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Dual Pmag, radio interference?

C

CamTom12

Active Member
I’ve got a non-RV (has an RV-6 engine cowling!) with an O-320 and dual pmags.

I get a crackling over the radio that matches RPM and when I toggle the p-leads the tone of the sound changes slightly. At ~2300 and below the radio (icom A200) is able to squelch most of the sound out but if someone else transmits I can hear it in the background. It’s especially annoying when listening to a quiet ATIS. At higher RPM the radio auto-squelch cannot keep up and it’s quiet loud. I can turn off all other potential sources and it’s still present (unless I turn off the radio).

Non-shielded power and p-lead wiring on the pmags. Tach output is from the right pmag and is shielded. I for-sure grounded the engine side, can’t recall if I grounded the engine monitor side of the shielding.

I noticed a little bit of deterioration in the plug wire boots on my older set of wires so I ordered some new ones but haven’t installed them yet.

Anything else I should look for/change?

Radio and ICS are well shielded. RG-400 for the antenna wire.
 
Pretty sure, it doesn’t sound like alternator whine. It’s absolutely crackly, sounds like ignition to me.

Also disappears when the radio is off / ICS is on so it’s not bleeding into wiring that I can tell. It breaks squelch on the radio, makes me feel like it’s getting picked up at the antenna, but I’m not 100% sure.
 
If you are using spark plugs with screw on terminal caps make sure they are tight. Loose terminals are one of the causes for the noise you describe.
 
I would ohm out the spark plug wires. I have not had the best experience with pMag supplied wires. I’ve had broken wires on more than one new set. Ended up making my own wires with MSD wire, terminals and boots. No issues since in 400+ hours.
 
KatanaPilot said:
I would ohm out the spark plug wires. I have not had the best experience with pMag supplied wires. I’ve had broken wires on more than one new set. Ended up making my own wires with MSD wire, terminals and boots. No issues since in 400+ hours.
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I’ve got a brand new set in my hands I intend to install tonight (on the “road” for a work trip), so I’ll buy a cheap multimeter and ohm them beforehand. If that’s the solution, I’ll definitely build a set of MSD spares when I get home.
 
E-Mag wires

Im dealing with this problem now. I found that not separating the spark wires in their runs was causing cross-over ignition. I also had some bad wires in the original ones from E-Mag. What part number wires did you buy to replace the E-Mag ones? There is a certain Ohms value that E-Mag recommends.
 
Flying EMT said:
Im dealing with this problem now. I found that not separating the spark wires in their runs was causing cross-over ignition. I also had some bad wires in the original ones from E-Mag. What part number wires did you buy to replace the E-Mag ones? There is a certain Ohms value that E-Mag recommends.
Click to expand...

I got two of these: https://www.aircraftspruce.com/catalog/eppages/emagplugs07-02716.php

Just finished swapping them. RTV is curing then maintenance operational check tomorrow.

I tested 3 of the old ones and two of them had tons of resistance. On the order of 1.6k ohms for the short ones. After that I stopped testing and just started swapping.

The new ones all tested around 170 ohm/ft.

My top wires definitely come together in the baffle pass-through. For now I just followed the old runs but when I get home I’ll make some new pass-throughs and keep them more separated. On the bottom wires I used some plug wire separators that I bought from autozone.


EDIT: I just got bored in my hotel and ohmed all my old wires. Here’s what I got for ohm/ft. These are randomly numbered, no intended correlation to cylinders:

1. 174
2. 171
3. 174
4. 174
5. 1239
6. 170
7. 1707
8. 702

So it looks like I had 3 bad wires.

I left the tach signal shield connected at the pmag, so if I get noise on my runup tomorrow then I’ll pull the cowl and cut that before I depart on the next leg of my trip.
 
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I too had some bad wires from pmag and replaced with "racing" wires - all good now.

While troubleshooting this problem, measuring the resistance, sometimes it was necessary to bend the wires to get them to show the high ohm reading on the multimeter. I'd get the strange crackling noise intermittently.

Glad you found the problem!
 
The noise is gone!

Still on the road, but I’m definitely going to build my own wires when I get home as others have done.
 
CamTom12 said:
I left the tach signal shield connected at the pmag, so if I get noise on my runup tomorrow then I’ll pull the cowl and cut that before I depart on the next leg of my trip.
Click to expand...

electronic, square wave tach signals are NOT prone to producing noise.
 
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