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Rotax 912uls Failure of both CDI modules

in General Tech Discussion 12 years ago
We recently have experianced two seperate failures of cdi modules on engines fitted into Tecnam Echo type aircraft.One engine has 1900 hrs and the other only some 350 hrs.
The older engine developed rough running in flight and outlanded,subsquent inspection bench testing found both cdi modules slow to kick in below 1000 rpm and erratic.
The low hour engine had flown to a maintnance site and subsquently would not start,again the same symtoms were found with the modules..For both modules to fail together would appear to be Heat related
Or suffer an electrical spike possibly.??

Has anyone in the states or Africa experianced similar problem as our ambient temps are somewhere around the same??

  • Re: Rotax 912uls Failure of both CDI modules

    by » 12 years ago

    Hi Richard,

    I live in Tucson, AZ in the US. We are desert and get temps to 110F. No issues with heat and I would suspect that if both modules had an issue at the same time something else electrical has happened before the modules. I can't imagine both modules going out at exactly the same time, heat or not.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Rotax 912uls Failure of both CDI modules

    by » 12 years ago

    Hi Richard

    What type of modules are they.

    Do they have the soft start or are they older ones without.

    Mark
    Thank you said by: Mike Arthur
      
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  • Re: Rotax 912uls Failure of both CDI modules

    by » 12 years ago

    Hi Mark,the cdi modules are the std unit prior to soft start.I have been working on Rotax 2st and 4st ultralight engines for 25 years and had never heard of simultaneous failure of both units.I have heard a whisper that Rotax may in the future require aircraft manufactures to attach heat sensitive tape to an area under the engine cover to verify temps..They do state in the manual that temps shoud not exceed 80c I believe.We also have recently experianced isolated camshaft lobe/hyd lifter failure to three 912uls engines built from 2009-2011..1300 to 1900 hrs.Anybody heard of similar failures in the states??
      
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  • Re: Rotax 912uls Failure of both CDI modules

    by » 12 years ago

    Richard,
    It is highly unlikely both SMD modules have failed simultaneously.
    I have seen a couple of cases where the modules struggle to produce a spark at low cranking speed as they age. If the threshold RPM where they start to spark is close to the cranking speed it only takes a slightly flat battery for it to appear that both modules have failed, you may get some days where the engine wont start & others where it will. The modules do seem to deteriorate in installations that the modules are subject to high under cowl temps & it may be this deterioration that suddenly shows as a "double" SMD module failure, but it is more likely one has not been sparking at cranking speed for some time, but once the engine is running it works fine. If the second module has deteriorated to the point where it wont produce a spark at cranking speed you get the impression you have a simultaneous double SMD module failure, however I am reasonably confident that both will spark if you manage to crank the engine a little quicker. When started both Mags will work fine (the problem is the modules not producing a spark at low RPM). Swapping out the modules (or sometimes one module) often fixes the problem as the new ones have not been exposed to high temps for long periods & haven't deteriorated.
    The temp limit for the modules has always been in the installation manual (or for as long as I can remember anyway), but it doesn't always get measured.

    There are other factors like the cold weather, inadequate battery size, old batteries etc that reduce the cranking speed that can suddenly bring on these problems in the winter.
      
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  • Re: Rotax 912uls Failure of both CDI modules

    by » 12 years ago

    We know of many rotax 912 CDI failures. The most usual failure is the unability to start the engine from cold, in this case the CDIs will work and the engine will start after applying a heat gun for 3 or 4 minutes to the CDIs.

    So, why did your CDIs failed at the same time? They didn't. One failed some time ago, but the other still worked, therefore you were able to startup your engine. But then, why didnt you detect the problem during the test of the ignition circuits? because after 3 or 4 minutes of warm-up the failing CDI works, thus hiding its problem. You only detect this problem when the second CDI fails and then the big problem is revealed.
      
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