I have a Continental IO-520D with Bendix mags and about 300 hours on the mags and wiring harness. Started having intermittent spark problems and tracked it down to a bad harness lead.

Any reason why I shouldn't just buy one individual lead from Kelly Aerospace to replace my bad lead?

You can, but I would buy a new harness... When you get it installed make sure all. The wires are tied so that there is absolutely no chafing against the engine, baffles or the wires themselves...

About a year or so ago one of my customers had engine troubles with his Grumman Tiger on a flight back east... Turns out that the ignition harness chafed through the wires close to the back of the mag and started to cross fire the plugs.... He taped up the wires with electrical tape and flew her home.. Replaced the harness a couple weeks later...

My 2C...

Brian

Thanks Brian. Appreciate the input.

Did you have Champion spark plugs installed?

I have champion fine wire plugs. Why?

Check your plugs for internal resistance I have probably 8 dozen Champion fine wire plugs with high resistance above 5K mostly 30K and above and or open resistors no connection through the plugs. That means the energy that was supposed to create a spark is used up trying to go around the dead resistors ends up still in the ignition leads and shorts to the shielding the same as in the Slick magneto it shorted out the magneto coils due to the high resistance of the Champion plugs. I have thrown out problem 300 to 400 bad Champion plugs compared to 1 bad Auto light / Tempest plugs in 15 years.
So please check your plugs internal resistance, I know that they are about $100.00 each but take a look all champion plugs have the problem.
I even sent plugs back to Champion and they replaced the resistors and the new resistors failed within 100 hrs.
By the way Champion has now copied the Tempest plugs after putting out bad resistors since year 2000.
If any one like’s Champion fine wire plugs I have 4 boxes that were just repaired by Champion I will sell for a great price.

Interesting. Thanks for the insight. When my mechanic pulled the lead off the plug to examine it, he said it looked kind of fried like it had been arc-ing under the insulator where the spring and wires are connected.

Per Aircraft Magneto Service's online troubleshooting guide, "most bad mag checks are spark plug related".
Not sure how you traced the problem to the harness, but maybe this will help.

http://www.aircraftmagnetoservice.net/# ... uide/c15r3

Spark plugs can look good, even excellent, and test OK in a bomb tester, but still fail the ohms test and be the problem. Esp with slick mags, they seem to throw a weaker spark than a Bendix.

Thanks for bringing up the info from Aircraft Magneto as I have been working with them for many years and a wealth of knowledge and yes if the person handling the plug leads has oils or other contaminants on his or her hands and touches the plug lead insulators and or spring and are not cleaned an arcing will accrue that will require the replacement of the lead spring and a cleaning of the plug inside the barrel at the spring contact that is now corroded in the spark plug due to the arcing witch will act as an insulator stopping the energy from getting to the all mighty spark plug electrode to make a spark so your engine can run smooth ???
PS the same happens to the plug lead at the magneto end also.

wow that was one long-ass sentence!