M20R 2 GX - Alt Volt warning light on

[Yariv]

Hi,

I noticed the Alt Volt warning light today for the second time (it happened also a week ago and I executed the steps described here and thought the problem was gone) when I started the engine and turned the Alternator on. I tried resetting the associated circuit breakers but that did not change anything. I then decided to check if the stand by alternator would charge the battery and it did. I increased the RPM to 2300, turned off the main alternator and engaged the standby alternator and it started to charge the battery and the Alt volt warning light turned off. I then disengaged the standby alternator and turned on the main alternator and this time the alternator was charging the battery and there was no warning light. 

is this a symptom of one of the sensors gone bad?

I proceeded with the short flight, initially around the pattern and then 15 miles a way and there were no further issues during the flight. But it still bothers me obviously. 

thanks,

Yariv

 

[mike_elliott]

Have your alternator drive coupling tested

[GeeBee]

I will second what mike_elliot said. You should not fly the airplane with a low voltage light warning, even if it goes out with the standby alternator. The standby is a "get you to a suitable and safe landing place", not an optional alternator to go flying. The reason is the main alternator coupling could be going bad, possibly shredding rubber into your oil or even worse, mechanical damage occurring to the gear set which could set you up for engine failure. More than likely your problem is either an intermittent field wire, or an internal problem in your main alternator like bad diode, worn brushes etc. None the less, until you verify the coupling is sound, do not fly.

Here is what can happen

 

[Jeff_S]

In addition, check for corrosion on all contacts.  I had an intermittent (really, a one-time) issue with the main alternator going out in-flight. I was close to home so I flipped on the backup and just completed the flight.  Once on the ground, I tested everything and it worked fine.  The A/P investigated and found corrosion in the wiring, which is pretty common here by the ocean.  We cleaned that up and have had no issues since.

Well, except for right after annual the Oil Pressure suddenly dropped to near zero while taxiing back to the hangar from the shop.  Freaked the technician out! But again, corrosion in the cannon plug running between the sensor and the G1000 component was the culprit.  That's my go-to first step now when anything wonky happens that could remotely be related to electrical.  Just something to keep in mind, since the OP lives in Washington and maybe in the rainy parts.

[Yariv]

7 hours ago, GeeBee said:

I will second what mike_elliot said. You should not fly the airplane with a low voltage light warning, even if it goes out with the standby alternator. The standby is a "get you to a suitable and safe landing place", not an optional alternator to go flying. The reason is the main alternator coupling could be going bad, possibly shredding rubber into your oil or even worse, mechanical damage occurring to the gear set which could set you up for engine failure. More than likely your problem is either an intermittent field wire, or an internal problem in your main alternator like bad diode, worn brushes etc. None the less, until you verify the coupling is sound, do not fly.

Here is what can happen

 

Just to clarify, I did not go fly with the warning light on, nor with the standby alternator on. I went for the short flight only after I verified that the main alternator was charging the battery and the warning light turned off. Thanks for this information, I will take the airplane to the shop.