alternator goes offline at high RPM

This happened in flight yesterday. About an hour into a 2 hour flight, I noticed that my voltage was starting to fluctuate between 14 and 12 volts. At first, it was very quick blips -- enough to make the electronic FADEC system toss up some warnings. Then after another 10-15 minutes, the alternator was more often offline than online. Reducing the RPM seemed to bring it back online. Since I was close to a larger airport, I decided to shut the alternator down, land and investigate.

On taxi to a maintenance shop, i turned the alt field back on to see what I'd get. Surprisingly (or maybe not), the alt was working OK. Just for fun, I took the engine up to 2000 rpm -- the alt went offline. I thought maybe the field current terminal wasnt getting voltage (maybe a loose wire, etc).. checked it with a volt meter while running at 2000 rpm.. the field current terminal never lost power (no loose wires or switches)... even though the alternator was offline.

Is this alternator dead? (its got 1200 hours - from a suzuki samuri.. yea that one). I've never heard of an alt failing like this before. Should I get a new one from Vans? or replace it with the same part? Is there any other wiring that I should check before splurging?

Welcome to VAF!!

Rob, welcome to VAF

I would check the alternator, and the regulator.

Do you have an internal or external reg?

My basic gut feeling is regulator.

Could also be the CB, or even a battery going bad, but those are low probability issues.

replace regulator

and keep the old one in case. Easy to replace, easy to fail. While you have that assembly off check the brushes. If that does not solve problem; take the alternator to a reliable shop and have them check it. At that point it should be down to bad diodes, field or stator. If the regulator was not bad, keep it in your parts shelf. My brushes wore down in about 300 hrs. Planes wear out brushes much faster than the same alternator in a car, especially if you fly high. The basic Denso regulator is common to many of their alternators. Many alternator shops will sell you the parts at a good price; brushes $10-15, regulator about $20. You can also use the shell of the old regulator to convert to the external regulator from a 1975-80 Ford. Just modify the brushes because the Ford system regulates the positive side of the field and Denso regulates the ground side. A good alternator man can help you do this right.
photo:


link to pics and notes:
https://picasaweb.google.com/110878516619960001571/AviationExternalVoltageRegulator#

Replace the alternator

The Suzuki alternator that I have used have an internal regulator. Last one I got from Advanved auto parts for around $100 with a life time guarantee. Be sure you have a blast tube blowing on the regulator section of the unit. They have one version that is a 55 amp unit.