Overvoltage problem
 

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?

Hi Jon. You don't say what year your plane is. Mine is a 1969. On that plane, I have replaced the regulator selection switch several times. This switch can degrade over time, and when this happens, the resistance in series with the regulator input causes a voltage drop that causes the regulator to overshoot the correct operating voltage. One symptom of this is that toggling the switch will typically cause this to settle a little bit, however its not a fix. Generally, the resistance of the path from the system buss to the regulator sense input must be low. Any issue with this, most likely the switch, but can be caused by poor crimps etc. will cause the system voltage to creep up and show instability.

Concur B2c2
 

As I recall, one half of the master switch is a sense line to the bus, as well as each regulator switch provides regulator voltage sense.

Under no load their resistance will eppar to be "0" (or damn close), but under any load the resistance is actually more. Just old contact surfaces.

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The most extreme example of this I have experienced was when I used regular automotive battery cables on my boat. A wire is just a wire, right?

A few years later at start of one season, the starter wouldn't turn. I measured cable resistance, which was zero. I measured voltage at battery and going IN to starter, of course was 12... something. But somehow, magically, when hitting starter, the starter end would drop down to ~6 volts. The battery cable had corroded INSIDE the insulation somewhere, and when hit with 300 AMPS became a big resistor.

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Similarly, when those switch resistance increases under load (1-2 Amps I think), even 0.1 ohm can throw the sense voltages off, and the remote regulators and alternator control signals start chasing ghosts.

I recall seeing upwards of a volt across the 'closed' switches.

Being more lazy than cheap, I partially lifted the switches out, to expose their guts, then used contact cleaner squirted above and below and into the switches, and cycled the switches a LOT of times to 'wipe' their internal contact surfaces.

The 'closed' (under load) voltage across the switch contacts dropped down from around 1 volt to some teeny tiny fraction of a volt, "and all was well in the land,"

Hi there

I had an over voltage issue a couple of years back, with some great guidance, hours were spent testing every circuit breaker, all terminal connections and switch’s.

Although all switches, circuit breakers and terminal connectors were replaced during the restoration process some 5 years earlier.

What I found was a circuit breaker that was creating resistance, leads from the alternators to the charging control were also creating resistance, including some terminal connectors.

In effect we found 1350 ohms of resistance, once corrected the issue went away. Post the correction, I also decided to change the regulators and the over voltage relay.