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Radio noise

Like tick, tick, tick with the time between tick following the rpm change.

I’m going to say yes, but the next time the weather is nice I’m going to listen with a more educated ear.

so when I turn the key to R the noise goes away, on an 0320, in what order should the plug wires be tested?
 
I’m going to say yes, but the next time the weather is nice I’m going to listen with a more educated ear.

so when I turn the key to R the noise goes away, on an 0320, in what order should the plug wires be tested?
There are only four wires. The sequence does not matter. This is done with the bad mag running. It can be done on both since you are looking for the noise to stop. Don't forget to hold the cowl and stay back from the prop. If you loosen the plug nuts before you start the engine it is easier. Be Careful.

You are looking for breakage in the shielding of the wire or possible leakage through the insulation. The latter would show a higher mag drop or intermittent plug firing.
 
There are only four wires.

You are looking for breakage in the shielding of the wire or possible leakage through the insulation. The latter would show a higher mag drop or intermittent plug firing.

The mechanic friend that will likely be helping me may know, or can look it up, but these ignition wires are snaked through spiral shielding and are not easily traced to their respective mag without undoing a lot of packaging. Thought I would save him some trouble. More specifically my question was, “Since they are difficult/impossible to tell what wire comes from which mag, which plugs are connected to my bad mag?”

And - none of my plugs are not firing as they should. Zero roughness. Engine monitor confirms all plugs are firing.
 
I thought I had the noise fixed earlier in the year and the it came back. After further inspection of the com wiring, one had the coating cracked exposing bare wire. Once this was fixed the noise went away. Leakage through cracked wiring coating will do it.
 
I thought I had the noise fixed earlier in the year and the it came back. After further inspection of the com wiring, one had the coating cracked exposing bare wire. Once this was fixed the noise went away. Leakage through cracked wiring coating will do it.

Let’s the smoke out!


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My noise is fixed... or it is gone because a mechanic tried to listen for it.

Mechanic wiggled some wires, said hmm a few times. Said he didn’t like the way/location the ground wire from the key switch that grounds the p leads shielding to nearest ground point from the switch end was done, and he re secured it. Then ran the engine to listen for the noise. No more noise.
 
My noise is fixed... or it is gone because a mechanic tried to listen for it.

Mechanic wiggled some wires, said hmm a few times. Said he didn’t like the way/location the ground wire from the key switch that grounds the p leads shielding to nearest ground point from the switch end was done, and he re secured it. Then ran the engine to listen for the noise. No more noise.

Ya. They say to ground the switch to airframe. But that’s just asking for trouble. Then when you have a bad engine to airframe ground the plead shield becomes the ground path and melts the pleads and makes the mags go quiet....


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run new ground wire direct from engine to firewall small stud thats welded to tube(there's 3 of them) 8GA works..... too much slop in original engine mounts to be a good ground system over time

So, The original ground was just the mounts, but no wire? Or where would I look to check existing ground?

Ya. They say to ground the switch to airframe. But that’s just asking for trouble. Then when you have a bad engine to airframe ground the plead shield becomes the ground path and melts the pleads and makes the mags go quiet....


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never mind... time for cheep insurance. Just gonna add a wire if I don’t find one there already.
 
Mojo

The best way to wire p-leads is to connect the ground tab of the switch ONLY to the case of each mag. NEVER to the airframe. I like using Shielded wires for p-leads and connect the center conductor to the p-lead terminal and the appropriate tab on the switch, Then connect the braided shield to the mag case and the ground tab on the switch. This allows the p-lead circuit to function as a stand alone circuit without injecting noise into the airframe circuits and also protects it from becoming the ground path for other circuits as Mike points out.

Web
 
E91D01CF-B08B-4A08-B7B9-6B0FDE394967.jpegI confirmed the instructions for my key switch calls for this wire
 

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The only thing that short ground wire does is window dressing. There is no electrical need for it. Wire it as per that pic but do not connect it to airframe ground.

Web
 
Basic circuitry:

A ground path is the path current (measured in amps) takes to return to the source that produced that current. Since a magneto produces it's own power, the ground path for current it produces will have to lead back to that specific magneto. There are a few tricks to minimizing the possibility of electrical noise such as shielding or short lengths of wire and not bundling 'noisy' wires with other wires. Also avoiding 'mixing' ground paths. Keep the p-leads as short as practical to eliminate the possibility of picking up or radiating noise. NEVER bundle p-leads with any other wires. Keep them away from avionics and coaxes. Distance is your friend. Using the shielded wire for p-leads gives a specific path for current generated by the magneto to return to that magneto without going through the airframe. Airframe circuits will ground back to the battery and have nothing to do with the magneto circuits. Connecting the ground tab of the switch to airframe ground will allow a different path for return and 'mix' magneto and airframe circuits, increasing the likely hood of noise generation.

Loose connections at the mags or at the switch can cause problems. I'm glad the noise has been eliminated but if connections have not been changed and wires were just moved around, I'm pretty sure a connection was just tightened.

Where is that diagram you showed from? It's not from Bendix and I don't think it's from ACS.

Web
 
Connecting the ground tab of the switch to airframe ground will allow a different path for return and 'mix' magneto and airframe circuits, increasing the likely hood of noise generation.

