over voltage

[Flyingtodd47]

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

[rzbill]

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.

[airguy]

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.

[scsmith]

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

[JonJay]

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.

[krw5927]

Originally Posted by scsmith

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

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.

[BobTurner]

Originally Posted by scsmith

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

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.

[PCHunt]

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

[krw5927]

Originally Posted by PCHunt

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

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

[Fred.Stucklen]

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.

Originally Posted by BobTurner

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.