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  #1  
Old 11-24-2021, 10:28 PM
mfleming's Avatar
mfleming mfleming is offline
 
Join Date: Sep 2016
Location: Joseph, Oregon
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Default Slick Mag P-lead Question

I wired my P-lead to the left Slick mag today (P-Mag on the right side)

I'm using a SPST upside down toggle switch on the panel for the mag. I'm using a 18AWG shielded wire from the switch to the mag. On the switch end the center conductor goes to one side of the switch and the shield has a pigtail to the other side of the switch. On the mag side, the center conductor goes to the main post and the shield goes to the ground screw next to it. The mag side is depicted in the first photo (not my photo but the wiring is the same).

I tested the switch function before connecting to the mag and confirmed switch down is a closed circuit and up is open.

My question and confusion is that the main post of the mag shows continuity to ground...so in effect the switch appears to do nothing...the mag is grounded all the time

What am I missing?
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RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring and Avionics harness completed 9/2/2021
FWF Started 9/3/2021

Donated for 2021 and so should you
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  #2  
Old 11-24-2021, 10:42 PM
BobTurner BobTurner is offline
 
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Default

The points must be closed. Rotate the prop (carefully) while watching your ohmmeter.
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  #3  
Old 11-25-2021, 12:10 AM
PilotjohnS PilotjohnS is offline
 
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Location: Southwest
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Default Switch ground

I think it is important to make sure the switch is only grouned to the mag and not grounded to the airframe too. This way, if the engine ground falls off, the alternator and starter return current doesnt go thru the mag ground to aircraft structure.
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  #4  
Old 11-25-2021, 08:22 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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The P lead post is actually the end of a capicator and it is wired in parallel with the coil's primary winding output on one side of the points (the other side is ground). That post should show 0 ohms to a mag ground when the points are closed and around 4 - 8 ohms to ground when the points are open (the resistance through the coil primary winding). The fact that you get continuity doesn't mean it doesn't work as intended - externally grounding this capacitor prevents the primary field collapse (normally caused by points opening) that causes the energy on the secondary winding, when the points open. You are essentially bridging the points closed when the P lead switch is closed. In order to spark, the primary winding circuit must get opened.

Larry
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Last edited by lr172 : 11-25-2021 at 08:30 AM.
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  #5  
Old 11-25-2021, 10:14 AM
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mfleming mfleming is offline
 
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Location: Joseph, Oregon
Posts: 809
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Quote:
Originally Posted by lr172 View Post
The P lead post is actually the end of a capicator and it is wired in parallel with the coil's primary winding output on one side of the points (the other side is ground). That post should show 0 ohms to a mag ground when the points are closed and around 4 - 8 ohms to ground when the points are open (the resistance through the coil primary winding). The fact that you get continuity doesn't mean it doesn't work as intended - externally grounding this capacitor prevents the primary field collapse (normally caused by points opening) that causes the energy on the secondary winding, when the points open. You are essentially bridging the points closed when the P lead switch is closed. In order to spark, the primary winding circuit must get opened.

Larry
I should have remembered this.

Looking at this diagram itís obvious how the points being closed cause the continuity I was observing.
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__________________
Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring and Avionics harness completed 9/2/2021
FWF Started 9/3/2021

Donated for 2021 and so should you
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