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Well folks, here's my update on this issue. It's been several months however I'm cautiously optimistic that this problem has been resolved. Time will tell as I fly along but here is the story.
I flew around 10 or so hours last November (2016) running the electrical system on one alternator or the other. The symptom would only appear while running on the front. Usually the over-voltage sensor would trip within an hour of running on the front. Switching to the rear alternator would yield several hours of trouble free electrical system operation. That seemed like reasonable evidence the problem was with the front side of the electrical system.
Also what I observed on my digital volt meter during operation on one alternator versus the other was that the front alternator was indicating plus/minus 2 to 2.5 volts rapid fluctuation but while running on the rear alternator, the voltage would only fluctuate plus\minus 0.1 volts. The rear was very stable. While running both alternators simultaneously, the wild fluctuation was present though not as amplified. Hmmmm, interesting.
The airplane went into it's annual inspection in December and that all went smoothly. My IA kept his eyes open for anything that looked unusual during the inspection regarding the electrical system. Nothing seemed amiss. For lack of a better plan, he removed the front alternator and sent it out for an IRAN. Neither one of us had much confidence that the front alternator was the problem but it is quite old so I was up for the experiment. Both of my alternators have been running trouble free for the life of my engines which are now beyond TBO. The shop (which I've used countless times with excellent service) opened the alternator and said "yes," it looks old in there but nothing inside the alternator was causing the problem. They cleaned it up, put it back together, and said "keep looking." Running the front engine with the freshly serviced alternator still yielded the same voltage fluctuations. The alternator shop was correct.
Well, one of the young ones in my maintenance shop who has been working under my primary IA went looking some more and discovered one of the diodes on the front firewall that is in the circuit which controls the over-voltage protection system appeared to have a defect. I'll attempt to describe.
The diode looks like a cylinder maybe an inch long and having a diameter of about a half an inch. The thing was enclosed inside a plastic case. There is a small metal pole protruding from this cylinder that a wire attaches to. Now for all I know the diode is encased inside this cylinder or maybe the entire cylinder is the diode. I have no clue how all this stuff works. That's why I pay these guys. Well, the little metal pole that is protruding from the cylinder was loose. You could actually wiggle it around inside the cylinder. Comparing it to the other diode on the firewall, that one was very tight. No play whatsoever. Replacing that $8.00 diode seems to have solved the problem. Voltage across the system running on a either single or both alternators is rock solid at 28.5 volts plus/minus 0.1 volts and no over-voltage tripping has yet to occur. Granted, I haven't flown much yet since the repair but so far so good. It's been a long winter. Better Wx ahead. Stay tuned.
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