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