A friend used to live in the shadow of the WBNS radio tower in East Columbus Ohio. He would get WBNS day and night on the TV with the TV turned off. He had to put a switch in line with the speaker, and that just cut the volume down. He also got WBNS in the silver ware drawer. If the radio sounds stopped, you just juggle the drawer to get different pieces touching each other and the radio came back on. Some people got it in their teeth fillings. Ladies picked it up in hair curlers. A crystal set kit worked before you assembled it. You didn't even need the tuning coil, so we had transistor radios before transistors were available.

 

In the plug wires you have 4 (or 6) radio stations. One end of the broadcast tower is a spark gap, so it broadcasts on all frequencies.

That means that no matter what length  wire is nearby, it is perfectly tuned to some of the noise you are broadcasting. So, no matter what any other wires in the airplane are supposed to be doing, they are always listening to the 4 radio stations, and in the case of a low voltage application, induced voltage from the stations may overcome the low voltage application.

 

A number of methods are used to eliminate or reduce this problem.

 

Distance. Magnetic coupling is inverse to distance. Keep everything away from the plug wires.

 

Antenna size. Keep plug wires as short as possible.

 

Shielding. As in shielded plug wires. And shielded low voltage wires.

I run my trigger wires for the MSDs through some dash 4 Teflon lined stainless braided wire, with the braid grounded.

 

Coil on plug systems. (No plug wires)

 

Smaller plug gaps. I start at .010" and go up to .015".

 

Inductive plug wires. Spiral wound Monel conductor wire from the hot rod shop. Eliminates radio noise in race cars. Otherwise the racing radios don't work. I have read that those wires also make for a longer lasting and hotter spark, but don't all aftermarket products do that?

 

Lynn E. Hanover   

 

Thought I had commented on this before.   This symptom is most likely caused by impulse noise from routing the EGT lines too close to ignition leads or other high noise Sources.  The thermocouples generate low level signals in the millivolt range so it doesn't take much to spike the reading.   

Tracy