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P lead

I can't seem to find this anywhere on the forum so here goes. I have a Slick Magneto model 4313 with impulse coupling installed on the left side of my new factory delivered Lycoming O-360. While checking out my wiring I noticed that the P lead seems to have no resistance to ground.
1. Do I have the correct connector for the P lead: I am using the nut and the fiber washer on the top of the magneto for the P lead and the shield ground is the screw marked GND.
2. Does the p lead normally have no resistance to ground?
3. Why?

I think the part that I can't get around is the fact that you ground the P lead to stop the magneto from firing so why is it seem to be grounded already. Help I am a product of the California school system.
 

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  • Slick P-Lead Fixation.jpg
    Slick P-Lead Fixation.jpg
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What was the ignition switch position when the resistance was measured? Ignition off is simply a bridge between that center conductor (P) to the shielding attached to the mag housing.
 
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Don't waste your brain cells on trying to hook up an ohm meter to the P-lead terminal. . It will work when you use a buzz box.. the resistance thing changes based on the position of the cam, if the points are open or closed. .it's black magic in there. .just use the buzz box, make sure you go past the impulse coupling (hear the snap) back up, then sneak up on the 25btdc mark. You want to back up just enough to take out the gear slack, not reset the impulse coupling.
 
I was measuring the resistance with the wires disconnected from the mag. Just the post and the ground. Thanks for the advice.
 
I did this too when I first installed my engine. Definitely don't read much into it. Just time it and all will be good. I do however think it's a good practice to take the wires off and make sure your switch in the off position is actually connecting the two leads. (grounding the mag)
 
I was measuring the resistance with the wires disconnected from the mag. Just the post and the ground. Thanks for the advice.

You’ll notice the resistance changes as you rotate the engine (and the attached mag) this isn’t telling you what you want to know.. borrow or buy a buzz box and don’t try to time it with an ohm meter. It is a good ides to verify grounding and ungrounding of the key or mag switches to the airframe with an ohm meter!
 
You definitely want to make sure your P-lead is grounded with the ignition for that mag turned OFF (zero resistance). That is the most important part. No resistance to ground is what you should see when the ignition is off. If you have an Cessna type ignition switch (L-R-Both), you should see zero resistance when the the selected mag is not selected (either the other mag selected, or off is selected). With the selected ignition on, or both is selected on (mag ungrounded), you should see no continuity at all (mega ohms - I don’t have an icon for that), meaning the mag is hot.

The P-lead will have zero resistance to ground when that mag is selected off, in other words, it is grounded and unable to send a spark to the spark plugs, when that mag is selected off because the energy from that mag, if the prop were to turn, is sent to ground - path of least resistance - to ground, vs, through through a resistive ignition lead to a resistive spark plug, in order to provide a high enough voltage to jump the gap on the spark plug to ignite that un-even fuel to air mixture and provide some sort of power. Could be a problem if you are doing maintenance in your hangar and your ignition system isn’t grounded.

By the way...... I like the way your P-lead is connected in your pictured post. The adel-type clamp secures the P-lead and shield ground, protects it from shorting somewhere, and provides a very robust installation. The two P-lead connection points can’t move, relative to each other, and should provide long term continuity.
 
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Points

I’m not an expert on Mags (don’t have any) but I think all the P-lead does is ground out the points. When you check the resistance from the mag contact to ground, you are reading the contacts on the points. If the points are closed, you read a short to ground.

-Andy
 
"A picture is worth a thousand words"
You can see from the diagram below that using an ohm meter to check the P lead will always show a ground thru the coil with the points open or closed.

yua8a.png
 
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