N

n505at

Member
While taxiing to the run up area I got a way high CHT reading on #1 while all others were nominal. During run-up I found the plug on #1 fired by the left mag did not work ? EGT went south. So it was back to the barn.

Bad plug is at the bottom of the cylinder. Wires from the CHT sensor were rubbing against the plug wire. I could see copper colored wire on the braid around the plug wire. I could not see defects in the CHT sensor wire.

I replaced the plug which did not look that bad. I made a spacer to keep the wires apart. I have not run the engine as I just ran out of time this afternoon.

I do not know the function of the braid around the plug wire. Is it reasonable to think this chaffing could cause both problems?

Thanks for the help

Roy Samuelson
RV7A at OAK
New Lyc O-360 Lasar ignition
 
I've never worked on a Lycoming but from my experience with cars and snowmobiles, It's possible that the insulation was damaged badly enough to allow the spark to grounds itself before getting to the plug.

On a car, you can get a nasty surprise if you touch a used spark plug wire : it looks fine but the insulation is so badly damaged by time(cracked) that the high current of the spark will arc between the conductor and your hand.
 
From workin on airplanes this is a common issue. I have seen cables rubing down to the condition that you state and have had some be just fine and others do not pass the current test. Look the cables over well, they make a test box to run a high voltage lead tester that you may be able to borrow from a local shop, this will tell you if there is a break down in the wire. If you are worried about it, replace it and be done. But I have seen some spark plug wires on Ag planes that looked like they were abour to break in half and worn from rubbing on baffling and they still passed the test.
 
Update 7/21/09

This must have been a puzzle as I only got 2 replies. I talked with my local mechanic who said the plug could be shorted out with strange current going to the CHT sensor. In talking with him and based on what I read on various other threads, it seems that routing sensor wires together with plug wires is not a good idea because of both chaffing problems and electronic noise. So I spent a few hours figuring out how to route the wires, keeping them separated. I fabricated tabs that hang from the bottom rocker arm cover screw which hold the sensor wires. I also fabricated some spacers that hold the wires apart with wire ties. I started the engine this afternoon. All seems well.
 
Similar Set-up; Replaced Harness

Last year I accomplished some work on my O-360 with LASAR ignition (installed higher compression pistons and new oil sump). I have over 500 hours on the engine and upon re-install I had a cold spark plug. Checked several things and noted chaffing (and some exposed wire) on the harness leading to that spark plug. Bit the bullet and purchased a new harness - all works very well now. If you need it I'll look up the part number, but it is the ignition harness from Aircraft Spruce.
 
n505at said:
I do not know the function of the braid around the plug wire. Is it reasonable to think this chaffing could cause both problems?


The braid around the plug wire is for noise suppression. Without it (or with the ground(s) at each end loose or missing) you may be able to hear nothing on the radios except the spark noise.

As far as the high temp reading, I would guess (although it's only a guess) that ignition noise was causing a false reading.

In my profession that is known as "Electromagnetic Interference".
Click to expand...
 
"Black wire spacers" from Lightspeed!

I used the "black wire spacers" from Lightspeed to separate the leads. You can use a screw through the middle of them to attach with adel clamps and stand the leads off so they will not rub against anything. It makes a real professional installation.

See link here: http://www.lightspeedengineering.com/Products/Accessories.htm

The harness came with four installed, I purchased another 6 to do the complete installation.

Regards, Tonny.
 
Glad I could help. Expensive for what they are (probably cost $ 0,02 to make), but compared to what other stuff cost for this fr*&^%n plane, it is peanuts, I guess.

Regards, Tonny.
 
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