over-voltage
 

Hello all. I am new to this forum and though I have been reading your many posts over the past six years of Skymaster ownership, I have not felt the need to contribute until now as most of my maintenance issues have been pretty standard. Anyway, the following may be useful to anyone who has experienced an over-voltage situation where the over-voltage sensor trips off the alternators.

My airplane is a 1973 337G (no-turbo, no pressure). The cause of this problem I’m certain started to develop the day the airplane left the factory and took 35 years to get bad enough where a problem was evident. Anyway, the symptom was seemingly random over-voltage conditions causing the sensor to trip off the alternators. By random, I mean few and far between and no common action that caused the condition. I purchased the airplane in 2002 and the events occurred once or twice a year flying 75 to 100 hours a year on average. The pilot manual even addressed the condition stating, “After resetting the sensor, if the condition was temporary, then the sensor should not trip the alternators again.” Well, I reasoned that once or twice a year sounded pretty “temporary” to me.

As time passed the condition slowly got worse and the events reoccurred more frequently although no common action would cause it. Sometimes I would raise or lower the gear and it would happen and then not. Sometimes just sitting there in cruise, it would happen and then not. Running on one alternator or the other made no difference. Sometimes it would trip and then not.

Well, I trust my mechanic implicitly but at $75/hour I prefer to not send him on an “Easter egg hunt.” Fortunately, he trusts me also so he did not mind if a buddy and I went on the hunt for him and reported our findings. Well my buddy is much more knowledgeable about electricity than me so he took the lead. He went on the hunt searching for evidence of heat reasoning high resistance was causing excessive draw on the alternators in turn causing the over-voltage condition. He turned out to be mostly correct.

After poking around various locations, he opened up the circuit breaker panel located by the pilot’s left knee. He found the connection clearly showing heat damage. Forgive my layman’s terms here but maintenance tech I am not. On my airplane the entire buss assembly is placed in a recessed box in the side panel. This box is lined with a plastic container isolating it from the metal airframe behind. The main (thick) wire that comes from the front of the aircraft and is attached to the buss was the damaged one. What occurred over 35 years of vibration was that the connection to the buss and the wire was rubbing against the plastic liner, slowly but surely wearing a small hole in the plastic box. Eventually the connection was making intermittent and random contact with the metal airframe behind the box and slowly getting worse with each passing hour. When the connection contacted the airframe, the electricity that was supposed to power the buss was heading off to ground so the system cranked up the voltage from the alternators in an attempt to make up for the loss. The connection that would contact the airframe actually arced enough to melt a small pencil tipped size hole in the outer skin of the airframe. On the up side, the over-voltage protection system in the airplane worked flawlessly protecting my alternators from burning themselves up which most certainly would have otherwise occurred.

Well, the solution was simple. Repair the wire and repair the plastic box. The box is actually made of a much thicker and tougher material now though at the moment I cannot recall what my mechanic called the material. Suffice it to say, I am very confident that nothing behind that circuit breaker box will ever touch the airframe again. It has been about 6 months and 50 hours with no over-voltage condition occurring and I am certain it will not be happening as a result of this cause again.

I certainly do not wish any electrical problems or any problem on anyone but if someone out there is experiencing this problem hopefully this post will make your life a little easier.

Thank you so much for sharing this with us. At some point it will surely be of help to someone else. And I commend you for your initiative in tackling the diagnostic part of the work. In these tough economic times, that attitude can spell the difference between keeping and selling your Skymaster.

Ernie

You are welcome and thanks to everyone who has posted useful information. I have and continue to read as many as I can!

Thanks for the posts. I have a 1973 - same set up as yours. Ill check for the problem you posted. In my case some of the connections worked loose in the box to the point the avionics bus ground opened up. Fix was to just tighten up the contacts. Yours is probably tight and good to go after the work.

Hi John,

Have you had problems with over-voltage conditions occurring?

No over voltage in 10 yrs. now. Did have some intermittent low voltage over the years. It would show one alternator off and voltage would drop a little. That was fixed a year ago when an alternator drive failed in the rear engine. I take some credit for finding out how to adjust the voltage regulator and working with an A&P adjust it - problem is gone. I got to believe that the droping on and off due to poor voltage adjustment helped fail the alternator drive.

Over voltage
 

Hi,
This is good information. I also have had both circuit breakers pop. In the last 100 hours I had 2 overvolage problems. One was on take off after a 2 hour flite to Lake George,NY. I push the breaker in and turned the alt. switch back on and it happened again. Of course you loose all electric power. I waited 1 minute and reset breaker and this time did not turn on alternators. Electric power came back and I flew for 30 minutes just on battery. I told ATC I had a electric problem and if I lost radio or transponder I would use a portable com I had on board. After 30 minutes they said the transponder was failing. I than turned on 1 alternator and the breaker did not pop. After 5 minutes I turned on the other and all was OK.
I told my AP and he looked and found nothing. That was a year ago. I will pull the breaker panel and look for the problem you had. Thank you, Dale Campbell

Hi Dale,

It sounds as if your system is somewhat different than mine. By all means I would go ahead and check behind the circuit breaker panel but for what it is worth, my problem never caused any circuit breakers to trip. The only symptom was the over-voltage sensor shutting down the alternators which is exactly what the system is designed to do. To reset the system requires turning off the alternator switches and turning off the master switch for a few seconds followed by turning the master and alternator switches back on to restore normal operation of the electrical system. Assuming no problems, the system should obviously continue to function which was not the case for me until the problem behind the circuit breaker panel was uncovered.

Good Luck.

