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  #1  
Old 04-06-2018, 12:46 AM
Paul Eastham's Avatar
Paul Eastham Paul Eastham is offline
 
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Default Inflight spark plug wire failure (Lightspeed/automotive style)

Thought others might be interested in this recent maintenance issue, didn't see anything like it in the archives.

Was on a short fuel run a few days back when I noticed something painted yellow on my Dynon. Cylinder #4 EGT was hot, 1500 degrees versus the usual 1200. Everything else seemed normal. Was almost home so didn't debug further, despite Mike Busch's admonitions...

On the ground, a mag check showed rough running on the right (Lightspeed ignition) and cylinder #4 going cold. Easy diagnosis, no spark, right?

On disassembly, spark plug tested good and wire tested good. Upstream of that is the coil, but the other cylinder that's fed from the same coil was firing normally, so the coil seems good...gotta be connectors I figured. After a bit of wiggling the wire around at different angles I saw the resistance spike and eventually cut out entirely.

I put in the order to Amazon for new cable and connectors and crimp dies -- and while waiting for that to arrive cut open the bad cable. The crimp doesn't seem to have fully captured the black/white conductor (see photo - barely), and when I poked around in the crimp a bit with my multimeter probe, a piece of thin wire broke off and fell out. Turns out these cables use a wound thin wire instead of a central conductor, and I guess it failed.

Interestingly, in the lightspeed manual, it says replace these cables every 10 years. Time on this cable: 11 years! FWIW, the cable was secured at two spaces along the engine pushrod, though I think some vibration probably happens. You'd think the larger crimp would protect this area ... maybe the cable was just defective from the start?

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Last edited by Paul Eastham : 04-06-2018 at 01:17 AM.
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  #2  
Old 04-06-2018, 01:24 AM
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Paul Eastham Paul Eastham is offline
 
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...looking a bit more in the archives, looks like a coil can fail for only one cylinder. So perhaps my reasoning is not quite right above, but hopefully reached the right conclusion anyway
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  #3  
Old 04-06-2018, 02:53 AM
Ron B. Ron B. is offline
 
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For what a coil is worth, might be wise to replace it along with the ign. leads!
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  #4  
Old 04-06-2018, 07:29 AM
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Thanks for the heads up Paul. I ordered a new coil this week to replace the one connected to a wire that fell off recently, per the manual. I did not order a new set of wires, they are 14 years old but did not show any issues when I put a meter on them. Based on your experience I will replace the entire set.
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  #5  
Old 04-06-2018, 08:16 AM
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I have made many sets of wires between hot rods, boats and airplanes and have found that some wire stock is more forgiving of assembly errors than others. Generally, the center conductor is folded back along the outside of the insulator jacket and then captured there by the closed back of the terminal (opposite the crimp). In some cases, the slightest nick of the center conductor will create enough of a stress riser to break outright when the conductor is folded back during the assembly/crimp process. In any case, it looks like yours was hastily assembled because only half of the crimp has captured the outer jacket. I'm suprised it lasted this long.
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  #6  
Old 04-06-2018, 09:34 AM
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Attaching better photo. It looks wrong to me, but according to the crimping instructions (bottom right of page 2) apparently that's how it's supposed to be on this style terminal.

Pushing the conductor underneath is the procedure for single crimp terminals, so it says.

If you zoom in to the photo you can see the black wire wound around the center (toward the top) and the missing wire lower down. It seems to have failed right where the crimp starts to make contact with the conductor.

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Last edited by Paul Eastham : 04-06-2018 at 09:45 AM.
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  #7  
Old 04-06-2018, 11:02 AM
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As many of these as I've made, I have yet to run across the dual crimp versions. Good stuff there. Makes a lot of sense for the really large center conductor variants of the wire. Hard to say if the original crimp was sub optimal, but I guess 11 years' service is pretty respectable. Something to pay attention to at assembly though. Appreciate the thread!
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #8  
Old 04-06-2018, 11:46 AM
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MSD Superconductor wires are assembled like this. They don't fold the conductor over like many other wire sets. You do have to take care when capturing the conductor and we recommend you use the proper crimping tool (MSD 35051).

http://www.sdsefi.com/msdwires.pdf
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  #9  
Old 04-06-2018, 11:58 AM
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Interesting, rv6ejguy, your link says not to use a crimper for the conductor. My link says to use the crimper Also seems to conflict with the youtube videos that MSD have posted.

" Following is a new crimp procedure for the Dual Crimp Terminal. ... Do not use the MiniCrimper to crimp the conductor."
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  #10  
Old 04-06-2018, 12:36 PM
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You can accomplish the closing of the 2 "wings" around the conductor in several ways. I find the end jaw on the MSD tool may not squeeze the wing quite tight enough on the conductor. You can start it with that jaw but I usually give the terminal an extra squeeze with some pliers. You want to be sure the spiral wound black conductor is firmly touching all 3 surfaces of the terminal.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 450.6 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
https://www.youtube.com/channel/UCiy...g2GvQfelECCGoQ


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