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Old 04-21-2022, 11:22 AM
DavidHarris DavidHarris is offline
Join Date: May 2018
Location: Upland, CA
Posts: 166
Default High Spark Plug Wire Resistance

I've found three spark plug wires with unusually high resistance and am looking for advice.

I'm doing my second condition inspection on a Vans RV7A with a Superior XPIO360 engine and dual pMags. I'm using BR8ES automotive plugs. The aircraft is flying well and has no obvious ignition issues. The spark plug wires are supposed to have about 180 ohms/foot of resistance, and were in that ballpark on the first condition inspection.

Last night I observed 400-600 ohms resistance for five of the wires, but 1.1k for another, 2.4k for another, and 10k for another. I repeated the 10k measurement many times and kept getting exactly the same value.

I don't know too much about spark plug wires. I'm hoping there might be something simple to repair rather than just replacing three wires. Does anyone on the group have advice of common issues that might lead to these symptoms?

Thank you!

David Harris
David Harris
Vans RV-7A N47HM
First Flight March 2020

Last edited by DavidHarris : 04-21-2022 at 11:28 AM.
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Old 04-21-2022, 11:56 AM
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bjdecker bjdecker is offline
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 1,157

My MSD 8.5mm wires measure 50 Ohms/Foot (3 foot wire is ~150 Ohms).

By any chance were you inadvertently measuring the spark plug resistance also?

In any case, plug wires are not serviceable -- but very simple to make new ones. Buy bulk wire, terminations, weather boots, and crimp away.

Good thing to check on a CI -- excessive resistance will damage secondary windings in the coils.
Brian Decker
RV-7 (Flying)
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Old 04-21-2022, 02:07 PM
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Ironflight Ironflight is offline
VAF Moderator / Line Boy
Join Date: Jan 2005
Location: Dayton, NV
Posts: 13,022

I had this a few years ago on one of our dual-PMag planes. Discovered it when I was trying to chase down a vey slight roughness that hadn’t been there before. Discovered that it seemed to be due to a little corrosion on the ends of two of the wires where they plugged into the PMag - but this was after I got new leads from Emagair. It’s worth checking for corrosion, but it’s also really simple to replace the leads with new ones.
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
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