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Ignition noise in headset

PCHunt

Well Known Member
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If I receive a weak transmission, I can hear ignition noise in my headset, which makes it difficult to copy the incoming transmission.

I have discovered that the noise disappears if I switch the mags to R. On the Left mag, there is noise.

I've checked the harness out with a tester, and a megger. Moved the plugs from left mag to right mag, and the noise stayed with the Left mag.

The P-Lead Shield is grounded only at the Mag end. All connections look good.

Have tried 3 headsets, same result. Passenger hears the same thing.

The Slick Mags are both relatively low time. This is not a new issue, I'm just finally getting around to tackling it.

Looking for ideas from the gurus.... :confused:
 
couple of items that my be of help.

1. check the resistance of the spark plugs with an Ohm meter. Anything over 5000 ohms, toss it!
2. Any long run of wire should be shielded (they can act as an antenna), back to nearest point of power or master ground.
3. sometimes automotive spark plugs with screw cap tops can get loose, cause noise.
4. Make sure any shields are NOT shielded on both ends! That often causes a lot of noise in the headset.
5. Check intercom for proper operation, should filter out minor artifacts.

DAR Gary
 
Disconnect the p lead from the magneto that is generating the noise to determine if it is an issue with the p lead shield. Try grounding both sides of the p lead shield. and see if there is any change.


Good suggestion on plugs. If you have Champion plugs installed, check to see if they are the old style plugs with the removable resistor in the center. If they are, pitch them. If any plug is over 5K resistance, replace it.


Check the resistance of each plug wire to confirm you don't have an open plug wire.


If none of these things help, then noise could be generated internally in mag. possible rotor or center brush wear, weak capacitor etc. Pull the mag and inspect. You can download the service manual from Champion website
 
Thanks for the suggestions. Next step will be to disconnect the P-lead and go flying. Awaiting good wx.

No Champion plugs, all are Tempest REM-37BY. All tested below 2K ohms. Leads tested 3 ways, continuity, lead tester, and megger.

Could anything in the radio installation be an issue? Antenna or antenna connections?
 
Did you measure the resistance of the plug wires? You mentioned continuity, and that's good -- but what about ohms?
 
try some capacitors, they clean dc up nicely. Easy to install and remove. worth a try.

I assume you aren't talking about the capacitor that is part of the mag.....? Could you be more specific on which capacitor to buy, and where to install it?

Thanks!
 
Ignition capacitors, put one on the B+ lead on the alternator, i have one on each p-leads. They are cheap and clean dc up nicely. Just my 2 cents worth.
 
Don't get distracted by trying to mask an issue. If one mag does not exhibit this issue and the other does, and it did not do this before, then it is probably something with the mag and associated wiring and hardware. If you have exhausted all the external issues, then pull the mag and have it inspected. BTW, check your engine to air frame ground strap.
 
Update

Updating progress:

For Bill.Peyton, the noise got noticeably worse with the P-lead disconnected at the mag. :confused:

For bjdecker: Great Idea, thanks! I tested the center conducter on 4 leads on the bad mag, and 3 leads were at about 2 ohms, and steady. The 4th lead was 42 ohms, then when I intentionally moved the lead around, the resistance jumped all over the place! I have made up a new lead and installed it.

An Avionics Tech at my airport (NZAR) suggested that I test the shield on each lead as well, by taking the lead off the spark plug, then see if the shield goes to ground. Again 3 were OK, one was open. It turned out that the ferrule holding the lead into the mag cap had come loose. I trimmed the spark plug lead, and reassembled with a new ferrule.

So, two issues found, but due to weather, haven't been able to do a flight test.

Thanks for the great suggestions......... :)
 
The fact that the noise increases when the p lead is totally disconnected is could be an indication of an engine grounding issue.



BTW, the p lead should be a shielded wire. The shield should be grounded at BOTH ends, the mag and the switch. The p lead shield should not be grounded to the central aircraft ground bus.



You should verify that your engine has an independent ground strap to the airframe and the connections are clean and proper.


It does sound like you may have found your issue with a broken shield on one of your ignition wires..
 
Hi Bill, I respectfully disagree about grounding the P-Lead shield at both ends. Everything I can read on the issue, including Mr. Nuckolls, says that the shield should be grounded only on one end, usually the Magneto end.

I have checked my engine ground cable, which goes from the engine to the airframe, and it appears and tests good.

