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  #1  
Unread 11-10-08, 09:22 PM
edasmus edasmus is offline
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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.
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  #2  
Unread 11-11-08, 12:22 AM
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Ernie Martin Ernie Martin is offline
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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
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  #3  
Unread 11-11-08, 07:09 AM
edasmus edasmus is offline
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You are welcome and thanks to everyone who has posted useful information. I have and continue to read as many as I can!

Last edited by edasmus : 11-11-08 at 11:24 AM.
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Unread 11-11-08, 09:37 PM
John Hoffman John Hoffman is offline
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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.
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Unread 11-11-08, 09:44 PM
edasmus edasmus is offline
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Hi John,

Have you had problems with over-voltage conditions occurring?
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Unread 11-11-08, 09:56 PM
John Hoffman John Hoffman is offline
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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.
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  #7  
Unread 10-04-10, 08:59 PM
Mark Hislop Mark Hislop is offline
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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
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Unread 10-04-10, 09:36 PM
edasmus edasmus is offline
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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
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  #9  
Unread 10-05-10, 01:30 PM
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Great news Ed
Thanks for posting all the trouble shooting. This will be of great help to someone else in the future.
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  #10  
Unread 10-29-17, 10:24 PM
edasmus edasmus is offline
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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....
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  #11  
Unread 02-24-18, 05:19 PM
edasmus edasmus is offline
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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.
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  #12  
Unread 04-24-18, 08:55 AM
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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
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Last edited by general : 04-24-18 at 11:02 AM.
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