Web

excellent post, and even if the diagram that comes with the mag switch tells you to ground it to airframe DON'T... for the reasons you mention... plus then the pleads will never become a ground path should the engine ground to airframe be bad, and melt your pealed(and shutting you mags OFF)
 
There is a half inch bonding strap from engine to frame located near oil pressure outlet. Where would enough juice in the p leads to melt wires ever come from?

That strap, soldered to the engine mount works well when everything else is in a perfect state. The problems come from crusty attach point at the crank case or paint/powdercoat on the engine mount, at the firewall. Both of these conditions create high resistance to current flow. Also, when the installation is new, everything may be clean enough to work for the time being. As the aircraft ages, corrosion will take place at both of these points and increase resistance. Once the resistance is high enough, the electrical system will look for lower resistance ground paths. This is where current flows through things like primer lines and p-leads (when grounded to the airframe). Think about the current flow when cranking the starter or when the alternator is charging. If the p-lead shields are grounded at the mag case and at the ground tab at the switch, there is no physical path for current to flow if the crank case ground fails. As for the crank case ground itself, a lot of us here favor a strap directly from the crank case to the airframe. This system takes the ground from the crank case to the airframe instead of from the crank case to the engine mount to the airframe. Eliminates an extra seam in the ground path.

Web
 
I've been following this with interest, and have a question. In my plane at least, the key switch is mounted directly to the instrument panel, which is screwed to the boot cowl, which is - - etc. So my question, is - - Is the ground tab on the mag switch isolated from the switch body?
 
Another source of RF noise can simply be loose spark plug wire caps. When using radio tracking equipment around 50 and 150 Mhz the slightest looseness of a cap on a spark plug created pulsed static on the radios that synched with engine rpms. The caps ground the plug wire shields there and contain ignition noise from getting out with any smoke. P-lead filters can help as well if they're a source of interference.

Gary
 
I disagree with using filters. Magnetos, alternators, etc, in good working condition won't make enough noise to warrant filters. If an item is generating electrical noise, fix it. Filters simply mask a problem.

Web
 
Here's a new issue for my plane. I recently installed a UAvionics wingtip unit, and I'm experiencing intermittent radio noise associated with its strobe. The unit is wired in accordance with the instructions and operates correctly.

I know that strobes are the culprit because if I turn off all strobes the noise stops. I know that particular strobe is the culprit because if I turn off the ADSB unit by turning off the nav lights (which also turns off that strobe) while leaving the remaining strobes on and operating, the noise stops.

My radio is a Garmin SL30

The noise does not occur on all frequencies, and it will come and go on some frequencies. Freqs I have heard it on are 121.1, 124.2 and 126.5 - all Olympia Wa area freqs for Seattle Center and Seattle approach.

But the noise is intermittent - seems to be location dependent, and also seems to be when the transmitting station is a bit distant, based on voice clarity from that station. The noise has not shown up on 122.8, 122.9, 124.4 (Olympia tower), 121.6 (Olympia ground) or 118.1 (Portland Approach) , for example. The noise goes away when someone is talking on the frequency. It acts like something is "helping" the strobe hiss to break squelch at some times / places and not others.

I have not called UAvionics yet, but am thinking someone here may have experienced the issue and can offer advice. Hope so!

Edit: The noise is also new. The ADSB unit has been installed for almost two months, but the problem has cropped up in the last couple of weeks. Could there be some sort of issue with the transmitting stations which have been problematic?

Further edit: Two days ago I was at an Olympia avionics shop (unrelated reason involving a friend's plane) so asked the techs there. Unfortunately, the problem would not replicate on the ground there. Of course!

Thanks - -
 
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Noise on some freqs but not on others or worse on some, is a fairly common complaint. Electrical signals spill over onto other frequencies when they get powerful enough. And they get worse on 'multiples' of frequencies. So if you multiply or divide up or down far enough you will usually arrive at the most troublesome frequency. Think of the frequency of the noise OR the receiver. Now multiply/divide by any number (2, 4, 6 . . 5, 10, 15, etc).

If I were troubleshooting this particular installation, I'd start by checking and cleaning the ground for the light and the uAvionics unit. Then I'd check to see if the Com antenna has a good ground plane and is not mounted on paint or powder coat. Then I'd separate the com coax from any power wires as much as possible. Worse case is that you simply have a strobe that makes noise. It's not common but I have seen LED strobes that make a ticking noise when the flash. Replaced the unit and the noise went away.

Check all the above stuff first. Then, if the noise continues, contact uAvionics for some help or even a replacement unit.

Web
 
I disagree with using filters. Magnetos, alternators, etc, in good working condition won't make enough noise to warrant filters. If an item is generating electrical noise, fix it. Filters simply mask a problem.

Web
I have older Bendix mags with the coaxial style p-lead connectors. The mags had the coaxial p-lead filters when I bought the plane. A couple of years ago I overhauled the mags, and looking at the filters, decided they were too worn out to use (disintegrating shields, unknown age).

A new PMA filter (MF3A) with the braided shield to fit the connector is virtually impossible to find. A non-PMA filter is available at Aircraft Spruce for $260, and you need two. A filter without the braided shield is $80, but it seems to me that the shield is needed between the capacitor and the mag case.

Anyway, after removing the filters, there was a slight increase in magneto noise when receiving weak signals.

Interestingly, the noise dramatically increased twice, when a plug was fouled, and when the internal capacitor was about to fail (an early life failure on a newly rebuilt mag). So, mag noise can be a diagnostic tool.
 
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