Alternator variable voltage

Background: A few weeks ago, I had an alternator issue, basically a blown field fuse. Replaced, checked, and tested ok.

(http://www.vansairforce.com/community/showthread.php?t=81202)

I went up today for a Sunday afternoon flight. Cruising around at about 9000ft, I noticed that the buss voltage is varying between about 14.1-14.9V (as indicated on Dynon D120). I suspect that an overvoltage condition caused the field fuse to blow a few weeks ago, thus it looks like a similar situation may be occurring, i.e., replacing the fuse did not fix the fundamental problem. So, I'm looking for thoughts on what would cause the fluctuating voltage.

Plane power, 60A alternator; flying straight and level, no changes in electrical load. Voltage varies 14.1-14.9; sometimes it holds steady around 14.3-14.4 (which I consider normal from the previous 400+ hours of flying this airplane) for a while, then starts jumping around again. Ammeter bounces around a bit showing -2 to +5A, but generally +1 to +2A. I checked the belt recently, so I don't think it's a belt issue (but not absolutely certain of that).

I'm interested in any suggestions as to why this is happening, and how to fix it. I didn't find anything obvious in the archives.

Thanks,
Greg

Check Aerolectric Connection.

The ground for the alternator regulator must be common with the ground for the alternator. If not, the regulator can get bad ground info due to loads on that same line, or bad connections (thru a switch?). Resistance in this path can cause the regulator to sense the wrong (low) voltage causing Over Voltage symptoms.

Good Luck

A local pilot had voltage fluctuations much greater than you are seeing. An A&P checked it and saw debris (some possibly metallic) around the brushes I believe. That was cleaned and it works ok now.

Yes..that A&P is really good.

I agree with frazitl that a bad connection can cause over-voltage. Check the regulator ground and voltage-sense wires.
A loose connection will cause intermittent problems.
Joe Gores

Thanks for the replies. I'll check those things and see if it fixes the problem.

Greg

On a rainy day water dripped on my master switch. (split cessna type)

A small amount of water that got into the switch created the problem you describe.

voltage variance

I had a similar problem 100 hours ago on 729 total hours now. After reading an article by a Piper guru, he pointed to the split Cessna type master and he changed my operating procedures by starting the engine with the alternator switch off. In any event changing the switch stopped my regulator from varying the volts thru a switch that had varying resistance that caused the alternator volts to jump around. Forgot to say I put in a new switch. That solved the problem.

Update: finally got some time to check this and found a loose spade on my switch. Thought it was fixed, but then ended up with a loose battery connection as well. Short test flight and voltage is steady at 14.1, so I think it may be fixed.

Thanks for the input, and happy it was something simple.

Greg

Alternator ground

I had fluctuating voltage; checked alternator ground wire and ran a wire from the regulator base to the alternator ground site - problem fixed. It may have been the original alternator ground didn't have a good contact, or it may have needed the ground wire from the regulator that I added.

Thanks to fraziti for the heads up on this one - without such help, I'd probably still be buying new parts!

Barney, in Memphis
RV-3 & -4 flying.

Greg Arehart said:

Update: finally got some time to check this and found a loose spade on my switch. Thought it was fixed, but then ended up with a loose battery connection as well. Short test flight and voltage is steady at 14.1, so I think it may be fixed.

Thanks for the input, and happy it was something simple.

Greg

Click to expand...

Greg:

Years ago, I had a lose field connection. It caused intermittent over voltage spikes taking the alternator off line.

Your loose connection is a reason that one will get poor regulation and over voltage trips.

My most recent alternator issue was also intermittent. Was flying IFR and got a low voltage indication and battery discharge. Was only airborn about 20-minutes so asked to return to where I had just taken off. After two days of troubleshooting including replacement of 5-year old battery and 15-year old regulator, the intermittent issue still existed. Everything tested OK on the ground, runup, takeoff, but would trip turning crosswind. Bottom line, the 3-year old 410 hour alternator was replaced and everything worked well on the 1.3 hour test hop. I sent the alternator back to B&C for test, inspection, and repair. I hope to learn WHY it was intermittent. Have never seen an alternator intermittent that used an external regulator before.

IMHO, the only thing worse that a charging issue in VFR conditions is a charging issue in IFR conditions. Yes I was nervous but knew that I could make it back safely as the ceiling was high and visibility was good under the overcast.