Thoughts on Plug Wires

Change the plug harness or not?

  • Yes. Change it. It's a good time to do it.

    Votes: 7 70.0%

  • No. Just replace the one bad one.

    Votes: 3 30.0%
  • Total voters
    10
Dav8or

Dav8or

Final Approach
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Dave
I have one plug wire that has the outside shielding scuffed away to expose the plastic insulation. It was rubbing on a metal hose ID tag. I have since solved the rubbing issue, but I guess I need to replace the wire. The wires I have are the older exposed metal shielding type.

I plan to do the 500 hour mag inspection and so it got me to wondering about just putting all new wires on while the mags are out. I guess the newer style Slick wire harness is about $400. Do plug wires wear out much? These have about 500 hours on them. In my old automotive days, we used to change wires frequently as they were cheap and brand and type mattered. I'm inclined to change them, but am I irrational?

The question you all might have is, am I having a problem? The answer is yes, sort of. Two issues with my Lycoming IO-360 A1A with Slick mags-

  1. The engine has never run all that well LOP. The entire fuel system has been cleaned adjusted and inspected. The intake gaskets have been replaced. I installed new plugs. I have never done anything to the mags or wires. My injectors are stock and I'm not able as of yet to be able to run the GAMI lean test because of erratic fuel flow indications, but I'm working on that. The rough LOP could be the injectors, but it could also be ignition.
  2. I recently had the engine start running rough in cruise and continued through taxi. Number 1 cylinder showed normal EGT, but low CHT. That means either ignition, or valves. Did a compression check and it was normal. Number 1 is the cylinder with the damaged plug wire. Checked both plugs and found lead fouling in the bottom of the bottom plug. Cleaned it out and taxied back for a run up. Run up seemed improved, but unfortunately, I couldn't fly because on the last run up, my prop cable connection failed. When it rains it pours I guess. I hope to have the prop fixed this week and I'll fly it.
Anyhow, I'm inclined to replace the plug harness in the pursuit of reliability and a good running engine that can run smoothly LOP. Is this crazy? Would you just replace the one wire, or the whole shebang? Is there a brand/type of harness that is better than another? So the lurkers can participate, there is a poll, but I would rather have opinions.

Thanks for the help!
 
I had a plug wire go bad and elected to change the harnesses on both mags. The wires had at least 1,000 hours on them. The mechanic found the bad wire by running prolonged wire tests after we had noticed a slight roughness in engine operation. Both mags had been rebuilt (one on the roadside so-to-speak) and the plugs had checked good. The fuel system was new (STC'd upgrade).

Maybe overkill to change the both harnesses but it's nickles-and-dimes in the scale of things. I was a little gun-shy after the mag failure in flight. I will say that the old wires were not pristine and that seems important to some people. I just looked at it as one more item that the previous owner had gotten their moneys worth out of and it was time for new.
 
Dav8or said:
I have one plug wire that has the outside shielding scuffed away to expose the plastic insulation. It was rubbing on a metal hose ID tag. I have since solved the rubbing issue, but I guess I need to replace the wire. The wires I have are the older exposed metal shielding type.



Thanks for the help!
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Depending on how bad it is or what the rest of the harness looks likw, I'd be tempted to fix it using some bare shield and F4 tape (otherwise known as rescue tape or self fusing silicone tape)

Tape:

http://www.aircraftspruce.com/catalog/appages/uniwrap.php?clickkey=21224

I wonder how I've ever lived without that tape. It will come in handy if you persue a brand new harness as well.

Shield:

http://www.aircraftspruce.com/catalog/elpages/shieldbraid.php?clickkey=8356
 
Also, depending on how the leads are installed into the harness cap (that plugs into the mag) your mech may or may not have the capability to replace just one at a time.

I'd be guessing they aren't easily replaced.
 
Plug wires will deteriorate with age. Best to replace them at overhaul time. Once the outer covering is worn through moisture will affect performance of the lead.

