P Lead

[Zook]

I got used mags that came without p leads. S4LN-21 and S4LN-20. Is there a way to make your own p lead that have “slick” ends on them ?

[DENNY]

Here is a good thread. https://www.supercub.org/forum/showt...-lead-question pay attention on proper ground hook up!! Here is a good Video on the P leads.https://www.youtube.com/watch?v=D-GBt_4dvf4
DENNY

[wireweinie]

By 'Slick' ends, I'm assuming you mean ring terminals?

P-leads are just single conductor, shielded wire. Use a solder sleeve to attach a jumper wire to the shield at each end. Install #10 rings at the mag end and rings that match the screws on your switch at the other end.

At the mag; Center conductor to P-lead terminal. Jumper wire to the ground screw on the case.

At a key switch; Center conductor to the tab marked for THAT mag. Jumper wire to the ground tab.

At a toggle switch; Connect the center conductor and jumper wire to the two switch tabs that are CLOSED when the switch is DOWN.

Notes on installation.
-NO CONNECTIONS from ground tab on switches to airframe ground.
-Route P-leads separate from all other wires. Never bundle with other wires.
-Keep P-leads clearly marked so that they will always be connected to correct mags and correct switch tabs.

Or you can always contact me and I'll make them for you

Web

[Cub Builder]

Aircraft Spruce has an adapter they want a small fortune to buy. But Fresno carried the originals for $17. Uou could easily modify for your use. <https://www.fresnoairpartsco.com/product/p-lead-kit-magneto-bendix/>.

[DENNY]

Web, The ACS switch calls for the center ground to be connected to the airframe. I am pretty sure this is why I fried my points in the right mag this summer when I had a loose ground on the engine. Just to be clear the center ground #5 is where I should hook up the P lead grounds with no ground lead to the fuselage. The switch itself does not have to be grounded to anything for the starter to work correctly. Am I thinking right on this one? https://acsproducts.co/wp-content/up...stallation.jpg

DENNY

[wireweinie]

Web, The ACS switch calls for the center ground to be connected to the airframe. I am pretty sure this is why I fried my points in the right mag this summer when I had a loose ground on the engine. Just to be clear the center ground #5 is where I should hook up the P lead grounds with no ground lead to the fuselage. The switch itself does not have to be grounded to anything for the starter to work correctly. Am I thinking right on this one? https://acsproducts.co/wp-content/up...stallation.jpg

DENNY

Correct. No airframe ground to the tab labeled (5) on that diagram. Connection to both P-lead shields only. The other ends of those shields will be connected to the ground screw on the side of the mag case. Also, no grounds are needed for starter relay or shower of sparks operations.

All electrical items need a ground path back to the source of their power. Magnetos produce their own power and have nothing to do with the ships battery or charging system (remember that you can hand prop with no/dead battery). When mag switches are selected to 'off', they short out the primary coil to prevent the mag from producing a spark. Since the mag produces it's own power, that means the primary coil needs to be shorted out to the mag. If you connect the mag switch to airframe ground and there is no ground path back to the mag, then you will have a hot mag and will not be able to shut it down with the switch. Using the P-lead shield as a ground path back to the mag has two uses. First is an unbroken ground path from the mag switch directly to the mag, for safest switch action. Second point is that it confines any electrical noise produced on those pleads to the leads themselves and doesn't inject that noise onto the airframe ground path. I.e., less chance of mag noise on your headset.

Web

P.S. I apologize for starting yet another argument. This time I brought up hand propping.

[BC12D-4-85]

^^^^We're spoiled by Web. Worth the price of joining the Forum. Try to get an A&P to explain this sometime. I have - no luck.

Gary

[wireweinie]

Thanks for the kind words. I got a good start when my uncle (Paul Williams of Williams Airmotive/Airframe Components) told me to be sure and specialize and not just be a general mechanic. Later a man named Larry Dumm sat me down and taught me about connectors and connector tooling. With all of the people like these in my past that pushed me forward, I feel obligated to share my experience.

Web

[BC12D-4-85]

I get a little engine noise on my ICOM A-200 (need to check that w and w/o antenna connected). I think my mag switch is grounded at the panel so undo that. Now I need to look see what the P-lead shields look like, where they grounded, and what other wires are tied up close by. My B&C 13A alternator (C-85-12 engine) has a filter cap on the firewall but need to check where the charging lead is routed.

Gary

[Bearhawk Builder]

Appreciate this explanation Web, I never thought about isolating the start switch from the rest of the electrical system for the reasons you mention. Most other references say to make sure that switch is grounded to airframe ground, including the switch diagram and the latest issue of Sport Aviation. However I can see the potential of introducing mag noise into your system that way and that the switch does not need to be grounded. I do think this highlights the importance of a good engine to airframe ground strap though.

Correct. No airframe ground to the tab labeled (5) on that diagram. Connection to both P-lead shields only. The other ends of those shields will be connected to the ground screw on the side of the mag case. Also, no grounds are needed for starter relay or shower of sparks operations.

