912 Help

[Bryan]

I have a 912UL in my Kitfox and last night while flying I got a loud noise thru my headset. It would come and go and everytime this loud noise would happen my tach which is a Rotax unit would peg to full RPM then the noise would stop and the tach would go back to normal. It would cycle back and forth. I cut off the alt. circuit and then turned it back on and it stopped. Several minutes later it started this again and I did the same thing and it stopped. I landed and am wondering where to start looking for a problem. Any help would be appreciated. Thanks

[jtpitkin06]

You didn't say what the loud noise sounds like. Is it a loud short bang or a long squeal? Does the frequency vary or is it constant? Does the noise change with engine RPM? What was the voltage or ammeter reading when this occurred?

Because the tach indication changed at the same time, and many tachs drive from an output on the alternator, that's where I would look first.

If a diode is failing in the alternator it can cause havoc with audio systems. An internal short can cause ground loops.

I'll take a shot in the dark here and say you should take your alternator off and have it tested.

JP

[Bryan]

Thanks for the reply, it is a long squeal that comes and goes at an even pace. The RPM's of the engine never change it is just that when the squeal happens the tach jumps to full rpm's, and the loud noise comes thru the headset. I actually do not have the external alternator so what I mean when I shut off the alternator I am just shutting down the elecrical system. Then the problem is gone when I activate it back on.

[jtpitkin06]

Bryan,

Let's take the alternator out of the system by disconnecting the batt lead on the alternator and unplugging any sense wires.

Then start the engine and run with battery power only. If the noise is still there we can look elsewhere, but the tach going wild at the same time puts my money on the alternator.


Let me know what you find.

John

[Av8r3400]

If you run a 912 without a battery in the loop you will fry the charging system. $$$$$$$

Not recommended.

[jtpitkin06]

If it isn't clear... remove the batt lead from alternator and run the engine with the battery powering the system. All were doing is removing the alternator from the electrical system to eliminate it as a source of noise.

We're also disconnecting the sense line(s) to the alternator so there is no input to the alternator. the altenator will spin but there's no input, no sense line and the output is not connected.

The ships battery stays connected at all times. If the battery is disconnected, you would have no way of starting the engine.

Clear?

[Dorsal]

Not to add to the confusion but I think you are suggesting to disconnect the regulator from the alternator, the alternator is integral to the engine. Whereas one should not run the engine with the regulator connected to the alt and not the batt I believe you should be able to safely run with the regulator disconnected from both the alt and the batt. I assume that is what you meant by disconnecting the sense wires.

[jtpitkin06]

Yep, were trying to get the alternator out of the system. Short of disassembling the engine to get the alternator out. To avoid that huge labor task we are disconnecting all the alternator connections.

Audio noise is a hard dog to chase. We'll do what we can to chase it down. Right now, my money is on the alternator but we're trying to save some cash and not replace it shotgun style. so let's confirm it is, or is not, the source of the noise.

JP

[Bryan]

WOW you are all talking over my head. I will be the first to say that the electrical side of a plane is my weak area. I usually get help in this department. However I am not sure that I would want to do what you guys are talking about. I can not just start the engine up and test for problems. This problem only happens ever so often. I may fly 30 min. and then all of a sudden the noise comes thru my headset and the tach goes crazy. (Not the engine). It may give me 3 or 4 pulses and then quit and may not do it again for the rest of the flight. Or it may do it in 5 min. So as you can see I would probably not be able to diagnose this on the ground very easy. Its one of those GREMLINS that may be real hard to catch. I was just hoping that someone else may have had this happen and would instantly know what to look for.

[jtpitkin06]

OK Bryan, some folks are just not comfortable with ergs, amps and ohms.

Your symptoms sound like an intermittent failure in the alternator. Your tach RPM signal likely comes from the alternator. When the alternatior goes blitzoid, it takes the tach with it. Because the output is now a chopped DC instead of smooth DC it feeds that signal all through the electrical system like an oscillator. That creates noise in the headsets or speakers.

the intermittent condition can be caused by temperature changes; either internal temps or ambient air temps. Other contributing factors may be vibration or G loading.

If you are not comfortable doing electrical testing to locate the problem, just find a local Guru. Any avionics tech should be able to help you.

You asked what the problem might be and we are just trying to save you money and time in the avionics shop.

The condition is highly likely to get worse. Then you will have a hard failure that is easy to identify.

Good luck.
John Pitkin
Greenville, TX

[Dorsal]

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.

[jtpitkin06]

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

[Bryan]

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.

[jtpitkin06]

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

[Dave Holl]

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

[Bryan]

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.