over-voltage

[rhurt]

Both alternators failed yesterday when I was ferrying a 337H from PA. In heavy haze over the mountains and the radio lights started getting dim. Neither alternator out light was energized, or the hi/lo voltage lights. The engine monitor showed 14 volts on the bus and there was enough power to cancel IFR before shutting off the master.

Got to better visibility and continued the flight while troubleshooting, finding the system would charge if the rear alternator was on, but would overvolt and pop off line if both alternators were on. The front would not charge by itself. The plane had voltage pulsing issues before and after the failure.

I suspect the front diode is bad and will let you know if that's correct. Maybe it will stop the pulsing issue also.

My 337C has diodes on the middle of the firewall just under the cowling. One diode/relay assembly and the over/under voltage assembly was replaced and the voltage flicker went away. It was likely the relay, but it is interesting the hi/lo voltage assembly is mounted under the rudder trim chain and lube drops on it after the chain is serviced.

[n86121]

While a diode, that is technically a high current rectifier. Allows each alternator to 'pull' current onto the power bus. Yes, if it is internally loose, then its shot. It could have been unloading and reloading the load and all sorts of mischief.

If you have trouble finding a 'cessna part,' I might think any common 100 AMP rectifier will do the trick, anything way above the current an alternator can actually produce. Modern rectifiers will also have less of a voltage drop, so everything works easier.