All electrical items need a ground path back to the source of their power. Magnetos produce their own power and have nothing to do with the ships battery or charging system (remember that you can hand prop with no/dead battery). When mag switches are selected to 'off', they short out the primary coil to prevent the mag from producing a spark. Since the mag produces it's own power, that means the primary coil needs to be shorted out to the mag. If you connect the mag switch to airframe ground and there is no ground path back to the mag, then you will have a hot mag and will not be able to shut it down with the switch. Using the P-lead shield as a ground path back to the mag has two uses. First is an unbroken ground path from the mag switch directly to the mag, for safest switch action. Second point is that it confines any electrical noise produced on those pleads to the leads themselves and doesn't inject that noise onto the airframe ground path. I.e., less chance of mag noise on your headset.

Web

P.S. I apologize for starting yet another argument. This time I brought up hand propping.

[DENNY]

I called the last IA to work on my switch (SB rebuild) He did not remember offhand if it was grounded to the frame but if it was in the instructions most likely done. He said the only reason he could think of that they wanted the switch grounded to the airframe was a backup to the mag ground that likes to get damaged at the mag end. So less chance of a hot mag. He agrees with our assessment of the fuselage grounded switch and loose ground causing the mag issue. I am going in for annual this month and will most likely pull that frame ground off the switch as long as IA doing sign off agrees. I do mag checks on every flight just as part of the flow so I am not that worried about missing a hot mag. Web thanks again for holding our hands through the world of electrons!!
DENNY

[wireweinie]

I do think this highlights the importance of a good engine to airframe ground strap though.

Bad ground from engine to airframe will let you know real quick. High resistance will show as slow starter speed. Broken/no ground strap will show as random wires burning/smoking/melting. Sometimes, even the primer lines heat up.

Web

[BC12D-4-85]

On some (more than one or two) of the planes I've owned and flown the P-lead braided shield was loose at the magnetos. Just pulled up close and frayed, or sometimes they had a few strands loosely captured by the nut assembly that connected the lead to the mag (example below). Eventually from handling or vibration they would separate from the mag. Nobody either cared or knew better how to reattach them. Now we know there's a better way thanks to Web.

Gary

[skywagon8a]

I do mag checks on every flight just as part of the flow so I am not that worried about missing a hot mag.

Do you do a mag check at the end of your flight just before shutting down? Not many of us do. How else would you know if you had a P-lead wire failure during the last flight which left you with a hot mag? The airplane could be resting in it's hanger or on it's tie down with a hot mag which some unknowing person could find if they moved the prop.

[frequent_flyer]

On some (more than one or two) of the planes I've owned and flown the P-lead braided shield was loose at the magnetos. Just pulled up close and frayed, or sometimes they had a few strands loosely captured by the nut assembly that connected the lead to the mag (example below). Eventually from handling or vibration they would separate from the mag. Nobody either cared or knew better how to reattach them. Now we know there's a better way thanks to Web.

I had a far more obscure fault on a P lead of my PA-28-180. Back when when it was a 6 way partnership one of the members reported a 200 rpm loss of rpm on climbout. It spooked him and he wanted us to rebuild the mag or, even better, rebuild the engine. There was some resistance as none of the other partners had experienced the problem. However the mags were overhauled.

Much later I experienced the same problem and it was attention getting.

I eventually found out what was causing the problem. The bushing shown in the previous P lead photo was shorter on one mag than on the other. The P lead presses on a leaf spring in the rear mag housing and there were signs of arcing between the spring and the mag case. I replaced the abnormally short p lead bushing with a standard length bushing. That increased the separation between the P lead leaf spring and the mag case. Never saw the problem again.

[skywagon8a]

I eventually found out what was causing the problem. The bushing shown in the previous P lead photo was shorter on one mag than on the other. The P lead presses on a leaf spring in the rear mag housing and there were signs of arcing between the spring and the mag case. I replaced the abnormally short p lead bushing with a standard length bushing. That increased the separation between the P lead leaf spring and the mag case. Never saw the problem again.

There are two different leaf contact springs. One like yours requires the long P-lead bushing. The other leaf spring is long enough so that it makes contact with the magneto housing when the P-lead bushing is removed. That one requires the short bushing. When your P-lead is removed the mag becomes hot. When the short P-lead bushing is removed the spring grounds out the points making the mag safe.

[wireweinie]

Pete beat me to it!

Similar set up to what the stock Beavers had. Disconnect the p-leads at the firewall cannon plug and a contact inside that connector automatically grounded the mags. Worked great to keep mechanics safe but caused a couple of high laundry bills when it happened in flight.

Web

[DENNY]

Do you do a mag check at the end of your flight just before shutting down? Not many of us do. How else would you know if you had a P-lead wire failure during the last flight which left you with a hot mag? The airplane could be resting in it's hanger or on it's tie down with a hot mag which some unknowing person could find if they moved the prop.

I usually don't but it would not be hard to work it into the flow.
DENNY

[dirthog4]

I shut down with the switch tests p leads every time.