M20J -- P-Leads, magnetos and radio noise

[BaldEagle]

I have an annoying issue with background noise with incoming transmissions.  I narrowed it down to the left Mag (noise goes away when selecting R mag in flight) and when I pulled the cowling I noticed that the the shielding on the left P-Lead had separated from the nut with about an inch of the wire unshielded and with an ignition lead rubbing up against the unshielded part of the P-Lead.  There's about 220 hours on the dual mag since last overhaul so I'm assuming the mags are likely good and it's a P-Lead shielding issue that is causing the radio noise.  Both mags are grounded.  Alternator is a couple of years old; noise filter / capacitor is attached to alternator.  The shielding on the right P-Lead is starting to fray as well, so that needs replacing also.

Anyway, before I call my A&P for a hot date to replace the P-Leads, does anyone know the length for Left and Right P-Leads?  I believe this is the product I need:

https://www.aircraftspruce.com/catalog/eppages/d2-d3-pleads.php

For bonus points... does anyone know what ignition harness / end cap is needed for D3000 dual Bendix for A3B6D Lycoming?  I was planning on replacing this the next time the mags are due for overhaul, but I may have this done at the same time.

[N201MKTurbo]

If you are getting noise from the P leads, I would be more concerned with the condenser than the P leads. You can swap the condensers and see if the problem moves to the other mag. Along those lines, if they are long enough, swap the P leads and see if the problem stays on the same mag, which would indicate it is the P leads.

[EricJ]

The p-leads and their shields are routed through a cannon plug in the firewall, so the leads from the magnetos are not very long from the mag to the cannon plug.   A multimeter will show whether the shield is still grounded at the cannon plug or not if it has separated from the connector at the mag.   A quick test of whether the bare chunk is the issue or not can be done by wrapping a little aluminum foil around the bare spot and zip tie it in contact with the exposed part of the shield.

If that chunk needs to be replaced it may only need to be repaired from the mag to the cannon plug.   I'd double check the shields from the cannon plug to the switch, too.

When I sent my dual mag to Kelly Aerospace for the 500-hour inspection last year I sent it with the lid and harness since the harness was pretty janky.   They concluded my harness was "trash" and sent me a new lid with new harness with my rejuvenated mag.   It's been great.   I think there's a parts catalog on Kelly's website and price list for stuff like that.

And +1 that it could be a condenser, too.

[Ragsf15e]

Check the connection and condition of the harness at the spark plugs too.  One annual mine were left finger tight (they are supposed to be only slightly tighter), and I ended up with bad feedback, but my engine ran fine.  Weird.

[glenn reynolds]

spend some time on bogart aviation's website.  They seem to have great info on P leads and what connects to what.  I agree that turning off a mag may mean that the spark plug leads or magneto output is what is causing the noise, not necessarily the p lead.  Remember that the P lead is a ground, when the switch is open there is no ground via the p lead, not a spark transmitter.  The shield reduces it's tendency to act as an antenna for all the high voltage emf happening in the mag and spark plug leads.   And do note that i'm no expert on this stuff, just shooting my two cents.  

[BaldEagle]

Thanks all for the feedback and assistance.  I was poking around and gently pulled on the shielding of one of the P-leads to find that the shielding had separated from the nut.  It looks like a common fix is to pull the shielding up about an inch and to then ground it to a nut on the magneto case assembly, which is what the previous owner's A&P had done to the other P-Lead at some time in the past.  For those of you that do owner-assisted annuals, a line to add to the checklist might be "gently pull on p-lead shielding" as it might save you serious misery later...

While I was poking around I pulled the cap off the dual mag to check the two capacitors.  The service manual says to replace the capacitor(s) if it has a date code older than 9921.  One capacitor should have been replaced years ago but wasn't.  I don't have the equipment to test capacitance, etc., and I'm not an A&P in any case, so I'm just going to replace both of them as they are only about $100 each.  Though I'd report back here because there a Service Bulletin (SB651) regarding this on the dual D2000/3000 magnetos: it seems that A&Ps are pulling the dual mag and sending them off for the 500 hour inspection but are leaving the two capacitors attached to the ignition harness cap.  The SB reports of rough running engines because of neglected capacitor inspections and subsequent arcing, etc., which "will result in the destruction of the contact assembly, loss of ignition and loss of engine power".  Seems like $200 well spent to me.  Thought I'd mention it here because if you have the dual magneto, this might be something you want to check at some point.

[EricJ]

3 hours ago, BaldEagle said:

While I was poking around I pulled the cap off the dual mag to check the two capacitors.  The service manual says to replace the capacitor(s) if it has a date code older than 9921.  One capacitor should have been replaced years ago but wasn't.  I don't have the equipment to test capacitance, etc., and I'm not an A&P in any case, so I'm just going to replace both of them as they are only about $100 each.  Though I'd report back here because there a Service Bulletin (SB651) regarding this on the dual D2000/3000 magnetos: it seems that A&Ps are pulling the dual mag and sending them off for the 500 hour inspection but are leaving the two capacitors attached to the ignition harness cap.  The SB reports of rough running engines because of neglected capacitor inspections and subsequent arcing, etc., which "will result in the destruction of the contact assembly, loss of ignition and loss of engine power".  Seems like $200 well spent to me.  Thought I'd mention it here because if you have the dual magneto, this might be something you want to check at some point.

When I sent my dual mag to Kelly for the 500-hr inspection I confirmed that new caps were included with the quoted cost of the inspection.  If you don't send the lid with the harness (which most don't and likely don't need to), they just send you the caps loose.

 

[PT20J]

On my M20J the p-leads were grounded at the dual mag and also at the ignition switch. When I switched to the -A3B6 with two Slick mags, I didn't ground the mag end per Slick's recommendation. For shielding purposes, the shield only needs to be grounded at one end.

[glenn reynolds]

I had noise on my radios and when I spoke with my mechanic about it, he had me use his spark plug ignition lead tester to check each spark plug wire.  This box connects to ships power and then you put the third lead on the spark plug wire (plug end).  Two of my leads were damaged and so I replaced the harness and problems solved.  

[M20JBRA]

Em 22/05/2022 em 22:38, EricJ disse:

Os cabos pe seus escudos são direcionados através de um plugue de canhão no firewall, de modo que os cabos dos magnetos não sejam muito longos do mag ao plugue do canhão. Um multímetro mostrará se a blindagem ainda está aterrada no plugue do canhão ou não, se ela se separar do conector no mag. Um teste rápido para saber se o pedaço nu é o problema ou não pode ser feito enrolando um pouco de papel alumínio ao redor do ponto nu e amarrando-o com zíper em contato com a parte exposta do escudo.

Se esse pedaço precisar ser substituído, pode ser necessário apenas consertá-lo do carregador ao plugue do canhão. Eu verificaria novamente os escudos do plugue do canhão ao interruptor também.

Quando enviei minha revista dupla para a Kelly Aerospace para a certificação de 500 horas no ano passado, enviei-a com a tampa e o arnês, já que o arnês era bastante irregular. Eles concluíram que meu arnês era "lixo" e me enviaram uma nova tampa com um novo arnês com minha revista rejuvenescida. Tem sido ótimo. Acho que há um catálogo de peças no site da Kelly e uma lista de preços para coisas assim.

E +1 que poderia ser um condensador também.

Condensator... serious problem...