Davidv

Puzzling Electric Issue

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I’m hoping the collective wisdom of this forum can help me solve what has become an annoying issue.  A few months ago I was mid flight and I received a solid LEFT ALT indication on the annunciator panel and the left alt field breaker popped.  I continued on the right alternator and then had the problem diagnosed by my mechanic in FL.  They used a volt meter and determined the issue was in the left alternator with possible arching.  
 

I ordered a replacement alternator and had it installed a few days ago.  After installation the left field was not receiving any power.  We swapped the voltage regulator left and right plugs and sure enough the right was no dead and the left now had current.  The annunciator panel was now showing a flashing red indication for the left. We then determined that the voltage regulator needed to be repaired.  I went to a very reputable shop in Islip called Consolidated Aircraft Supply (at the recommendation of @kortopates and I was extremely impressed with their setup.  They had a test box that was built specifically for my regulator.  They found that a transistor in the left side was bad and had it replaced.  It was bench tested and worked perfectly in the shop.  
 

When I reinstalled the regulator in the plane the left side was still not functioning and the left alt field breaker popped as soon as I turned on the left rocker switch.  I brought it back to the shop and they retested the regulator to find that it was still working perfectly on both sides.

After many hours (and a lot of $), the issue is still not solved.  Has anyone seen this before or have any ideas?

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Does it blow the breaker on the ground without the engine running? If you disconnect the field wires from left alternator, does it still blow the breaker?

If it is blowing the field breaker, that means there is a problem with your regulator or its wiring. it is unlikely there was anything wrong with your alternator. It would need to be a short to ground in the rotor winding or between the two field wires. Although this is possible, I would consider this rare. It is also easy to trouble shoot with a meter.

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Electrical issues can be slow to track down, when looking for a short it’s very possible to move the wire and eliminate the short with out realizing it, but it will return. I would start at the circuit breaker and check the resistance across the terminals, high resistance equals heat which will cause the breaker to trip. If the breaker checks good look at the wiring diagram and see what is down stream from the circuit breaker, any component and/or wire could cause a short. I generally make a copy of the wiring diagram and use a highlighter to trace each wire I’ve checked, although not too many wires on that circuit. A quick way to check things is disconnecting everything down stream and reinstall things one at a time until the breaker trips, whatever was connect last will be more than likely the problem. Not a lot of help, but electrical problems are one of things you just have to jump in and start looking/testing 

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2 hours ago, N201MKTurbo said:

Does it blow the breaker on the ground without the engine running? If you disconnect the field wires from left alternator, does it still blow the breaker?

If it is blowing the field breaker, that means there is a problem with your regulator or its wiring. it is unlikely there was anything wrong with your alternator. It would need to be a short to ground in the rotor winding or between the two field wires. Although this is possible, I would consider this rare. It is also easy to trouble shoot with a meter.

Yes, it blew on the ground right after I hooked up the repaired alternator.  As I mentioned, the shop that repaired it checked it twice on their equipment and they claim it works fine.  I’m not sure about disconnecting the wires but I’ll troubleshoot that on Monday when I go back to the MSC that installed the alternator. 
 

Thanks for your help.

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1 hour ago, RLCarter said:

Electrical issues can be slow to track down, when looking for a short it’s very possible to move the wire and eliminate the short with out realizing it, but it will return. I would start at the circuit breaker and check the resistance across the terminals, high resistance equals heat which will cause the breaker to trip. If the breaker checks good look at the wiring diagram and see what is down stream from the circuit breaker, any component and/or wire could cause a short. I generally make a copy of the wiring diagram and use a highlighter to trace each wire I’ve checked, although not too many wires on that circuit. A quick way to check things is disconnecting everything down stream and reinstall things one at a time until the breaker trips, whatever was connect last will be more than likely the problem. Not a lot of help, but electrical problems are one of things you just have to jump in and start looking/testing 

Thanks, if I can’t solve it at first I may ask a friend who also has a bravo if I can put my repaired regulator in his plane to see if everything works the same. We will have to get out the wiring diagram as you suggest. 

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Grab a meter and one of the many good aircraft electrical troubleshooting guides and I am sure you'll find the issue soon. 

Here are some:

https://planepower.aero/aircraft-alternator-support/troubleshooting/

https://hartzell.aero/top-tips-for-troubleshooting-your-aircrafts-alternator/    and video: 

 

There are several good ones on Cessna owner page, but here is just one.

https://cessnaowner.org/everything-you-need-to-know-about-aircraft-electrical-in-1000-words/

 

 

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Alternator circuits are relatively simple. The regulator senses bus voltage and sends current to the alternator field in proportion to the differential between bus voltage and set point voltage. The circuit depends on sound connections in the field circuit, and in the ground paths from the alternator to the airframe. 

Dual alternator systems are only slightly more complex. Load sharing should be equal, and often times when under light loads one alternator will carry the full load, and the other will carry nothing, causing an inoperative light.When that occurs, adding significant load should extinguish the light. Turning on pitot heat is enough load to make the inoperative alternator come on line. If it does, then the system is normal. If not, you have to determine if the problem is in the regulator, the wiring, or the alternator itself. It is rarely the alternator (provided it has less than 1500 hours since new).

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