Magneto failing when hot

Discussion in 'Maintenance Bay' started by Jim K, Nov 18, 2021.

  1. Jim K

    Jim K Pattern Altitude PoA Supporter

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    Continuing the saga of my AOG back in August... we were flying back from AR and we started getting the surging engine again. We were pretty close to our fuel stop, so I continued, and the problem also continued to get worse. This time though I was able to isolate the issue to the left magneto. Engine ran smooth on the right only and nearly died on the left. All egt's would drop, which again pointed to a bad mag rather than a few fouled plugs.

    Here's where the plot thickens...at low power in the descent to land, the engine ran smoothly. I did a runup on the taxiway as soon as I pulled off and both mags ran smooth with drops within spec.

    Fueled up, took a break, and decided to continue home. I know this will be a controversial decision, but it was a 30 minute flight, it was running perfectly on the ground, I was on a hair trigger to abort, and I had 3 airports to fly over to get home. Of course the engine ran perfectly.

    At least it did until I was about 5 min out, at which point it began to display the same symptoms of the left mag dropping out. This seems to confirm my suspicion that something is causing the left mag to fail when it's hot. My money is on the coil or the capacitor, but I don't know a lot about magnetos beyond the theory of operation.

    The left set of points (bearing in mind this is a d3000 "dual mag") looked like it may have been arcing, but it's hard to tell. I believe that would point to the capacitor breaking down? Plugs, harnesses, and everything else looked good. Mag has about 125 hours since it was overhauled a year ago. We sent it back to poplar grove.... anyone hazard a guess on what they'll find?
     
  2. Dana

    Dana En-Route

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    Likely the coil, that's what mine was. It was a Slick 431. It would start missing when the oil temp got to around certain point, to make sure I taped a thermocouple to the mag and sure enough, as soon as it got to some temp I forget exactly, it started missing. On a single ignition engine, it got my attention! Replaced the coil and all was well.
     
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  3. RyanB

    RyanB Administrator Management Council Member PoA Supporter

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    I’d wager that it’s a bad coil as well. A bad coil will usually work fine until it gets hot and then that mag dies and won't fire any cylinder. After the engine cools off, it might start and run normally until the bad coil heats up again. What were your CHTs like when the engine was showing these symptoms?

    Have your A&P IRAN and be on your merry way, otherwise you’ll be waiting a month or more for QAA to get their act together for an overhaul. :)
     
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  4. Dan Thomas

    Dan Thomas Final Approach

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    My bet is on the condenser, based on the increased erosion of the left points set. That whole system relies on a functioning condenser. It does more than protect the points. When those points open, the coil's primary coil's field collapses through the secondary coil, generating the spark voltage. It has to collapse very quickly. If the condenser is dead or not even there, you get no spark, since the points start to open and the primary current will just arc across the gap as it widens until it can't jump anymore; that continued flow results in a slow collapse of the field and a weak or nonexistent spark. With the condenser good, the current will flow into it until the gap is too large to allow ionization and arcing to occur, and then the current stops and the field collapses. This takes microseconds.
     
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  5. Jim K

    Jim K Pattern Altitude PoA Supporter

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    I think a bit lower than normal, but to be honest I wasn't looking that close. I was mainly paying attention to egt.
    This was my train of thought, but my experience with points ignitions in automotive applications is that the condenser is either good or bad. Are they known to be heat sensitive?

    For that matter I don't really understand how heat would affect a coil.
     
  6. Magman

    Magman Line Up and Wait

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    I don’t disagree with any of the preceding possibilities .

    There are a few things peculiar to D mags you may want to check out.

    Assure the p-lead connection is tight . Some Bendix /TCM will ground out if even a

    little loose.

    The nut attaching the capacitor to harness must also be tight to assure ground.

    I believe there is a very old SB on this.

    it’s very easy to pick the wires from the points/ capacitor when installing

    the harness that will also give you grief.

    Toooo easy.

    Pitting on points is indicative of a bad capacitor.
     
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  7. wilkersk

    wilkersk Pattern Altitude

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    I remember reading an article recently, that talked about how doing the mag-check on the ground was a fairly un-demanding test and would only show really bad problems. The author ( I think it was Mike Busch) said that a real mag check would be to do it in-flight when you're trimmed up at cruise power.

    That being said, since you already know theres a problem, you might want to consider having your mags benched check. I think you can still find FBOs that'll do that for you. Any chance your mags are close to ready for their 500hr IRAN?
     
