Is my alternator going?

[cwaters]

Hey everyone, I have a '77 J that I got a little less than a year ago and have fallen in love with and put almost 100 hrs on it so far. It's my first plane and I don't know a ton on ownership yet. Last annual was just a few months ago and clean bill of health was issued. 

My wife an I flew about 1 hr to see some family for thanksgiving on Wednesday and had no issues. On the return trip the following day we had a low voltage warning light come on about 20-30 min into the flight. I went through the checklist and cycled power and the breaker and had no success. Landed at an airport about half way back home and cycled the checklist again leaving the power off for a bit longer and that seemed to do the trick. We continued the trip (only about 15-20 min left till home field) after an extended time on the ground with everything running and monitoring the EDM 900 voltage reading. We got the warning again about 5 min from landing. The voltage was reading about 13.5 steady for a few minutes then began to fluctuate down into the mid 12s then dropped to low 11V and stayed there wile amps were about -8 with minimal equipment on. 

 

I've got a call into the local mechanic on the home field to take a look but figured I'd come here and see if someone with some experience had any suggestions that might help narrow the search for the issue or tell me I'm looking at the wrong thing thinking its the alternator going out on me. 

[cbarry]

It may be the alternator, but it really sounds more like the voltage regulator is bad.  

[cwaters]

23 minutes ago, cbarry said:

It may be the alternator, but it really sounds more like the voltage regulator is bad.  

thanks, My mind when straight to Alternator but the voltage regulator makes more sense. 

[PT20J]

Maybe this will help.

[EricJ]

My first check would be that the field wire is secure at the alternator and the shield is not shorting to it.

 

[Guest]

Lots of possibilities for this.  How many hours on the alternator?  When were the brushes last checked? Broken field power or ground wires at the alternator. Loose or slipping drive belt.

Clarence 

[cwaters]

17 minutes ago, M20Doc said:

Lots of possibilities for this.  How many hours on the alternator?  When were the brushes last checked? Broken field power or ground wires at the alternator. Loose or slipping drive belt.

Clarence 

I don't have access to my logs at the current moment and won't for about 2 weeks. I'll have to check when I get back.

[PT20J]

Alternators are really pretty simple. Current is carried to the rotating field armature through a couple of brushes (wear item). The field current is controlled by the voltage regulator (sometime wires break especially where they connect to the alternator). The magnetic field from the armature excites current flow in a three phase stator that surrounds the armature. The three phase AC current is converted to DC by semiconductor diodes (failure point). The voltage regulator has power transistors that can fail. Check the easy things first.

Skip

[Bartman]

As others have written above and with several “like” comments, check the field wire before you do anything else. In 15 years of ownership I have experienced the exact scenario you described twice, and both times it was the field wire. 

[carusoam]

Field wires win for most probable cause of charging problems…

The wires have a tendency to vibrate, crack, and break…

The field wire is responsible for setting the alternator/generator’s output…

When the wire gets disconnected… the charging system stops putting out voltage…

 

Remember to put extra supports on your field wire when installing a new one…

PP thoughts only,

-a-

 

[Bartman]

Alright I jinxed myself.  
 

Right after posting on this thread the voltage light illuminated steady on initial climb. This was about 30 sec after electric gear retract and as I was setting up cruise climb. Pulled the field fuse and got the flashing voltage warning but after reset all was fine for the short flight to do 4 way GS calculations after adding shims to the lower engine mounts. I got 158kts 5,500 on 12gph @25/25. Not bad for the old girl in unmodified condition. I found the field terminal ever so slightly loose and crimped, but did not think it was the problem. 

It happened 2 times again today same circumstances. I set the JPI for continuous battery monitoring and it read 13.7 and usually 13.8 all the time otherwise an uneventful flight. Field wire good and terminal tight on post flight inspection.

I know this Kelly Aerospace alternator has at least 1,500 hrs so removed for rebuild but I see nothing obvious in the wires or bearings. New solid state VR about 10 yrs ago. 

[A64Pilot]

Alternators don’t usually come and go, they either work of don’t’, usually.

Loose wires / corroded grounds etc. do often cause intermittent problems.

I’d also be concerned with a battery who’s voltage dropped all the way down to 11V in only a few minutes with an 8 amp load though.

10.5V is DEAD and low 11 volt is very close to dead.

[RobertGary1]

1 minute ago, A64Pilot said:

Alternators don’t usually come and go, they either work of don’t’, usually.

Loose wires / corroded grounds etc. do often cause intermittent problems.

I’d also be concerned with a battery who’s voltage dropped all the way down to 11V in only a few minutes with an 8 amp load though.

10.5V is DEAD and low 11 volt is very close to dead.

In my case the bearings in the rear of the alternator were bad so it would drop in and out. Figured out that I could get it back by jockeying rpm.  

[BravoWhiskey]

I have a similar issue currently and just dropped the the plane off with a shop to troubleshoot shoot today.  In my case I drop to negative amps when taxi/landing lights are on and need to reset the alt to get positive back.  Once I have a diagnosis I will post to this thread.

[A64Pilot]

The field wire also runs through the split master switch, and I believe the spade connectors are known for building up some corrosion and causing intermittent operation, usually results in pulsating voltage, but the spade connectors are an easy, free check, try wiggling them and see if it makes an effect

[Bartman]

UMy problem was the alternator, but a simple fix. Before doing anything else we took a look at the brushes. My oil leak prior to last year's rebuild was “substantial” and required us to find new case halves. The alternator is the only thing we did not rebuild or send off for inspection, but I would have been money ahead for doing that. Lots of oil and gum and the brushes were compromised due to that, but otherwise plenty of meat on the bones. She is right as rain now, but needs a test flight.  Can’t do that in KY today. Below is a picture of my former hangar at KDVK that I vacated in 2018. The devising tornadoes that hit Western KY were of historic proportions. Pray for Western KY and Mayfield.