If my test flight still shows a problem, I will ground both ends of the P-Lead shield, but IMHO, that's not the problem.

Testing will tell the tale.

BTW, what is the logic for not grounding the shields to the aircraft central ground? My shields are grounded only at one place, on the magneto, at the screw provided for the purpose.
 
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Hi Bill, I respectfully disagree about grounding the P-Lead shield at both ends. Everything I can read on the issue, including Mr. Nuckolls, says that the shield should be grounded only on one end, usually the Magneto end.

I have checked my engine ground cable, which goes from the engine to the airframe, and it appears and tests good.

If my test flight still shows a problem, I will ground both ends of the P-Lead shield, but IMHO, that's not the problem.

Testing will tell the tale.

BTW, what is the logic for not grounding the shields to the aircraft central ground? My shields are grounded only at one place, on the magneto, at the screw provided for the purpose.

The p lead is noisy and you want to use the shield to go directly back to the mag to self-cancel, when grounded. One end of shield should connect to the mag ground screw and the other to the other pole on the mag switch.

That is best practice and would be surprised if knuckles doesn’t recommend this.

Larry
 
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The p lead is noisy and you want to use the shield to go directly back to the mag to self-cancel, when grounded. One end of shield should connect to the mag ground screw and the other to the other pole on the mag switch.

That is best practice and would be surprised if knuckles doesn’t recommend this.

Larry

He does recommend connecting the shield at both ends, but easy to misunderstand it the way it's written:

http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf

BobNuckolls said:
Note that I show shielded wire between the magnetos and the magneto switches. Spark plug wires and magneto p-lead wires are the ONLY wires in the airplane which are ALWAYS shielded. Further, p-lead shields should be grounded to the airframe/engine only at the engine end. The shield is used to provide a ground for the "GRD" terminals at the switch but no other connection to the shields should be made at the instrument panel end.

pmag grounding bob nuckolls.png
 
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Attached, Aeroelectric drawing..:

The P-lead is a single conductor, shielded wire, and the shield should be grounded at "both ends" -- magneto, and the GRD pole on the keyswitch -OR- equivalent ground on ignition switch.
 

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Completes the circuit, not grounding both ends

The diagram allows the grounding of the p lead back to the engine, what I believe to be the correct circuit path. Technically it’s NOT grounded at both ends, there is no tie to the airframe ground at the switch side.
 
The diagram allows the grounding of the p lead back to the engine, what I believe to be the correct circuit path. Technically it’s NOT grounded at both ends, there is no tie to the airframe ground at the switch side.

Of the two GRD terminals on the ACS keyswitch, the center GRD pin (the one with the STAR in the schematic) is attached to ground ALL the time.
The GRD terminal next to R terminal (Marked with an ASTERISK) is only connected to ground when the key is in the "START" position.
 
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Yup, I stand corrected

My bad, brain fart. I shouldn’t try to comment on three hours of sleep, or look at it as a thumbnail. ;)
 
Of the two GRD terminals on the ACS keyswitch, the center GRD pin (the one with the STAR in the schematic) is attached to ground ALL the time.
The GRD terminal next to R terminal (Marked with an ASTERISK) is only connected to ground when the key is in the "START" position.

When you say that the center GRD terminal is attached to ground all the time, do you mean that there is an external wire from the GRD terminal to airframe ground? Or is the GRD terminal grounded internally in the switch assembly?
 
Educate me please . . .

My airport manager has been going through this same thing with no cause yet found on a Citibria. But - it has Bendix mags. And an external center core capacitor. They are $$$$$. BTW it has shields ground at BOTH ends.

For Bill.Peyton - -if the p-lead is disconnected and the ticking remains, would that not point to high tension harness, spark plug or internal mag issue? If not can you help me understand? With no p-lead attached there would be no antenna for the RFI, right?
 
shut off the alternator
is it coming thru all headsets
I put a gizmo on my alternator eons ago for noise. worked great.:)
IMG-3169.jpg
 
Of the two GRD terminals on the ACS keyswitch, the center GRD pin (the one with the STAR in the schematic) is attached to ground ALL the time.
The GRD terminal next to R terminal (Marked with an ASTERISK) is only connected to ground when the key is in the "START" position.

The switch should be connecting that grd terminal to the other terminal with the plead attached and not to the frame ground.
 
Final Update! :D

The issues has been fixed!