  8. Jim K

    Jim K Pattern Altitude PoA Supporter

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    He's in favor of checking them LOP. If they'll light a LOP mixture at cruise, they'll light anything. Mine went out for the 500 hour almost exactly one year ago, and have ~125 hours since then. Critically, the capacitors did NOT go with the mags. Of course if they had, they probably would've bench tested fine. As I mentioned they've already been sent out; I'm anxious to hear back from them if they find a smoking gun.
     
  9. brcase

    brcase Pattern Altitude

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    I had a mag fail when hot on me this summer. Would run up fine would even mag check fine in the air but once we landed (fuel stop) and let the mag heat soak for a bit it would run very rough with very high EGT readings. Let it cool down for an hour or so and it would fine again.

    1st time we noticed it, we hadn't figured out it was the Mag as I only noted the EGT readings on #1 and #2 cylinder, not sure why I didn't notice anything on the other cylinders. Changed the plugs all was fine until we got to our next stop, fortunately our next stop was our destination and being paranoid I checked it again as we taxied in and sure enough it was running very poorly on that Mag.
    Not my plane but I think they replaced the condenser in the Mag, had no issues after that.

    Brian
     
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  10. skyking3286

    skyking3286 Pre-takeoff checklist

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    Having gone through this with two Bendix SF4 mags, it's probably the coil. The coil has a primary and secondary part and when it heats up, the fine wires can expand and fail. They work fine when cold. The starting preflight check is perfect. You only discover this when you do a post flight mag check when the engine is hot and then you get a misfire. The other part of the question is for the overhaul shop to make sure that they heat the whole mag up to 165 degrees or so and recheck it before they release it with a yellow tag. The Bendix overhaul manual requires it, but most shops don't see that part of the instructions. A shop usually throws the condenser and points in it while the mag is opened up and on the bench.
     
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  11. Clip4

    Clip4 Final Approach

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    Not that uncommon for D3000. Micro fractures in the block.
     
  12. geneseib

    geneseib Line Up and Wait

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    Had an interesting one once. No problems on runups or in the air. But occasionally one mag would drop out during the takeoff roll. We aborted a few takeoffs. Turns out there was a scrape in the insulation on the grounding wire to one mag. The takeoff acceleration would sometimes move the wire just enough to short out the mag.
     
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  13. Racerx

    Racerx Cleared for Takeoff

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    Time to go surefly or electroair. Oh wait dual mag. The red headed stepchild. Nice diagnostics to be able to narrow things down and keep everyone safe.
     
  14. Dan Thomas

    Dan Thomas Final Approach

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    Read this whole thread: https://www.pilotsofamerica.com/community/threads/hot-mag-question.134103/#post-3139982

    Stuff expands and contracts as it heats and cools. A weak connection in a condenser or coil can be pulled apart when hot. Coils can short and arc internally, too, as can condensers.
     
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  15. Jim K

    Jim K Pattern Altitude PoA Supporter

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    I was flabbergasted to get a call Wednesday that my mags were back. Poplar grove turned them around in less than a week, so I can't complain about that. They also said since they did the rebuild a year ago they'd "give me a break" on the labor. We'll see what that means.

    Apparently he did find one of the coils dropping out. He also replaced the left points. He said the caps tested good, but recommended replacing them due to the burnt points and them being 15 years old.

    Plane will be back together this afternoon. Probably won't test fly it for a couple days. I've not been feeling 100%, and I need to fly it a couple hours to make sure the issue is gone.
     
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  16. Gary Ward

    Gary Ward Pre-takeoff checklist

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    Good to hear about Popular Grove, sounds like great service.

    I hope that you get to feeling better soon.

    Couple year ago I had my D mag overhauled at a local shop my mechanic recommend.

    It was an hour drive to the other side of town. They called me 48 hours later and said they were ready for pick up . I was only down for 4 days. It performed like new. I would rather drive than ship when this close.

    Professional Aircraft Accessories in Amelia Ohio.
     
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  17. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Bad coil. I went through this with the old Bendix heavyweight (S6LN-50's) mags. The coils were bad and a lot of the new old stock ones were bad in the package as well. Oddly, it would only fail after you flew it and shutdown (heat build up) and then you could sit there and run up and do a zillion mag checks without problem and when you got 200 feet in the air it would start misfiring.

    I finally got lightweight mags on and added some blast tubes to cool the mags and that solved the problem.