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Thread: Radio noise

  1. #41
    Wag2+2's Avatar
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    Checked everything above and it turned out to be the ground wire nut on the left mag was loose. Radio works like it is supposed to now. Thanks for all the input. You guys are great!��
    Last edited by Wag2+2; 02-25-2019 at 05:01 PM.
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  2. #42
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    Bonus when the fix is easy!

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  3. #43
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Wag2+2 View Post
    Checked everything above and it turned out to be the ground wire nut on the left mag was loose. Radio now works like it is supposed to now. Thanks for all the input. You guys are great!��
    ummmmm.... ummmmm... that shouldn't be your problem, or let me rephrase that... if that solved your problem, then you have a bigger problem.... the P-lead ground should not be acting as an engine ground to your frame(it should NOT be connected to fuselage at key switch or toggle switch end ONLY to mag case.... EDIT: YES thats how Piper did it on newer cubs, but they should NOT HAVE... when you have a bad main ground and p lead shield serves as engine ground, the plead wires insulation melts and the engine gets shut off!! )

    you have a bad ground between your engine and fuselage frame, add a 8 ga wire from an engine stud on back to one of the 3 #10 studs coming through your firewall from fuselage.... to original grounding on a cub is horrible going through mount bolts and tabs....

  4. #44
    skywagon8a's Avatar
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    mike, I think that he was talking about the P lead ground from the switch to the mag. Not the engine ground to the airframe.
    N1PA
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  5. #45
    Wag2+2's Avatar
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    I think the engine ground to frame is solid. You can just see the nut behind the oil temp ground wire.
    599FB26D-5247-44A8-B953-777B929DEEDC.jpg

  6. #46
    www.SkupTech.com mike mcs repair's Avatar
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    Radio noise

    Quote Originally Posted by skywagon8a View Post
    mike, I think that he was talking about the P lead ground from the switch to the mag. Not the engine ground to the airframe.
    Exactly. And that should not be a noise factor since p lead ground was not connected to condenser when running. If plead ground was source of noise because of the loose nut that means it was acting as the BEST poor airframe ground at the time. Noise is just a variation the quality of a connection/flow. If that makes sense...


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  7. #47
    wireweinie's Avatar
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    Electrical noise = rapid change in current flow.

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  8. #48
    Wag2+2's Avatar
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    Thanks for your input Mike and Wireweinie. I bought this Experimental so I am learning things as I go and electrical is not my strong suit. I checked the ground and the negative from the battery goes to the engine block. A similar sized cable then goes from the block to the frame at the motor mount. The connections have an oil film on them from a slight oil leak misting the area. Don’t know that that would effect grounding.

  9. #49
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    If the connections are tight, that oil film will not affect them.

    Electrically it's good that the ground cable is attached directly to the crank case. The ground lead from the crank case to the engine mount should be moved from the mount to the air frame itself. The ground path has to many joints and seams to cross when attached to the mount. Also paint and powdercoat need to be removed from attach points.

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  10. #50

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    I’m having a similar issue. Not bad, but noticeable. All connections are tight, but left mag is making a snapping crackle in the radio. Brand new slicks. Key switch is fairly new too.

    connected in the system is an electric tachometer too, signal from p leads at switch.

    Since these wires are routed together, left and right leads, why is my left mag making noise?

  11. #51
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    Does the noise go away when you run on right mag only? Is the noise continuous or only present at certain RPM's?

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  12. #52
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    Quote Originally Posted by MoJo View Post
    I’m having a similar issue. Not bad, but noticeable. All connections are tight, but left mag is making a snapping crackle in the radio. Brand new slicks. Key switch is fairly new too.

    connected in the system is an electric tachometer too, signal from p leads at switch.

    Since these wires are routed together, left and right leads, why is my left mag making noise?
    Ignition harness new too? Could be a leak in the shielding of one spark plug wire.
    Could be a leak in the P lead shielding of the other mag.
    N1PA

  13. #53

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    Quote Originally Posted by skywagon8a View Post
    Ignition harness new too? Could be a leak in the shielding of one spark plug wire.
    Could be a leak in the P lead shielding of the other mag.
    When I select R on the key switch, all noise quits. I’m going to say it’s at all rpms, but definitely discovered during runup 1700 mag check. Hmm....Old plug wires, and they would not win a beauty contest. If I’m not mistaken, isn’t there a tool to check plug wires?

  14. #54
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    I assume that you mean the leakage checker. Similar to a megger. It's connected to the center conductor and shield. A high voltage is then placed on the center conductor to see if it will jump to the shield. It's purpose is to test the condition of the insulation.

    In your case, I'd check for a loose connection in one of your plug wires. Connect an ohmmeter to each end of the center conductor of one plug wire. Then move the wire around and see if you lose continuity. Do this to each plug wire and don't be gentle. Any issues with testing or condition, just put on a new harness.