Over Voltage
 

I did shut every thing down and wait, as you mentioned, and followed the power back up procedure. But, as soon as I put the first alternator back on, the breaker poped again. That morning when I started my flight, I had to get a battery jump to get it started. I did not fly for 6 weeks and the battery was dead. I though when it started to charge,it over charged.
Because it was 2 hours into flight before the over voltage light came on and breaker poped.
After I drained down the battery by not turning on alternators for a half hour the battery was no longer over charged and when I turned alternators back on after that all was normal again. Problem never came back, so I forgot about it. I did purchase a battery charger with maintance feature that GMAS recommended on this site and I leave that on now when not flying. It seems to work great. Dale

Well folks, just a brief note. Almost two years and over 200 hours since I last posted on this thread about my "over-voltage" tails of woe and the ugly beast has reappeared in my Skymaster. Same symptoms as last time. Random "popping" in my headset and on the last flight during the landing gear up cycle, the over-voltage sensor decided it was time to shut down the alternators. After resetting the over-voltage relay per procedure in flight, for the next 20 minutes, the beast caused no further problems. The problem has appeared on my last two short flights (about an hour total) with the over-voltage protection tripping on the last flight.

Of course, the first place I went was behind my circuit breaker panel and all was in perfect condition. Now the airplane is all opened up looking for evidence of a short circuit and arcing. Myself, a friend, and my IA have been everywhere you can be within reason without completely dismantling the electrical system and can see nothing obvious.

The plan now is to put the airplane together enough to go fly with my IA early next week and hope the beast shows it's ugly head while my IA intends to be crawling around the cabin looking for sparks while I try to isolate the issue. Currently, the "haystack" so to speak is quite large. I'll keep you posted.

Any suggestions?

Ed

Ed...

YOU are the same age as my Skymaster. I wish I was in that age group!

Thanks Jim....Yes, it is kinda funny that me and my airplane are nearly the same age. In April of 2009 I had the opportunity to fly my Skymaster down to Springfield, MO to meet the first owner back in the early 1970's. I actually met him through this website. Anyway, his name is Paul and he is a super nice man. He gave me photo copies of his logbook that noted the hours that he flew the airplane, as well as the original invoice that showed all the options the plane had when it was new, and lastly a photo of the airplane when it was new. It was very cool and the fact that I was in the 1st grade at the time makes it even more special. I can clearly remember being in the 1st grade.

As for my electrical issues, I just returned home after flying with my IA and fortunately the electrical system misbehaved immediately upon gear retraction on take-off. He had his volt meter attached to the buss to monitor exactly what was happening. One thing that was clear before we even departed is that the alternators are set too high. They are making 30.5 to 31 volts and I guess it should be more like 28 to 28.5 volts. The fact that the voltage is so high leaves very little margin to the 32 volt limit for the over-voltage sensor during normal voltage fluctuations. He also suspects something is up with the alternator field circuit breakers but at this point I am not exactly sure what it all means. He feels the system is enough out of spec that after making some adjustments he wants me to fly some more and see how it goes. He cannot rule out arcing somewhere in the system similar to what caused the beginning of this thread but after searching the last 4 days and finding nothing he is going to make these adjustments and we will see what happens. Stay tuned.

Ed

Circuit breakers can surely be a problem. When I first got our Skymaster (17 years ago) we had problems with the main alternator bus breakers blowing. Everything would run fine for a while, then one of the mains would blow. Shortly after that, the other one would pop. There was just no way you could keep the alternators on line.

I finally purchased two new main breakers (35 amp as I recall), replaced the originals and the problem went away. Since that time, I have upgraded the alternators to the 60 amp units (The original were 38 amp) and, of course, have upgraded the wiring, breakers, etc. to support the higher current capability.

I gues the object of this reply (if there is one) is let you know that circuit breakers can weaken with age. The breakers are not that expensive and could save you some trouble. It does seem that you and your mechanic have most likely identified the real culprit with your voltage checks. I would expect that adjustment of those would eliminate the issue if you are simply having the electrical system shut down. The overvoltage system should not blow the field breaker when it activates. If the breaker is popping, I would suggest that you do both the regulator adjustments and replace the field breaker.

Good Luck and Blue Skies!

Over Voltage
 

Take a volt meter and check for resistance drops at each point in the field circuit...master switch, alternator switches, alternator circuit breakers, and any wire junctions or connectors. The smallest bit of resistance can cause you to have to adjust the VR's too high. Adjust the VR's to 28.5 volts. I find the best way to adjust them is to remove them from the firewall, and mount them on a board. I have a wiring harness that connects to the wires in the engine compartment, and I can have the VR's in the cockpit with me while I make the adjustments. The adjustments are VERY sensitive

Thanks Mark...

I have been meaning to update for the last day or two but just been short of time. The voltage has been successfully set to 28.5 volts. We also found signs of arcing on the master switch contacts so that was replaced. One alternator circuit breaker was replaced because of wandering resistance or voltage variations through it. You could tap the breaker and watch the variations on the meter. A diode was also added to the landing gear power pack because a consistent "pop" was heard when the gear hit the stops on the up cycle. That was the moment I was getting the over-voltage system to trip off the alternators. Since this work has been completed, the electrical system has been behaving normally. Nothing but the sound of silence in my headset other then the voice of the friendly ATC. I have about 6 or 7 hours on it in the last week with no problems so she is slowly regaining my confidence.

Ed

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.

https://www.slideshare.net/mobile/St...oting-test-box
If you're working on electrical systems on Skymasters this PowerPoint from my friends at Airscan in Florida is well worth looking at. There are actually quite a few power points that they've done. The one for the IO 550 engine conversion for the 337 is pretty interesting also