alternator issue or just a hiccup?

[JamesMooney]

Curious what MSers think of this:

 

1976 M20F. Just had my voltage regulator reset during annual (2 hours ago, because it had historically been low).

 

Went flying on Tuesday PM (daylight). During run-up, I had VOLTAGE annunciator light up and amps showed discharge. USB voltage meter showed ~12.5-13V. (Should have shown 14-14.2V)

 

I pulled the Alt Field circuit breaker twice. I also reset the Master switch. Each time, the voltage went back up to 14.1-14.2, the amps were charging, and annunciator went off. After 15 seconds, it all came back - discharging, 13V, annunicator lit.

 

The 2nd time I reset the Master, everything was fine. I also unplugged the USB charger, and noticed the volts went up from 13.7 to 14.2 (I was charging my iPad and iPhone). I waited 10 minutes, and then continued the fight. 2 hours flying, and the volts were 14V, amps charging (or neutral), and no annunciator lights.

 

Thoughts? Random post-annual gremlin? Alternator? Other?

[ArtVandelay]

Voltage regulator would be my 1st guess.

[Ragsf15e]

Well once, maybe.  Twice?  I wouldn't be going IFR until I was pretty confident in the electrical system.  The intermittant problems are the worst, but that doesn't seem right.  I'd start with the wires you guys messed with at the annual at the regulator then very carefully check the wires into and out of the alternator for any grounding.  They live in a tough spot down there.

[Marauder]

Voltage regulator would be my 1st guess.

I second your guess. When my VR went out, I had similar symptoms. My plane never (this over 30 years and 2 alternators) ever reached 14V. I would see a solid 13.8 but in the week before it failed, it would drop down in the low 13 range and occasionally down in the high 12s. Resetting the field breaker would bring it back up for periods of time back to 13.8. I was convinced it was the alternator going bad.

Then it hard failed. I replaced it with a Zeftronics and couldn’t be happier. Rock solid at 14.1V and has been this way for the past 3 years. Even at idle it maintains the voltage.


Sent from my iPhone using Tapatalk Pro

[EricJ]

Zeftronics has the awesome diagnostic LED, too.    That's very useful.

 

[Justin Schmidt]

Have a look at the Field Wire connector on the alternator. They get old and brittle, happened on mine.

[A64Pilot]

13 hours ago, jpschmidt said:

Have a look at the Field Wire connector on the alternator. They get old and brittle, happened on mine.

Agreed, especially right where the terminal crimps the wire, can’t hurt and costs nothing to verify 

Edited May 15, 2022 by A64Pilot

[N201MKTurbo]

My alternator became intermittent when I was on the road. A shop said they were too busy to fix it, but they were gracious enough to let me use some hangar space and tools. 
 

I removed the brush holder, one of  the rivets on the back of one of the field studs was very loose. I re-set the rivet and then soldered the stud to its wire lug. This fixed the problem and got me home. I ordered a new brush holder, but the repaired one would have worked for a very long time.

[Guest]

Start with the easy and work to the difficult.  Loose or slipping alternator drive belt, loose connections at the alternator, poor wiring, loose or poor connection at the alternator field switch, worn alternator brushes, bad alternator, bad voltage regulator.

Clarence