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Yet Another Magneto Noise Thread

Skywagon185

Skywagon185

FRIEND
Bigfork, MT
I hate to do this but I'm out of options. I have read every thread on radio noise and have employed most "fixes".

The problem: Magneto noise on weak stations and open squelch. It's definitely the mags as it changes with RPM and goes away when I shut down the engine. Close signals are fine.

This is a 1954 PA-18 with a newly overhauled engine. Bendix mags with the old style filters and caps. New P-leads.

I have done the following:
  • Installed new ground from engine case to firewall
  • Ensured p-leads are separated from all other wires and do not route near radios or coax
  • Wired switch according to previous radio noise posts
  • Wired a ground for the antenna
  • Removed the new p-leads and installed the old p-leads
  • Isolated the electronic tach
  • Isolated the alternator (breakers)
  • Checked all spark plug leads for tightness
I'm perplexed and the wife is pissed (her airplane).

Should I replace the filters with new? Should I remove them completely (I'd need new p-leads)? Should I sell the airplane?

I'm running out of ideas. Help!
 
One test is to remove the P-leads one mag at at time at the engine and run the engine. Careful=HOT ENGINE. Does the noise remain with either or both removed? If it's reduced then the P-lead/filter if present/mag switch have issues. You can also then reconnect to the mags and remove them one at a time at the mag switch to see if the P-leads are antennas. Some choose to not ground the shield at the mag switch because of unwanted noise. Experiment.

Another test is to remove the coax first at the com or nav antenna via the connector, then at the radio if it's not hard mounted. If the antenna is removed stepwise from the circuit and the noise then quits at one stage, or persists, then the source is either the feed coax (antenna removed), or electrical sourced to the radio (all coax removed).

If you have a handheld AM radio or aircraft receiver they can sometimes be used to focus on the noise source. Find the best frequency for hearing your noise then move the radio around the plane and over the running engine so the antenna can pick up the loudest point of the noise source. Removing the external antenna can help find it when it gets close. Using a headset connected to the handheld helps.

Spark plug to plug wire connections (wire spring and cap threads for example) that are corroded can cause noise. Same for slightly loose wire cap on plug. I found that bad plug leads affected the antenna system more than other sources attached to the radio. My experience while aircraft tracking critters with transmitters attached.

Gary
 
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Another source of "static" I've found (excluding the internals of the magnetos) are the insides of the spark plug cavities and the insulating ends of the wires (used to call them cigarettes). Sweat and salt from hands or contamination from thread lube (if used) can cause low level arcing. If that gets to the outside of the plug wire's shield it causes problems. Clean with rubbing alcohol if suspected.

I made up a remote antenna to chase static. Extend the removable antenna from an aircraft band handheld with a piece of coax cable fixed to a wood dowel. Safely move the antenna near potential sources. If too strong a signal, remove the antenna and just use the coax open end to find a source.

Gary
 
Skywagon185 said:
I hate to do this but I'm out of options. I have read every thread on radio noise and have employed most "fixes".

The problem: Magneto noise on weak stations and open squelch. It's definitely the mags as it changes with RPM and goes away when I shut down the engine. Close signals are fine.
  • Isolated the alternator (breakers)
I'm running out of ideas. Help!
Click to expand...
What do you mean by "Isolated the alternator (breakers)"? Relocated them? Or shut them off to eliminate any possibility of it being alternator noise? Alternator noise does this too: "changes with RPM and goes away when I shut down the engine."
 
If you‘ve tried everything listed in the posts above and still can’t eliminate the problem you could take the plane to Michael Haisten at Ruby Valley Aviation in Twin Bridges. I realize KRVF isn’t exactly next door, but Michael is an excellent diagnostician and WILL find and fix the problem.
 
Check your plug wires to be sure there are no cracks or places where the insulation has been worn through to the shield. I chased noise in my Extra 300 for quite a while doing all the normal things, including overhauling the mags to be sure the condensers weren't the issue. It got to be so bad the radio was almost useless from the noise. Nothing worked until I found a few places where the shield was showing through the outer insulator and changed the plug leads. Bingo. No more noise. To be clear, the shield was not broken, only showing through the insulator. That was enough to broadcast a tremendous amount of static.
 
