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Bad Contactor Diode?

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stearman456

Joined: 14 Aug 2010
Posts: 49

Posted: Tue May 08, 2012 5:56 am Post subject: Bad Contactor Diode?

Recently a friend of mine purchased an RV-6A, and shortly after buying it he turned the master switch on one morning, there was a "poof", and the wire supplying the ground to the battery contactor burnt up.  



The airplane needed some major electrical work (even prior to this).  It was wired basically according to Z-11 (I recognized the architecture straight away) but CRUDE!  Cripes...  Anyway, we changed out the contactor as it was pretty rough looking and I had a new one on the shelf from Spruce.  But it didn't have a spark catching diode so we used the old diode that the previous owner had made up.  This time when he powered it up (hanging onto the ground wire) he felt the wire getting hot immediately and killed the power before anything flamed.  Changed the contactor completely and put a new one from B&C (with the supplied diode) in and the problem went away.  So my question is, if the diode goes on the contactor, do you normally end up with a shorted out contactor?  And if you do, is there a fix or a safety, like putting a fuseable link in the contactor ground wire?



Thanks for all the help.

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nuckolls.bob(at)aeroelect
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Posted: Tue May 08, 2012 9:35 am Post subject: Bad Contactor Diode?

At 08:56 AM 5/8/2012, you wrote:

 --> AeroElectric-List message posted by: "stearman456" <warbirds(at)shaw.ca>



 Recently a friend of mine purchased an RV-6A, and shortly after buying it he turned the master switch on one morning, there was a "poof", and the wire supplying the ground to the battery contactor burnt up.  



     This is an exceedingly rare event. So rare that    legacy design goals in TC aircraft do not incorporate    any form of protection for this particular wire . . .



 The airplane needed some major electrical work (even prior to this).  It was wired basically according to Z-11 (I recognized the architecture straight away) but CRUDE!  Cripes...  Anyway, we changed out the contactor as it was pretty rough looking and I had a new one on the shelf from Spruce.  But it didn't have a spark catching diode so we used the old diode that the previous owner had made up.  This time when he powered it up (hanging onto the ground wire) he felt the wire getting hot immediately and killed the power before anything flamed.  Changed the contactor completely and put a new one from B&C (with the supplied diode) in and the problem went away.



     Helpful observations. These contactors don't generally    short. Their most common failure mode is errosion of contacts    followed in frequency by open coil. About 15 months ago we    did a tear-down inspection on a new-contactor failure. The    thread of this discussion can be found at:



  http://tinyurl.com/6mu7gba



     with pictures at:

 



  http://tinyurl.com/7petyz8



     In this case, the contactor suffered from assembly error    where the coil wires were not properly soldered.



 So my question is, if the diode goes on the contactor, do you normally end up with a shorted out contactor?



     Do you still have the old parts? Measure the resistance    of the contactor coils. Odds are high that they're okay    and will measure something on the order of 15 ohms. Also    check the diode with an ohmmeter. I'm betting that you    see equally LOW (short) resistance both ways through the    diode.



 And if you do, is there a fix or a safety, like putting a fuseable link in the contactor ground wire?



    You can do this. A 5A in-line fuse right at the contactor   would prevent a repeat . . . but I'd be interested in   knowing what the part number and source was for the diode.   It may be little comfort to say that this is a rare event . .

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stearman456

Joined: 14 Aug 2010
Posts: 49

Posted: Tue May 08, 2012 6:37 pm Post subject: Bad Contactor Diode?

Hi Bob,



I know I have the diode and the first replacement contactor, and I'm pretty sure the original contactor is still in the bottom of the garbage can in the shop.  I'll box it all up and get it headed your way this week.  I'd appreciate reading the autopsy report once you've had a look at it.  



Dan Jones

warbirds(at)shaw.ca



---

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nuckolls.bob(at)aeroelect
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Posted: Tue May 08, 2012 9:11 pm Post subject: Bad Contactor Diode?

At 09:23 PM 5/8/2012, you wrote:

 	  Quote: 	
		  



Hi Bob,



I know I have the diode and the first replacement contactor, and I'm 

pretty sure the original contactor is still in the bottom of the 

garbage can in the shop.  I'll box it all up and get it headed your 

way this week.  I'd appreciate reading the autopsy report once 

you've had a look at it.


	


   Very good.

   Bob . . .

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stearman456

Joined: 14 Aug 2010
Posts: 49

Posted: Thu May 10, 2012 6:25 am Post subject: Re: Bad Contactor Diode?

Hi Bob,



Found it all and it's boxed up, ready to ship.  Can you just email me the address that you'd like it sent to, please?



Dan

warbirds(at)shaw.ca

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nuckolls.bob(at)aeroelect
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Posted: Thu May 10, 2012 8:53 am Post subject: Bad Contactor Diode?

At 09:25 AM 5/10/2012, you wrote:

 	  Quote: 	
		  



Hi Bob,



Found it all and it's boxed up, ready to ship.  Can you just email 

me the address that you'd like it sent to, please?


	


box 130, Medicine Lodge, KS  67104

   Bob . . .

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