Great news Ed
Thanks for posting all the trouble shooting. This will be of great help to someone else in the future.

Hello all! Well, it looks as though it has been 7 years since I last posted on this thread about my over-voltage issues! Well a Halloween horror story, the beast is back. Dang it!

Started two flights ago. In cruise climb about 10 minutes after departure all is well and a little zap and pop in my headset and sure enough, the over-voltage relay has tripped and the alternators are off. Crap, hmmm, well I reset and all goes well the rest of the day for the remaining two hours of flying. Passing anomaly I hope. Time will tell.

Next flight this past weekend, in cruise, level flight on auto-pilot for 30 minutes or so with absolutely nothing going on other than to enjoy the view. Then, out of no where, little zap and pop and there is that damn beast. Reset, complete the flight and while I'm taxing to the ramp, the beast hits again. WTF.... I say.

Well, I enjoy my Halloweenie party for the night, next morning, fly home. First 30 minutes, all is well, and then in cruise, on auto-pilot, doing nothin, the beast strikes again. I reset and 5 miles from home while leveling to slow for the pattern, he strikes again. I do a quick reset to get the gear out, fly the pattern turning from base to final all configured for landing, the beast strikes again. He's an active mother f---er. It's simply fascinating to me how for 7 years and probably 400 hours of trouble free electrical operation and then all of the sudden out of no where the monster awakes.

Well, Ill be headed to the shop in the morning but in the mean time, if anyone has ever had this symptom (and I realize many of you have, I have read all the horror stories on this forum) and found a "SMOKING GUN" that cured it (as I have when I started this thread), please do tell. I'm sure we will do all the usual connection cleaning and ground checking, etc. That zap I hear in my headset, is an arc. I need to find that arc. The breaker box will be the first place I go, but the problem will likely not be there. I remember that repair like it was yesterday. That insulator box that holds the breakers will last 1000 years.

For all that are interested, stay tuned for how this Halloween horror story evolves!

Note: I love my Skymaster, the Halloween story is for dramatic effect! ;o)

Bye for now....

Well folks, here's my update on this issue. It's been several months however I'm cautiously optimistic that this problem has been resolved. Time will tell as I fly along but here is the story.

I flew around 10 or so hours last November (2016) running the electrical system on one alternator or the other. The symptom would only appear while running on the front. Usually the over-voltage sensor would trip within an hour of running on the front. Switching to the rear alternator would yield several hours of trouble free electrical system operation. That seemed like reasonable evidence the problem was with the front side of the electrical system.

Also what I observed on my digital volt meter during operation on one alternator versus the other was that the front alternator was indicating plus/minus 2 to 2.5 volts rapid fluctuation but while running on the rear alternator, the voltage would only fluctuate plus\minus 0.1 volts. The rear was very stable. While running both alternators simultaneously, the wild fluctuation was present though not as amplified. Hmmmm, interesting.

The airplane went into it's annual inspection in December and that all went smoothly. My IA kept his eyes open for anything that looked unusual during the inspection regarding the electrical system. Nothing seemed amiss. For lack of a better plan, he removed the front alternator and sent it out for an IRAN. Neither one of us had much confidence that the front alternator was the problem but it is quite old so I was up for the experiment. Both of my alternators have been running trouble free for the life of my engines which are now beyond TBO. The shop (which I've used countless times with excellent service) opened the alternator and said "yes," it looks old in there but nothing inside the alternator was causing the problem. They cleaned it up, put it back together, and said "keep looking." Running the front engine with the freshly serviced alternator still yielded the same voltage fluctuations. The alternator shop was correct.

Well, one of the young ones in my maintenance shop who has been working under my primary IA went looking some more and discovered one of the diodes on the front firewall that is in the circuit which controls the over-voltage protection system appeared to have a defect. I'll attempt to describe.

The diode looks like a cylinder maybe an inch long and having a diameter of about a half an inch. The thing was enclosed inside a plastic case. There is a small metal pole protruding from this cylinder that a wire attaches to. Now for all I know the diode is encased inside this cylinder or maybe the entire cylinder is the diode. I have no clue how all this stuff works. That's why I pay these guys. Well, the little metal pole that is protruding from the cylinder was loose. You could actually wiggle it around inside the cylinder. Comparing it to the other diode on the firewall, that one was very tight. No play whatsoever. Replacing that $8.00 diode seems to have solved the problem. Voltage across the system running on a either single or both alternators is rock solid at 28.5 volts plus/minus 0.1 volts and no over-voltage tripping has yet to occur. Granted, I haven't flown much yet since the repair but so far so good. It's been a long winter. Better Wx ahead. Stay tuned.

Interesting.

Could you grab a picture the next time you have the cowl off?

Thanks,

John

Here's your photo....

Thank you.

I have been looking at my firewall trying to see where this is and no luck. Could you tell me where this is located on the firewall. Inside outside? Thanks. I have this problem intermittently.

Hi Mike

On my 74G; this part is located mid firewall, basically above the vac pump. It is part of the dual alternator control system.

Ed or one of the other members can tell you if this part is even relevant to your plane as I am not familiar with the earlier models.

Dave

Ps: Welcome to Skymaster ownership and this forum, members in the forum are all very helpful and passionate about Skymasters. No matter how trivial you may think your request may be, ask away.

Ditto SkyMac's post. Remove front cowling halves and look top dead center of the firewall. My airplane is a 73G model and would be identical to SkyMac's 74G. I have no idea what a 65 model would look like. Might be different. The diodes are enclosed in a case though so likely difficult to see anyway.

over-voltage--DIODES
 

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

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.

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?

Had the issue yesterday
 

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.

Image
 

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.

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.