Check your Overvoltage Relay!
 

For early Skymasters out there - add another check to your annual inspection. FUNCTION CHECK YOUR OVERVOLTAGE RELAY (Thru 1967 models).

I experienced an electrical burning smell in the cockpit a couple weeks ago while in flight. I was flying back from the avionics shop after an install of a Garmin GTX 345 and GTN 750. I was very quick to shut everything down, so did not take notice of any indications. Long story short, I got everything powered back up and landed safely with no further signs of problems. The burnt out piece of equipment was my avionics cooling fan which the manufacturer said had internally overheated. I dug a little further downloading my Engine Monitor data and saw I had 20 seconds of over voltage (32 volts) leading up to me smelling the burning and shutting it all down.

So - took a while to find the culprit as it was intermittent, but it turned out to be my #2 voltage regulator was failing. I have replaced it with a new Zeftronics unit, but the real issue I had is why the over-voltage relay did not trip the alternators at 31.5 volts. I pulled it out and bench tested it the other day per the manual, and I discovered 2 problems. 1- the voltage sensor was not tripping until 34.5 volts (could hear it click). I was able to adjust that and had it clicking at 31.4 volts. 2 - as i was trying to understand what exactly the relay should do, I discovered the clicking I was hearing was not actually removing the output from the relay to the regulator. I opened it up and found that the relay coil had failed and not opening the circuit.

The point of this long winded post is: the over-voltage relay is installed in your aircraft to protect your very expensive avionics from experiencing a high voltage. Make sure this equipment is functioning on an annual basis so you don't let smoke out the wires like I did! Anyone interested in more info, let me know.

Jeff
1966 Cessna 337A

Great info, will take note. Thanks

One of first things I did when I got my 337 was replace the old fashioned voltage regulators with zeftronic units. Sure, you want a safety system to work. Better to limit need to activate safety system with updated parts.

Those units are great. I just replaced my old one with 2 new. They both put out a steady 28.5 volts 👍

Protip - If you don't have a voltage display, buy a USB to cigar lighter adapter that includes a digital voltage display. They are cheap, maybe $10 on Amazon. The voltage at the bus is slightly lower than the voltage the alternator is producing, but its close enough!

+1 for the solid-state regulators. But, they still require the rest of the system (overvolt, paralleling circuit board, overvolt test, alt restart system, diodes, etc), on my G model.

So I got all of old Tom Carr’s electrical system 337s and wrote a 337 eliminating these components and using Plane Power R1224 regulators. A fed buddy ran it thru Wichita ACO and local FSDO for field approval. The feds were enthusiastically supportive and helpful.

Of course, during this, the charging system has been behaving very well, but at the next hiccup, we are doing the mod.

Voltage Regulator - True Paralleling Conversion Kit
 

For anyone interested, I sell a $400 kit that includes two Cessna 400-series aircraft voltage regulators, a wiring diagram, instructions, and an approved 337 form that has been successfully copied word-for-word.

True parallel alternators and voltage regulators is the way to go. It assures the load is being shared, not flipped completely from one alternator to the other.

Not sure what Cessna was thinking when they installed the Rube Goldberg approved factory system

The kits installs in under an hour, and the only NEW wire added to your aircraft is a paralleling wire that connect the two new voltage regulators.

Everything on your panel works the same, and there is no change to the POH.

The only difference is you won't see the alternator lights go on and off during your flight!

Can we use your wire diagram with the zeftronic regulators or do we need the 400 Cessnas ?

I'm interested, PM sent.

Did you replace the over voltage sensor? If so where did you find a new one at?

Thanks.