For bjdecker: Great Idea, thanks! I tested the center conducter on 4 leads on the bad mag, and 3 leads were at about 2 ohms, and steady. The 4th lead was 42 ohms, then when I intentionally moved the lead around, the resistance jumped all over the place! I have made up a new lead and installed it.

An Avionics Tech at my airport (NZAR) suggested that I test the shield on each lead as well, by taking the lead off the spark plug, then see if the shield goes to ground. Again 3 were OK, one was open. It turned out that the ferrule holding the lead into the mag cap had come loose. I trimmed the spark plug lead, and reassembled with a new ferrule.

So, two issues found. Tested everything on a ground run today, tuned the radio to a very weak ATIS signal that was breaking squelch about 75% of the time. No ignition noise! Listened with engine not running, and with engine running on each mag, and both mags. No noise.

Having said that, the issue about wiring the P-Lead shield is a bit confusing:

Nuckolls sayeth:

"One final point. Note that I show shielded wire between the
magnetos and the magneto switches. Spark plug wires and
magneto p-lead wires are the ONLY wires in the airplane
which are ALWAYS shielded. Further, p-lead shields should
be grounded to the airframe/engine only at the engine end.
The
shield is used to provide a ground for the "GRD" terminals at
the switch but no other connection to the shields should be
made at the instrument panel end.
"

From that I infer that if the shield is used to provide a ground to the center GRD terminal on the switch, then NO OTHER ground wire should be attached to that terminal.

In my case, the shield is only grounded at the engine (mag) end, AND there IS a wire going from the GRD terminal on the switch to airframe ground.

I think either one would work, and that they are both electrically the same.

"WHEW!":rolleyes:

Thanks to everyone for the thought-provoking suggestions. What a great forum DR has created!
 
Final Update! :D

The issues has been fixed!

For bjdecker: Great Idea, thanks! I tested the center conducter on 4 leads on the bad mag, and 3 leads were at about 2 ohms, and steady. The 4th lead was 42 ohms, then when I intentionally moved the lead around, the resistance jumped all over the place! I have made up a new lead and installed it.

An Avionics Tech at my airport (NZAR) suggested that I test the shield on each lead as well, by taking the lead off the spark plug, then see if the shield goes to ground. Again 3 were OK, one was open. It turned out that the ferrule holding the lead into the mag cap had come loose. I trimmed the spark plug lead, and reassembled with a new ferrule.

So, two issues found. Tested everything on a ground run today, tuned the radio to a very weak ATIS signal that was breaking squelch about 75% of the time. No ignition noise! Listened with engine not running, and with engine running on each mag, and both mags. No noise.

Having said that, the issue about wiring the P-Lead shield is a bit confusing:

Nuckolls sayeth:

"One final point. Note that I show shielded wire between the
magnetos and the magneto switches. Spark plug wires and
magneto p-lead wires are the ONLY wires in the airplane
which are ALWAYS shielded. Further, p-lead shields should
be grounded to the airframe/engine only at the engine end.
The
shield is used to provide a ground for the "GRD" terminals at
the switch but no other connection to the shields should be
made at the instrument panel end.
"

From that I infer that if the shield is used to provide a ground to the center GRD terminal on the switch, then NO OTHER ground wire should be attached to that terminal.

In my case, the shield is only grounded at the engine (mag) end, AND there IS a wire going from the GRD terminal on the switch to airframe ground.

I think either one would work, and that they are both electrically the same.

"WHEW!":rolleyes:

Thanks to everyone for the thought-provoking suggestions. What a great forum DR has created!

From an electron flow perspective, they are thr same. However, if the electrons flow back to the mag via the shield,, that surrounds the center core, it helps to cancel out EMI. The benefits only exist when the mag is grounded and not sparking. In normal operation, the shield is only grounded on the mag side.
 
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...
In my case, the shield is only grounded at the engine (mag) end, AND there IS a wire going from the GRD terminal on the switch to airframe ground.

...
Glad you found the problem with the headset noise!

Just be aware that if you lose your ground from the airframe to the engine, when you start the engine, the starter electrons may try to flow through this path. Another reason that 2 grounds are recommended between the engine and the airframe.
 
Glad you found the problem with the headset noise!

Just be aware that if you lose your ground from the airframe to the engine, when you start the engine, the starter electrons may try to flow through this path. Another reason that 2 grounds are recommended between the engine and the airframe.

Also throttle, mixture, and CS prop cables, plus any miscellaneous Bowden cables…zzzzzzt! 😱
 
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