Check your Overvoltage Relay!

[JAG]

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