[edasmus]

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.