over voltage

I have noticed at take off very briefly I get a loud whine in my headset, it vanishes pretty quickly. But Saturday when I was flying, it kept coming and going. Though not as often at slightly lower rpm. After a couple of times I noticed that my volt meter was pegged when it whined. It vanished when I turned the alt. field current off. I've checked for a loose wire or dirty connection, everything looks good. I am using a cessna type split master switch, and have an internally regulated 60 amp alternator from Van's. It is 6 or 7 years old but has only been in use 20 hrs. Any suggestions would be appreciated.
Thanks Todd

You are on the right track in regards to tracing breaks and dirty connections. The symptoms lead one instantly to think the regulator is loosing its voltage sensing connection to the bus. It thinks zero volts and maxes out the alternator. Yours is obviously intermittent so the break could be inside the insulation of a wire.

Can be either in the positive voltage sensing line or the ground connection - equal likelihood. Both must be a good connection to maintain the voltage differential target for the regulator.

overvoltage with internal regulator?

This can't happen with an internal regulator, unless there is a bad wire/connection internal to the alternator, right?

Makes sense Steve. Troubleshooting internally regulated alternators can be fun. I made this jumper to completely isolate the airframe wiring from the alternator for troubleshooting things like this. If the problem is in the regulator circuit your symptoms would still be present unless the symptom of an over voltage is falsely caused by something other than the regulator circuit, like something throwing noise on the buss that might be fooling your voltage meter, and causing the noise.

scsmith said:

This can't happen with an internal regulator, unless there is a bad wire/connection internal to the alternator, right?

Click to expand...

Probably so. If however there was somehow a bad ground connection through the alternator case, to the engine, and then to the ground block which should be serving as the reference ground for the volt meter, the voltage swings could occur.

Do you have a ground strap from your ground block or ground plane to the engine case? If so, it seems probable that the issue is internal to the alternator.

scsmith said:

This can't happen with an internal regulator, unless there is a bad wire/connection internal to the alternator, right?

Click to expand...

IF it's a Plane Power alternator, they pick up the voltage reference off the field line. So some resistance in that line will cause the alternator output voltage to go up - assuming there's still enough current to run the field magnets.

Isn't a whine noise sometimes caused by a bad diode?

PCHunt said:

Isn't a whine noise sometimes caused by a bad diode?

Click to expand...

Yes, but the OP also said the volt meter pegged which "probably" wouldn't be a symptom of a bad diode.

OverVoltage

Sometimes the Cessna slip master can have a high resistance in the 'FIELD' portion of the switch. If the regulator (either external or internal) uses the field wire for the voltage reference, the the switch resistance will cause the regulator to increase the alternator output voltage.

You can check for this condition by shorting out the 'FIELD' contacts on the split master.

BobTurner said:

IF it's a Plane Power alternator, they pick up the voltage reference off the field line. So some resistance in that line will cause the alternator output voltage to go up - assuming there's still enough current to run the field magnets.

Click to expand...

Fred's right on. I've fixed a dozen Cessna alternator problems by replacing that red split switch. I got a new one if you want it.

So here is my over-voltage issue --

So, here is my over-voltage issue.

First,
I have a Plane Power internally regulated alternator. 400 hrs.
I have an Odyssey PC-680 battery.
My voltage is indicated by the Dynon EMS D-120. I know that typically it reads a little bit low compared to a direct measurement of buss voltage, I have always assumed that's because of the small-gauge power wire to the D-120 causing a small voltage drop to the meter inside the D-120.

In the past, while cruising, the voltage indication typically floats around 14.7-14.8, with no accessories on other than the avionics buss, which powers the radio, transponder, Trutrak ADI-pilot, and Garmin 396. If I turn on strobes or nav lights, the voltage indication drops to 14.4 or so.

Last flight, I got an intermittent high-voltage alarm, with the indicated voltage floating around 14.9 to 15.2 (I have a 'yellow' warning set at 14.9 and 'red' alarm at 15.2. When I turned the strobes on, the indicated voltage dropped to about 14.4 and all appeared well.

Today on a ground run-up, I duplicated the high voltage alarm symptoms, with EVERYTHING turned off except the Dynon itself, so....

1) I don't believe it is noise, since everything was off
2) I checked the ground strap from the engine to the airframe
3) I checked the plug on the back of the PP alternator. (although I didn't pinch the contacts as Dan H. described)
4) the field circuit switch has been cycled a few times, so I doubt there is any sudden change in the resistance of the field circuit.

Note that the voltage is not pegged high, it is floating around from just below to just above 15V.

Questions:
is this an early sign of impending failure?
Can it hurt anything to have 15V on the buss? If so, I'll just keep some load on the system with the strobes or something. ( I know....I'm supposed to have the strobes on all the time anyway)
Aside from buying a B&C alternator, should I do anything about it?

Note to those that frequently suggest switching to B&C, this is a non-trivial change because of the need to install an external regulator. Pretty significant surgery at this point to mount one on the bulkhead behind the panel, there's not really any room left there. And of course, figuring out and hacking into the existing wiring.

15 volts is too high for a maintenance charge of the battery.
I'd look for a poor connection somewhere in the field current line.