PA-34 Seneca III “ALT INOP” Lite…

HGeorgeC*

Hi All!
Quick Question…
This has become more of a nuisance then a big problem, but I’d like to find out why?
Enroute, everything NormalNormal, I’ll occasionally lose the Left Alternator.
It will go off line and illuminate the “ALT INOP” light on the System Annunciator.
Prior to the failure the Ammeters will both be at approx 20, paralleled nice.
Upon failure, the Right Alternator picks up the load and I’ll comply with the POH Non-Normal Checklist and Reset the Left Alternator Switch, No Help. I’ve even pulled and reset the Left Alternator C/B and Rest the Switch again, No Help.
About 3-5 minutes later I’ll reset the Switch again and the Alternator comes alive! Then it may drop offline again…
This has happened several times now, only on the Left Alternator.
The Electrical Load has been nominal with only minimal components running, no Hot Props or Windshield. It been both day or night VMC.
I recently installed a new Concorde Battery.
I pulled the cowling to check the Alternator connections and all appears tight.
In checking the Logbook there doesn’t appear to be any history of it prior to me buying the airplane last year…
Thoughts??
Thanks in Advance!
George

empannin

George, I would suggest during ground operations at high idle and ~ 30 amp load you run the left alternator only (right off). I suspect it will give you Alt INOP in about 20 min or so. If the connections are tight then the next likely is something internal to the alternator that is overheating or is load dependent.

You might also listen to both alternators individually via the headphones. It is possible the left one has more noise that varies with rpm which can be diode issues.

When was it last overhauled? If it is very old or very new it is more suspect. Brand new expensive alternators have early fails too, even worse for overhauled.

If the diodes are failing you could get an AC ripple that is tripping the over voltage relay. Another technique would be to hook up a DC monitor to the cigarette lighter plug and look at AC/DC signature as a function of the alternator and load. Stress testing the alternator closer to 40-50 amps at high idle will reveal weaknesses. It is supposed to be able to operate 24/7 at rated load

I don't think there is any connection to your battery replacement.

If you want to upgrade to new voltage regulators with integral OV protection I might have two for my Seneca III that I am not using. Due to some ordering issues I ended up with 4 when 2 are needed. The regulators on your Seneca III are at the base of the aft interior of the nose cone.

From your description though it would seem more likely it is the alternator vs the wiring unless there is a loose connection you have not found.

HGeorgeC*

Hi Eric!
WoW! Thank You!!
I will run the checks you mentioned, talk to my A&P and I’ll let you know on the VR’s! Thank You for the offer!
Thanks Again, I’ll be in touch!
George

empannin

I just bought a fun setup for probing electrical systems. It is a basic multimeter/oscilloscope + a lighter plug. Once I have it I will measure left and right alternators + no alternators at idle and run-up rpms at different loads. ~ $100 bucks on Amazon.
Every device likely has some signature imposed on the DC bus. Will also check transponder, fans, etc.

I will look for the VR - it is possible I only have one spare.

HGeorgeC*

Hi Eric,
A quick update on my Seneca Alternator issue…
I was able to take a better look at the Alternator connections on the back. I put a 5/16 on the Field Lead and sure enough it tightened up!
I then ran the Checks you outlined and both Alternators worked well at Idle and Part Power, Loaded up and Unloaded.
So thank you for your time and expertise!
I had just felt around there at first for a loose wire, but when you mentioned the connections I thought I better look again.
Thanks Again!
Stay Safe, Fly Fun!
George
N8434M KLPR

empannin

George, excellent news! Sounds like a likely suspect for your intermittent issues.

FYI: I suspect well over 20% of electrical issues that are "fixed" by replacing alternators, etc are actually just connector issues. The new alternator with a tight connection works great - but would also have worked great by tightening the old connection...

griff390

Eric beat me to it, and I think he was conservative with his estimate. Can't begin to count how many electrical problems in computers, cars and aircraft were solved by simply reseating or tightening a loose connection.

In this forum, you'll often read "...always start with the basics..". That's great advice, and when it comes to the basics of electrical problems, my advice is to start with solid ground connections. If you don't confirm good grounds, you're in for a long and frustrating day of troubleshooting.

George; great to hear the problem is solved!