From my experience ignition noise is usually a buzz or rapid popping sound. A series of quick impulses that changes character and frequency with rpm. Alternator noise from a defective diode pack or regulator/overvoltage system is a steady whine that varies in audio frequency with rpm and electrical load. They sound different. Pulling the alternator field breaker shuts that off.

Gary
 
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skywagon8a said:
What do you mean by "Isolated the alternator (breakers)"? Relocated them? Or shut them off to eliminate any possibility of it being alternator noise? Alternator noise does this too: "changes with RPM and goes away when I shut down the engine."
Click to expand...
Pulled the breakers.
 
I had a similar experience a few years ago on my C180. Newly re-built Bendix mags and constant radio noise. Radios checked fine at the radio shop. A knowledgeable A&P at Surfside in Lino Lakes, MN said it was probably the filter, which I believe is actually a capacitor. Capacitors either work or they don't. Try replacing them with known working filters.
 
MartyC said:
I had a similar experience a few years ago on my C180. Newly re-built Bendix mags and constant radio noise. Radios checked fine at the radio shop. A knowledgeable A&P at Surfside in Lino Lakes, MN said it was probably the filter, which I believe is actually a capacitor. Capacitors either work or they don't. Try replacing them with known working filters.
Click to expand...
Thanks. I'm thinking of either replacing the filters or buy new P Leads (again) and eliminating them all together as they don't appear to be needed.
 
Back when we used low frequency NDB's to get around Alaska, any interference to weak signal reception from static was quickly addressed. Today of course with GPS it's not much of an issue unless it breaks squelch on the Com radios or disrupts communications.

Gary
 
Skywagon185, let me ask about the new p-leads. Are they shielded? Is the shield grounded at both ends? (both near the ignition switch AND at the magneto's?

Try to disconnect the grounded shield at one end or the other and please let us know what you eventually find that cures your engine noise in the headset/radio. Even if you end up taking it to an avionics shop or repair facility.

It is your feedback THAT ALWAYS makes these threads useful to both current members AND future users that may not even be born yet...

Cheers!
 
FabricGATOR said:
Skywagon185, let me ask about the new p-leads. Are they shielded? Is the shield grounded at both ends? (both near the ignition switch AND at the magneto's?

Try to disconnect the grounded shield at one end or the other and please let us know what you eventually find that cures your engine noise in the headset/radio. Even if you end up taking it to an avionics shop or repair facility.

It is your feedback THAT ALWAYS makes these threads useful to both current members AND future users that may not even be born yet...

Cheers!
Click to expand...
Yes, the P-Leads are shielded. I purchased them here. I am going to remove the filters and change the P-Leads to these from the same outfit. If that doesn't work I will take the plane to Twin Bridges.
 
Skywagon185 said:
Yes, the P-Leads are shielded. I purchased them here. I am going to remove the filters and change the P-Leads to these from the same outfit. If that doesn't work I will take the plane to Twin Bridges.
Click to expand...
What I am proposing (was taught to me) is that only one side, on end of the shield should be grounded, the other end you should cut shy of touching anything at the termination and finish it maybe with heat shrink

??? I was not understanding and it was told to me like this, "if you connect both ends of the same hose to the output of a pump, the water is not going to move..."
 
Much has been said to agree with, so perhaps this will just reinforce what some others have mentioned.

I’m not sure that alternator noise always goes away by pulling the breaker. Disconnect at the alternator to test.

Ignition harness itself can be a source.

Verify again that it’s power based and not the something reradiating the signal to the antenna. Connect the com to an external coax and com antenna and put it 6 feet away from the fuselage.

Assuming this is a constant source and not just on the ground.

This all started with a new engine install?
Prior was perfect or untested?
What coms are installed?

There is no USB adapter or new plug installed or in a socket right? External devices can be problematic.
 
Farmboy said:
Much has been said to agree with, so perhaps this will just reinforce what some others have mentioned.

I’m not sure that alternator noise always goes away by pulling the breaker. Disconnect at the alternator to test.

Ignition harness itself can be a source.

Verify again that it’s power based and not the something reradiating the signal to the antenna. Connect the com to an external coax and com antenna and put it 6 feet away from the fuselage.

