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?