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Thread: P-Leads

  1. #81

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    Default P lead question

    Hello,

    i think the p lead on my pacer may be miswired. I have what I think is old wiring with external brading and slick mags. Only the internal core of the wire is attached to the magneto stud and the shielding is attached to nothing.

    What i I read on here was to connect the p lead to the stud and the shielding to the ground stud on the magneto. So that seems straight forward and uncontroversial.

    My question is what should the other end of the wires at the switch look like. I seem to hear different answers.

    First, should the shielding be connected to the ground stud on the switch or not? I have read some folks say what I learned at the eaa avionics course: the other end of the shielding shouldnít be connected to anything to remove emf. I have read others who say connect it to the ground stud on the switch arguing that when in the switch is in the off position thereís no emf since the engine isnít running and you wont get an emf ground loop. And when the switch is in the L,R, or B position, the shielding is disconnected from and terminates at the switch and thus only connected on the magneto end when running. Whose got this right?

    Secondly, right now my ground stud on the magneto switch is connected via a short wire to a screw bolted through the panel. Not sure thatís a great ground, and I also wonder if that could fry my electronics that are also bolted to the panel as current tries to find ground. Does anyone have an opinion? Is this a good idea to ground the magnetos into the panel?

    Third, does the switch ground stud need another wire connected to the frame, or panel? I read some folks say the shielding is only for emf and not part of the ground circuit, which leads me to think I need another wire to ground from the switch. I read others who said slick recommends grounding through the ground on the magneto, which leads me to think I donít need another wire and if anything it would undermine the emf isolation the shielding provides.

    For the third question, my thinking is that it depends on whether you connect the shielding to the ground stud on the switch or not. If you do, then in the off position you are grounded through the shielding back to the magneto, to the engine, to the frame. If you donít connect the shielding to the switch then you need to get it to grounded somehow like the firewall, panel etc. maybe that explains the different opinions I read. Did I guess right?

  2. #82
    Administrator Steve Pierce's Avatar
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    Default Re: P-Leads

    The shield is for RMI only and not to ground the magnetos. I use a seperate ground wire at the switch to my common ground to a ground lug under the panel usually welded to the frame. I merged your thread with two more with some really good information. I had to read through some posts to refamiliarize myself on the subject

  3. #83

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    Default Re: P-Leads

    Thanks Steve. i started looking through the posts but sadly lots of the links seems dead to sky ranch, etc. So, if I understand your post the ground wire at the panel I have is what you do and you would connect the shielding at the mag. What do you do,with the shielding on the other end? Terminate at firewall or connect to the ground switch?

    And , I kept researching this last night and found a post mention mag filters on the firewall. I didn’t know what I was looking at and assumed that the prongs sticking out of the firewall were a fancy way bring the p leads through the firewall but now think those are the original mag filters. I have slick mags and saw some posts say you shouldn’t use mag filters with slicks, but I co uldn’t find anything from champion on the subject or a manual. How are folks wiring slicks? Should I remove those filters and run wire to the switch?

  4. #84
    Administrator Steve Pierce's Avatar
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    Default Re: P-Leads

    I think Homer gives a good explanation of the system.

  5. #85

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    Default Re: P-Leads

    I don’t recall the explanation behind it at the moment but I was taught sheilding only gets connected at one end and I don’t think I have ever seen it done otherwise.

  6. #86

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    Default Re: P-Leads

    As a general matter, that is what eaa taught me when I went to their avionics class. But, some say ground the switch and don't connect the shielding these are two different circuits for different purposes (Steve seems to be in this camp). Others say connect the shielding to the switch but don't ground the switch and the shielding serves double purpose (Jim seems to be in this camp). When switch is off, it is the ground. When on, it is open and thus acting as the emf sink. And, still other folks say this emf ground loop issue was an issue with old electronics which has largely been designed out of newer radios and that it is better to ground both ends now for some reason I didn't quite get. So, I count at least 3 different answers.

    Then you have these firewall mounted filters. Some seem to have 'em. Some say after 50 years they are trash and should be removed. Some say you only need them for Bendix and that the extra capacitor can harm slicks due to how they are designed and/or the plugs. Others, of course, disagree.


    Maybe I should have just asked if wheel landings are better than three points.



    Quote Originally Posted by Jeff J View Post
    I donít recall the explanation behind it at the moment but I was taught sheilding only gets connected at one end and I donít think I have ever seen it done otherwise.

  7. #87

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    Default Re: P-Leads

    Quote Originally Posted by Tnathan View Post
    Maybe I should have just asked if wheel landings are better than three points.
    Thatís easy! A proper 3-point is harder to do so I always wheel land.

  8. #88
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    Default Re: P-Leads

    I just wire it like Piper did, figure as many airplanes as they build they knew what they were doing.

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