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Thread: 912 Help

  1. #11
    Senior Member Dorsal's Avatar
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    Default Re: 912 Help

    John / Bryan,
    The following is my understanding of the Rotax system.
    The alternator on the Rotax is a fifth pickup coil along with the four ignition coils integral to the engine. Two leads come from this to the regulator which is a separate electronics module which does the rectifying (ac to dc) and regulating (determining when to charge the system). I personally expect that if there is a failure in the charging system that is is with the regulator module. The good news is that unit is external to the engine and fairly easy to replace or possibly remove and have tested. One possibility is that the system only squeals when charging, therefore it should occur more frequently if you put more load on the electrical system (lading lights etc). Another possibility is the big-ass capacitor that is connected to the output of the regulator (cylindrical part ~1" diameter couple of inches long) has failed or become disconnected. I do not how likely that is to be the issue but if it is it is cheap and easy to fix.

    Intermittent gremlins are the worst, good luck.
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  2. #12
    Senior Member jtpitkin06's Avatar
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    Default Re: 912 Help

    Thanks Dorsal for chiming in on this one. I had assumed he had the external alternator. I didn't even think about the internal variety which is not really an alternator, it's a dynamo.

    In any case the hunt is on for the noise.

    Bryan, if you have the internal generator you can start by checking the output voltage with a simple volt meter. The Ducati regulator should hold the output at 14 volts DC. If the voltage is either above or below by more than +2 to -1 then the regulator is suspect. It is a less expensive place to start if you are going to shotgun the fix and it's plug and play.

    Failures of the dynamo rectifier/ regulator, be it Ducati, Deere or Kubota is pretty common. I think they are all made by Denso with different labels pasted on them. but they get hot by design and can fry themselves. The intermittent characteristic of the noise leads me to think it is heat related.

    The tach jumping up and down when you get the noise indicates a high frequency AC current is getting into the system. The Rotax uses a pickup coil for the tach. The tach sensor measures pulses from crankshaft rotation. If a sudden rush of high frequency AC current gets into the system from a bad regulator, the tach thinks the engine has gone wild and pegs. At the same time the audio system is telling you it doesn't like the AC current and it lets out a squeal. That stray AC current is not good for your radios, transponder intercom, etc.

    If you have a friend with an oscilloscope it will be worth your while to buy him lunch. A scope can spot a failing rectifier/regulator where a simple voltmeter will not. Have your friend test the electrical system with the engine running and the scope set to AC. The DC bus should ripple a small amount but the voltage should not vary more than 5 milivolts. If it does, your rectifier/regulator is bad. The scope will also show each coil in the dynamo. If one of those is bad it will show a repeating pattern like blip, blip, thud, blip, blip, thud.

    Failing to find a local guru with a scope, I encourage you to take the aircraft to an avionics shop. Even at $100 and hour it's less than replacing any one component in the blind. I'm also concerned that the spikes you are getting will damage other components and that could get real expensive very quickly.

    Good Luck
    JP

  3. #13
    Bryan's Avatar
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    Default Re: 912 Help

    My concern is that as it gets worse am I going to have an engine quit? Like when I cut the electrical system out to get it to stop, how long is the engine going to keep going. Right now I am a little affraid to leave the comfort of the airport area. I flew two nights ago for not quite an hour above the airport and it never acted up at all. Thanks for your help.

  4. #14
    Senior Member jtpitkin06's Avatar
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    Default Re: 912 Help

    It is not likely you will have the engine quit because of a dynamo failure. The Rotax 912 has three separate electrical generating systems. Two for ignition and one for the dynamo. The dynamo can fail and the ignitions will continue to operate.

    The internal dynamo is rated at 18 amps maximum and 14 amps continuous. If you put too much load on the dynamo it can overheat and burn out the stator coil. Just to be sure you are not overloading the dynamo, add up the amps of all the electrical items. The radio, lights, instruments, sirens, etc. Make sure they do no total over 14.

    The regulator is a combination rectifier and regulator. the dynamo puts out AC current, the rectifier changes the output to DC and the regulator limits the voltage to 14 volts.

    The only failures you can get are:
    Failure of the stator coil. Charging system will not work at all.
    Failure of the regulator. Charging system will show over volt or under voltage or complete failure.
    Failure of the rectifier. Wild AC current into DC system. Possible noise in audio system. Voltage appears normal or low. Fault can not be detected with a simple voltmeter. Can damage electrical components like radios.

    The rectifier portion is made of 4 diodes. One or even two diodes could fail and you will still get reduced charging but it could induce noise into the electrical system.

    It is still worth your while to have the electrical system checked with an oscilloscope; particularly your rectifier/ regulator. Your engine will not quit, but you could damage some other items.

    Let us know what you find.

    John

  5. #15
    Dave Holl's Avatar
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    Smile Re: 912 Help

    All the replies look to be good advice, I would also check the large capacitor on the output(normally blue in colour about 1inch in dia) this smooths the o/p voltage, check the connections and do visual chk for overheating.I have had one fail on another a/c type with similar indications
    Dave
    Dave Holl
    Building Kitfox MK7
    Rotax 912ULS

  6. #16
    Bryan's Avatar
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    Default Re: 912 Help

    I flew it for about an hour the other night and no problems. Like I stated you never know when it is going to happen. I am concerned that it may start doing this and I can not get it to stop. Or even worse cause an engine to stop. It is getting a little old circling the airport all the time just in case this happens.

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