Overvoltage problem

[JoninNY]

I'm getting 'overvoltage' flashes and my A&P--who loves the 337 and knows it pretty well--is stumped. He says the regulators are fine, the fields work fine; he says it "ohms out but the voltage controls aren't working. Goes all over the place to 4 to 5 volts sometimes."
Anyone have any suggestions?

[RandyArizona]

I'll give it a shot...
Questions first:
Model and year of your aircraft.?
Original electrical system or changed?
All wire connections are clean, and strongly connected?
Have you isolated each engine to find out which one is performing the OV?
Alt. failure warning sensor is clean? (figure 15-6 service manual)
All fuses are good?
All connections are tight?

Now... there are diodes in the warning sensor, which after a few years kind of "wear out". Heat, vibration.... etc.
the ov relays themselves are sealed units. If you knew which alternator is giving the bad display, you could swap them out, and find out right quick which is the bad unit, or if it remains the same, the bad OV relay.
**** Dont swap out alternators... swap the OV relays... Had to edit to make it clear...****
Let me know how that goes.
Good luck.