Assuming proper mechanical function and fuel flow, plugs, leads, or mags could be the cause of rough LOP. Bendix mags are better altitude mags than are Slicks.
 
Replace with Monster cable.
 
GeorgeC said:
Replace with Monster cable.
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Ooooo yeah! If there were Monster Harnesses with gold plated connectors I wouldn't even be posting this. Those babies would already be on there! I really don't want anything to degrade the digital spark signal and prevent me from having the optimum HD engine experience! ;)
 
One factor might be, can you tolerate a rough engine or being stuck at a remote airport with a failed mag check - if not, then changing them routinely based on age or hours....before they are obviously toast could pay off.
 
Let'sgoflying! said:
One factor might be, can you tolerate a rough engine or being stuck at a remote airport with a failed mag check - if not, then changing them routinely based on age or hours....before they are obviously toast could pay off.
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I agree with this thinking, but what I'm trying to get at is, are there many wire failures? I know plugs fail and mags fail, but the wires? How frequent is that?
 
When I sent my mags off for IRAN, I sent the plug wires too. A good mag shop can test the whole system and replace (only) the wires that are bad.
 
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Individual leads can be replaced, so as long as the rest of the harness looked good I'd just replace the one that was bad.

If they are all in tough shape and/or have bad springs then I'd just bite the bullet and replace them all.
 
Anybody know if there has been any significant improvement in wire technology in the last ten years or so? Is one type of wire better than another?
 
Dav8or said:
Anybody know if there has been any significant improvement in wire technology in the last ten years or so? Is one type of wire better than another?
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Start looking at prices... I like the Skytronics stuff. They send a pretty nice bag of hardware to install them.



You may have a choice between threaded magneto plates which cost a few more $$ or swaged plates.

Threaded plates allow changing individual leads with a wrench and you don't have to remove the entire harness from the engine to do it.

I doubt many mechanics have the tools, parts or experience to replace a swaged spark plug lead, meaning the harness would have to be removed (hours $$) and sent out (shipping $$) and re-installed (hours $$) for a lead replacement.

Probably 90% of the slick/champion harnesses are swaged FWIW.


Starting to look like Skytronics is now being sold by Kelly Aersopace. I have no idea if that is a good or bad thing. :confused: Things have been juggled again since I bought a harness.
 
EGT and CHT wires routed right alongside ignition leads? :confused: Might make for some funky readings if the shielding on the ignition leads is not 100%.
 
Norman said:
EGT and CHT wires routed right alongside ignition leads? :confused: Might make for some funky readings if the shielding on the ignition leads is not 100%.
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I'm aware of HIRF, EMI & RFI issues. Never had any problem with this aircraft.
 
Dav8or said:
Anybody know if there has been any significant improvement in wire technology in the last ten years or so? Is one type of wire better than another?
Click to expand...


There is a lot of science to take into consideration when choosing wire for any given application. I've yet to see it all in a single design guide. I have folders stuffed with articles that highlight specific points for specific applications.

Off the top of my head, i would say metal braided shielding on a mag lead is primarily to control radio frequency interferance. Understand that electrical noise couples several different ways, RFI is the radiated higher frequency noise.

A good repair might be a scheme that makes the braid intact again. Maybe a length of braided sleeve somehow electrically bonded to the existing slieve. I have not checked what AC43.13 says on this. I dont know if a couple of solder sleeves would work (make a good bond, not damage dielectric qualities of insulation, how it would endure temps near engine).

Regarding gold plating, from what i've read, gold only makes sense in applications where voltages & currents are very small, it's not very durable, not the best conductor, but does not tend to develop surface corrosion that higher voltage & current applications would easily control by burning it off (whetting current).

In my job as an avionics engineer at an airline, one day i decided to just go through an see how many different wire types Boeing used for my system, just on the avionics rack. I came up with something like 7 different types. Different metal compositions, different gage, different insulation, different shielding, etc. That's just on the rack, a very mild environment, dry environment.

I wish i had a copy of their design guide.


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