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  #1  
Unread 11-14-17, 08:39 AM
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Brief Electrical System Malfunction

I had a strange situation on a recent cross country flight. First I noticed the radios “getting stcatchy” and they all lighting flickered a bunch and soon after that all electrical died entirely. I was a little concerned so began an orbit before I progressed any further into controlled airspace.

I checked all breakers - all good. Next I tried AV master Off then ON - nothing. After that I did AV master, BATT and ALT masters all OFF then ON and things slowly came back up. Within a minute or so all was working again and I continued gingerly on my way. It might be my imagination but the ONE instrument that didn’t seem to loose power entirely was my DME. Weird.

John K figures I maybe tripped the alternators and didn’t notice... This could be as I recently mounted my full sized iPad on the yoke and it sort of covers those lights. They are so bright that I definitely notice them when they occasionally flash on the ground but perhaps my position and/or focus is different in flight and I missed them. I hope this is true cause that will be much cheaper than other options...

Thoughts..?
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  #2  
Unread 11-14-17, 09:26 AM
edasmus edasmus is offline
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I cannot say for any certainty what occurred however the fact that the electrical system returned to normal after you turned the master switch off and then back on implies that you reset the over-voltage sensor. Read the POH on the over-voltage sensor operation.

I am currently struggling with this for the 3rd time in my ownership experience since 2002.

Simply put, the over-voltage sensor is designed to shut the alternators off if the sensor senses more than I think it is 32 or 32.5 volts across the electrical system. It is in place to protect your avionics from getting fried by too high of a voltage. Normal voltage should be about 28.5 volts across the system if all is set up and working properly.

If the over-voltage sensor trips, on the panel you should see the annunciator lights illuminated indicated the alternators are off and the red volts high light should also be illuminated indicating an over-voltage event occurred. The procedure to reset the over-voltage sensor included turning the master switch off for a few seconds to reset the sensor. The procedure is all spelled out in the POH.

In my POH for my 1973 G model, the manual states "if the condition was temporary, resetting the over-voltage sensor will return the electrical system to normal operation."

This implies to me that for some reason (please note I do not understand electricity) Cessna thought when they built the airplane that a temporary condition could exist that could cause this at some random time.

My personal experience with these conditions has been a malfunction is occurring in the system. They have not been temporary at all. The answer will come to you if this event reoccurs frequently as you fly along. In the mean time, I would fly along cautiously avoiding IFR conditions until you fully understand what is going on here. You wouldn't want to be in the clouds without electricity.

I have written posts on my experiences with over-voltage conditions on this forum that you can read. I am currently working on this again for the 3rd time on my airplane and do not have the answer yet for this latest round.

Keep in mind, that I have no clue what happened in your case. I simply am basing what I said on the behavior you described with the limited information in your post. If this happens again, see if the red "volts high" light is illuminated before turning the master switch off. This would verify if it was in fact an over-voltage event. Actually, let me rephrase..., it would indicate that the over-voltage sensor tripped. Maybe an over-voltage event occurred or maybe the sensor is faulty, tripping when it should not have.

Electrical problems in my opinion are the worst maintenance events to have. In my experience, nothing is more difficult to resolve.

Good Luck,

Ed
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  #3  
Unread 11-14-17, 12:11 PM
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A former member Pete Somers, God rest his soul, was a great contributor to this forum concerning electrical glitches

Tom, you may find some very interesting dialog in Pete's posts. I did a search on Over Voltage and there is lots of goods available on this forum. These events are never new to this forum and because we have about 15 years of posts on this forum there is probably an answer somewhere in the old posts. I didn't have time to drill down today on this issue so when you have time here is some info to research and learn

http://www.337skymaster.com/messages...archid=1327591
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Unread 11-14-17, 12:13 PM
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Quote:
Originally Posted by hharney View Post
A former member Pete Somers, God rest his soul, was a great contributor to this forum concerning electrical glitches

Tom, you may find some very interesting dialog in Pete's posts. I did a search on Over Voltage and there is lots of goods available on this forum. These events are never new to this forum and because we have about 15 years of posts on this forum there is probably an answer somewhere in the old posts. I didn't have time to drill down today on this issue so when you have time here is some info to research and learn

http://www.337skymaster.com/messages...archid=1327591
Thanks for this Herb. I will do some digging as I hope to avoid this predicament in the future.

TC
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Unread 11-22-17, 02:49 PM
B2C2 B2C2 is offline
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It has been pointed out in several posts but bears repeating that the system voltage will get unstable if you have more than the nominal resistance in the field circuit to the alternators. I have also several times had this issue. The first time all crimps were replaced behind the panel in the circuit feeding the field circuit for the alternators. The second time it was fixed by replacing the regulator selector switch. In my 1969 T337D this switch feeds either Reg 1 or Reg 2. Both regulators were showing jumpy voltages. Occasionally lighting the red overvoltage indicator. Replacing the regulator switch caused the voltage to be rock solid. After this was done I disassembled the switch and sure enough the terminals inside were badly worn. I would guess you had an overvoltage condition causing the alternators to go offline. then on recycling everything you got the circuit back. If you have a voltage indicator in your plane look at this. If its jumpy you probably have a field circuit issue.
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Unread 11-22-17, 06:07 PM
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Quote:
Originally Posted by B2C2 View Post
In my 1969 T337D this switch feeds either Reg 1 or Reg 2. Both regulators were showing jumpy voltages. Occasionally lighting the red overvoltage indicator. Replacing the regulator switch caused the voltage to be rock solid.
1973 G models and on, DO NOT have the regulator switch. Just so you are not confused the front / rear regulators are automatic in the G and H model Skymasters system. The older system is different and uses a pilot selectable regulator switch.
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Unread 11-23-17, 05:00 PM
edasmus edasmus is offline
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Thanks Herb...., you just answered my question!
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Unread 12-08-17, 03:00 PM
B2C2 B2C2 is offline
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I looked at the schematic for this system and it looks to me like both regulators are normally active, with the alternator generating the greater output actually acting as the charging alternator due to the diodes in series with the alternator outputs. At similar output voltages both would be active. This is nice as it would cause no issue with a regulator failure, but would limit you to one alternator in that situation. With the older design one regulator feeds both alternators, so if one regulator fails the regulator switch brings both alternators back on line. The design in the G and H models does make it so that if one of the alternators is spiking it will take down the system, and the regulators will still have the path resistance sensitivity, with the alternator field switch as a possible source of higher path resistance similar to the regulator switch in the older system. If you continue to have problems I would say a look at all the connections along the field circuit path starting at the buss bar is worthwhile to see if you have a source of high resistance.
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