Just giving an update on this thread -- apparently one of the two new magnetos I purchased has a manufacturing defect in the rivet that attaches the points. I've noticed that the left mag is a little more rough than I would think was normal, so I'm looking forward to having this swapped out and hopefully I'll be running like a champ.

I also broke down a bought a new wiring harness, since one of my leads is stripped (thanks previous owner). I figure if they are changing out the points in a mag, perfect time to have the harness changed.

I'm finally starting to understand all those threads I read about owning not saving you any money...
 
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