over-voltage

cessnadriver · #1 03-09-18, 10:20 AM

Hello. I have a 1965 C337 and I know exactly where to look for the diodes. Open the avionics panel (the large panel outside-top of the front engine cowling) look outboard along the inside left sheet metal side of the fuselage, you see some large shunts about 1 1/2" wide and about 6" long, they have a wavy appearance. Just forward of this are two diodes screwed to the side of one of the shunts. To test these diodes your mechanic will have to cut the wire on the top of the diode and check it with an multi-meter WITH A DIODE SCALE. The meter should read flow through the diode with meter leads connected one way and NO FLOW with the leads connected the opposite way. The diodes are hard to remove but doable! Remove the two 6-32 screws to get at the nut on the bottom of the diodes and if you're smart your have your A&P mechanic re-locate these diodes for better access. The diode PART NUMBERS: original-6F20-D, superseded by 20H3P, superseded by (1N3881 this one is more common in non-aviation appliances and can purchased at www.mouser.com for about $6 each.)
I hope this helps, I've attached a charging system diagram the diodes are ITEM 10. This diagram is for the 38 AMP alternators the diodes are the same for the 30 AMP alternators!
regards, BILLS

gunrunner99 · #2 03-09-18, 11:50 AM

Thank you so much for all the help. She has been a good plane for te last couple months taking me to New Orleans Texas and a bunch of other places. I just want to make here a great cross country plane again.

GAdams · #3 03-12-18, 11:02 AM

I'm having a similar problem locating the diodes. Does anyone know the location for a 1969 D Model? An/or have a wiring diagram?

rhurt · #4 03-13-18, 03:17 PM

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 · #5 04-01-18, 02:01 PM

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.

hharney · #6 04-04-18, 07:53 PM

David,
Big Cheese

Bill S post above has all the info on the diodes. These are readily available and cheap. I ordered two just to see and they are real nice.

Quote:

Originally Posted by cessnadriver

Hello. I have a 1965 C337 and I know exactly where to look for the diodes. Open the avionics panel (the large panel outside-top of the front engine cowling) look outboard along the inside left sheet metal side of the fuselage, you see some large shunts about 1 1/2" wide and about 6" long, they have a wavy appearance. Just forward of this are two diodes screwed to the side of one of the shunts. To test these diodes your mechanic will have to cut the wire on the top of the diode and check it with an multi-meter WITH A DIODE SCALE. The meter should read flow through the diode with meter leads connected one way and NO FLOW with the leads connected the opposite way. The diodes are hard to remove but doable! Remove the two 6-32 screws to get at the nut on the bottom of the diodes and if you're smart your have your A&P mechanic re-locate these diodes for better access. The diode PART NUMBERS: original-6F20-D, superseded by 20H3P, superseded by (1N3881 this one is more common in non-aviation appliances and can purchased at www.mouser.com for about $6 each.)
I hope this helps, I've attached a charging system diagram the diodes are ITEM 10. This diagram is for the 38 AMP alternators the diodes are the same for the 30 AMP alternators!
regards, BILLS