Electrical/electronic theory question - Defender Source
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  #1  
Old December 20th, 2011, 11:12 AM
Roverlab
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Trevor Griffiths
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Electrical/electronic theory question

At 96k miles, I fitted a set of Magnecore wires, a pair of ignition coils and the customer supplied spark plugs. Several hundred miles later, the customer calls up and says the he sees little wisps of blue electric activity along the ignition wires, but no symptoms. He returns, Atlantic British warrants the wire set and we replace them. Now, @106k, the condition has returned, in the dark, one can see electical activity along the ignition wires.
The Rover has never set a CEL during this period, logged any pending codes, and drives very well ( except for a vibe that is driveline/bent wheel).
I'm about to pull the spark plugs and confirm they are correct for the application as I, unfortunately, don't think I paid much attention to what they were when installing them.
Spray down the wires with water and the activity seems to increase, but doesn't create a misfire.

Thoughts?

------ Follow up post added December 20th, 2011 11:17 AM ------

Follow up: the plugs are Champion RC11PYPB4
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  #2  
Old December 20th, 2011, 11:40 AM
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Crack in the dizzy cap, likely. That or it's St Elmo's Fire
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  #3  
Old December 20th, 2011, 11:54 AM
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Ooops, forgot to mention it's a 03-04 D2- distributorless igntion.
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  #4  
Old December 20th, 2011, 11:56 AM
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The simplest explanation is that the coil voltage exceeds the dielectric strength of the insulator. Breakdown of insulator+air gap is occurring.

If indeed a "blue electrical activity", then I believe the effect is self-repetitive corona with positive source, or current leakage through the breakdown gap. Assuming Townsend discharge, then the corona represents leakage with current in the range of 1 microamp to 1 miliamp. Current at the spark plug gap is much higher, in the 100-1000 amp range. This may be why ignition is unaffected.

Paschen curve of air at sea level says 30kV to 40kV will traverse 1cm air gap or roughly 2.8mm silicone RTV (due to linearity of dielectric strength with respect to thickness.) This is for a perfect insulator. If there are surface features, such as microcracks, or even bends, which cause the insulator to thin on the outside of the bend, then the equivalent gap decreases significantly.

Basically, I suspect that the plug wires you are using, if you cut them in half and look at the cross section, have relatively thin insulators, with a wall thickness close to the Paschen gap of roughly 2.8mm.
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  #5  
Old December 20th, 2011, 12:09 PM
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Have you checked the ground strap on the engine?
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  #6  
Old December 20th, 2011, 12:42 PM
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did you gap the plugs? friend of mine bought a VW at auction that was doing the same thing. thought it was the wires and changed those, problem did not go away. checked the plug gap and it had not been adjusted, just taken out of the box and installed. gapped them and all was well.
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  #7  
Old December 20th, 2011, 01:38 PM
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Trevor Griffiths
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We ck'd them when we installed the plugs. I pulled a couple randomly and they are at .040". Spec is .038-.041".
Haven't confirmed ground integrity, but I would think that starting, a/c, headlights, other high load stuff would contribute symptoms. The car has no other symptoms or abnormal issues. ( Well, at least during this saga. But it is a Rover, after all.)
Ed, although I'm not familiar with the technical jargon, your explanation seems reasonable. I'm stumped as I have used all of the suppliers of these parts many times over with never a complaint or failure. Why now? Admittedly, I've never spent much time looking for trouble on a car with no driveability sypmtoms! I imagine 2 sets of wires could be defective, but my mind says unlikely; what else could cause the electrical corona besides poor jacket insulation properties?
I removed the SAI tube and physically confirmed the routing of the wires as clean and tidy in looms, away from a/c pipes, away from SAI crossover pipe, even fitted a couple of MSD igntion wire looms to correct alignment to near perfection. Still, I know that I have never had to be this meticulous to obtain good results. Come on , night fall.
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  #8  
Old December 20th, 2011, 02:05 PM
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corona formation is known to be affected by the following properties:

low atmospheric pressure
surface non-uniformity of conductors on either side of the air/insulator gap.
increased coil voltage
presence of non-conductive chemicals in air gap

does the vehicle have a fuel vapor leak, crankcase ventilation open to atmosphere, or exhaust leak?
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  #9  
Old December 20th, 2011, 02:43 PM
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Most GEMs trucks seemed to do this when the air was moist. I would say it was pretty normal and never had a misfire on either of mine with magnecores.
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  #10  
Old December 20th, 2011, 02:54 PM
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Quote:
Originally Posted by evilfij View Post
Most GEMs trucks seemed to do this when the air was moist. I would say it was pretty normal and never had a misfire on either of mine with magnecores.
Pretty much same here. Although, I'm not sure humidity had much to do with it. It just did it all the time, but with no ill effects.
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  #11  
Old December 20th, 2011, 04:27 PM
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Quote:
Originally Posted by evilfij View Post
Most GEMs trucks seemed to do this when the air was moist. I would say it was pretty normal and never had a misfire on either of mine with magnecores.
Yup, St Elmo's Fire.
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  #12  
Old December 20th, 2011, 09:42 PM
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Trevor Griffiths
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After dark, I let it run for awhile, drove it, ran through puddles -we've had showers all day- left it idle and nothing. No problem found, no reason to "fix" anything, no reason to keep the car, but no explanation. Time to move on to other Rovers with obvious problems- a 90 for a frame, a 110 for a spun rod bearing, a p38 for a BeCM.
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