    Also check the spark plugs. New plugs or old? Consistent plug gaps on each plug? Do they check consistently in a 'bomb tester'?

    If everything checks good, you should have the mag bench checked as there is the possibility that you have bad points/capacitor.

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  15. #55
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    Just a thought; did you verify that the p-leads actually connect to the correct mag? Left p-lead to left mag to 'left' tabs on switch, etc?

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  16. #56
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    Quote Originally Posted by MoJo View Post
    When I select R on the key switch, all noise quits. I’m going to say it’s at all rpms, but definitely discovered during runup 1700 mag check. Hmm....Old plug wires, and they would not win a beauty contest. If I’m not mistaken, isn’t there a tool to check plug wires?
    Put a qualified person in the cockpit listening to the radio with the engine running on the noisy mag. Then carefully remove one spark plug wire at a time from the spark plugs on that mag. Does the noise go away with one of the wires removed? If so, that is the leaky wire.

    The only time that there is current flow in the P lead wire is when that mag is turned off. If the shield on the P lead wire is the culprit, it will be the wire on the quiet mag.
    N1PA
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  17. #57
    www.SkupTech.com mike mcs repair's Avatar
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    Swap plugs from left mag to right etc. see if sound follows plugs


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  18. #58

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    Quote Originally Posted by wireweinie View Post
    Just a thought; did you verify that the p-leads actually connect to the correct mag? Left p-lead to left mag to 'left' tabs on switch, etc?

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    Based on a previous problem with a plug, and use of engine monitor, yes.

    Quote Originally Posted by skywagon8a View Post
    Put a qualified person in the cockpit listening to the radio with the engine running on the noisy mag. Then carefully remove one spark plug wire at a time from the spark plugs on that mag. Does the noise go away with one of the wires removed? If so, that is the leaky wire.

    The only time that there is current flow in the P lead wire is when that mag is turned off. If the shield on the P lead wire is the culprit, it will be the wire on the quiet mag.
    Would selectively removing plug wires only identify a bad plug?

    I have an electric tach that gets it’s info from the p leads, so not sure about no power through the p leads unless selected off.

    Quote Originally Posted by mike mcs repair View Post
    Swap plugs from left mag to right etc. see if sound follows plugs


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    Nothing on the backside of this cub is easy to get to. If the p leads are/were long enough, might see about having those swapped, but the plug thing is something easy that I can try.

    Thanks for all the ideas and education.

    It’s not bad/loud noise, but I plan to get to the bottom of this.

  19. #59
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    The power going through the p-leads is so small it's hard to measure. They only supply a signal to the tach, they do not power the it. And since the noise only occurs when the left mag is hot, it's not the tach/tach wiring.

    Process of elimination. Go with the easier stuff first; wires and plugs.

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  20. #60
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    Quote Originally Posted by MoJo View Post
    Based on a previous problem with a plug, and use of engine monitor, yes.

    Would selectively removing plug wires only identify a bad plug?
    That's the way we did it prior to the days of engine monitors. Rotating the plugs to the other mag would eliminate the plugs. If the wire isn't connected to the plug, the current flow isn't there producing the noise.
    Does the noise sound like a continual noise or does it sound like just one wire/plug? Like tick, tick, tick with the time between tick following the rpm change.
    N1PA
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    Quote Originally Posted by skywagon8a View Post
    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?

  22. #62
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    Quote Originally Posted by MoJo View Post
    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.
    N1PA

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    Quote Originally Posted by skywagon8a View Post
    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.

  24. #64
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    Quote Originally Posted by MoJo View Post
    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?”
    Can't tell that without looking.
    N1PA
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  25. #65
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    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.

  26. #66

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    Quote Originally Posted by Wag2+2 View Post
    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.
    Letís the smoke out!


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  27. #67

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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.

  28. #68
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by MoJo View Post
    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 didnít 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 thatís 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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  29. #69

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    Quote Originally Posted by mike mcs repair View Post
    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?

    Quote Originally Posted by mike mcs repair View Post
    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.

  30. #70
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    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.

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  31. #71

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

  32. #72
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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.

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  33. #73

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    Quote Originally Posted by wireweinie View Post
    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.

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    But it is likely that it cured my radio noise

  34. #74
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    nope

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  35. #75
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    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.

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  36. #76
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by wireweinie View Post
    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.

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    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)

  37. #77

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    This is where I found that picture.https://www.scribd.com/document/3650...Ignition-Noise Yes, acs instructions say ground the shielded wires from the switch end. 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?

  38. #78
    wireweinie's Avatar
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    Quote Originally Posted by MoJo View Post
    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.

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  39. #79
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    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?
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  40. #80
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    Yes. On a normal (for airplanes) key switch, all tabs are isolated from the case, and each other.

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