Assuming this is a constant source and not just on the ground.

This all started with a new engine install?
Prior was perfect or untested?
What coms are installed?

There is no USB adapter or new plug installed or in a socket right? External devices can be problematic.
Click to expand...
Yes, new everything firewall forward. Avionics are late model Garmin and were installed several years ago by Aerotronics in Billings. Never a problem until now. The coms on the ground are fine when the engine isn't running. Fire it up and open the squelch and you can hear it in the background. Strong/close transmissions are received just fine.
 
Is it affecting the audio while receiving a transmission, or actually breaking squelch during radio silence? What prompts you to open the squelch?
When you say late model Garmin do you know what model the com is?
Did you happen to ask Aerotronics about it?
 
Farmboy said:
Is it affecting the audio while receiving a transmission, or actually breaking squelch during radio silence? What prompts you to open the squelch?
When you say late model Garmin do you know what model the com is?
Did you happen to ask Aerotronics about it?
Click to expand...
Yes, when receiving a distant or weak transmission (say 40 miles). It generally doesn't break squelch. I was opening squelch on the ground to confirm I could hear it when the engine was running and couldn't hear it when it wasn't. Radio is a Garmin GTR 225. Only happens on radio receptions, not on the intercom (PS Engineering). I haven't asked Aerotronics (yet).
 
I recommend continuing the diagnostic path many have suggested above to rule out variables.
But, the GTR200 and the 225 are very sensitive coms in reception. Great radio but as an example I’ve seen alot of bleed over issues from a second com transmission bleeding over onto a GTR on a different frequency when their antennas are both topside.
Now, this isn’t your issue but its sensitivity range may be contributing to the effect.

I’d disconnect the alternator first but if that isn’t the issue I’d be curious if connecting to an antenna at least 6’ away made any difference. Based on the symptom it should not but I’m just a curious guy.

Sadly sometimes changes made much later will accentuate a previously unknown issue, grounds, shields and shield drains, etc. Avionics can be a fickle thing. Old ELT’s are often causes of Re radiation (in radio communication) so it’s an easy thing to check but disconnecting the elt for a test.

As others have suggested, alternator filters are available if it is that, but all the ignition items are ready sources to disrupt. Anything you can do to seperate firewall forward one item at time until it goes away requires patience and time but can identify the culprit.

With the new engine did you add an EIS system? Any new wiring between engine and panel? (I had mentioned the power port usb adapter before because the number of times a problem was solved by removing one)
 
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Skywagon185 said:
The problem: Magneto noise on weak stations and open squelch. It's definitely the mags as it changes with RPM and goes away when I shut down the engine. Close signals are fine.
Click to expand...
BC12D-4-85 said:
Here's one pilot's journey into the world of static. Read all 3 pages for his solution. https://mooneyspace.com/topic/22495-chopping-static-in-my-radios/

Gary
Click to expand...
What does the noise sound like? The noise in Gary's example is described as a "chopping static". Is this your noise, or is it more of a buzzing sound? The chopping sound is indicative of spark plugs or leads since they are intermittent in their use. A buzzing sound is more indicative of a rotating electric motor type of constant noise. I've never seen a capacitor cause a noise (that doesn't mean it can't). They generally show failure evidence with pitting on the points.
 
The Devil's Noisemaker in modern electronics is frequently a poorly designed switched-mode power supply. Those can be the common household "wall warts" universally used to transform one voltage to another, and often AC to DC current to charge devices. In aircraft they can supply lower voltage DC power. If noisy they emit a steady buzzing noise that Sky describes above....sounds like Frankenstein getting a software update.

Gary
 
I haven't seen you respond to the suggestion to "Un-Ground" one end of your shielded P leads...this is one I have seen a lot of. shielded ends terminated in ring terminals and some A&P's screw one end onto a mag screw and the other to a ground bus, thus creating a delightful ground loop, that keeps perfect whine with rpm's...annoying as hell.

I tend to like the shield grounded to a panel bus ground and terminate with a solder shield at the mag end, which I pull the pigtail out of to just keep the end tidy.

That has solved a lot of whines over the years.

Steve
 
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