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P-Lead Puzzle

Flying Canuck

Well Known Member
Patron
So, I just replaced my ACS keyed ignition with magneto toggles and a start push button. I understand the fact that a disabled mag is grounded and the circuit opens when you enable the mag. My switches are designed to do that. Problem is that I did a simple continuity test - p-lead to ground - switch in both positions. Result is the p-lead is grounded at both switch states, same on both mags. Then I checked the p-lead, not connected to anything at all an it is grounded. Same with both p-leads. Now I'm sure I'm just missing something here, I had no problems creating the open circuit and starting/running the engine with the key switch. How was that possible if the p-leads appear to be permanently grounded.

The p-leads are shielded wire with a ring terminal and the shield pulled out to another ring terminal. The central terminals were attached to the magneto positions on the key switch and the shield terminations were attached to the ground on the key switch.

I'm sure it's a simple explanation, but it escapes me. Can anyone educate me? I want to make sure I understand before attempting a start with my new setup.
 
It certainly sounds like you did the right things. Is it possible your switch internally grounds one of the terminals? And you used that terminal by mistake, for the P lead?
 
Check your switch and ensure it is open and closed when you think it should be.
You are confusing yourself by measuring the magneto. The magneto coil is tied to ground on the other side.
 
A continuity check is not reliable under this circumstance. If the points are closed in the mag, which they are most of the time, you will read a short to ground even with the switch open.
 
If you checking continuity on the mag switch, remove the connection to the mag, otherwise you are measuring through the mag coils like others said. .
 
Yes, no question I'm confused. My DPDT On-On switches are doing exactly what I expect them to, but when the p-leads are connected, I just get continuity between each pair of pins (1-2) and (2-3) regardless of switch position.

I've attached a schematic, the red battery path through the switches is so I can light the button when one of the mags is on. Ground is connected to the off position pin, so on should open the circuit.
 

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A continuity check is not reliable under this circumstance. If the points are closed in the mag, which they are most of the time, you will read a short to ground even with the switch open.

Thank you. That explains why I was getting the results I was. Of course I can't wrap my head around how the open circuit would actually enable the mag. I'll just trust my simple design and proceed with testing it this next week.

I just posted my schematic, if my simple design has a flaw, then that's a different issue.
 
Are both mags impulse mags?
If not, I think you have the starter circuit wired wrong.

I think Nuckols recommends you wire the starter in series through the switches, so the non-impulse mag must be OFF to engage the starter. Check AeroElectric Connection.
 
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No, only the left mag has impulse coupling. I was just going to use a procedure to start on left mag only. I do like the idea of enforcing that though, more than the idea of lighting up for a hot mag.

It's not a huge modification, bat comes out of start enable to R MAG, out of the OFF side of that switch to L MAG and out the ON side to the start button. And if you set the switches right you win the prize of a lit start button. I like that, I'll do it. I don't have any more terminals for the switches, but I can make the mod with just one splice.

Thanks for picking that up Mike.
 

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Okay, when the OP said the P lead was not connected to anything at all, I thought he meant not anything at all. If it was still connected to the mags, then that’s the issue. Very carefully rotate the prop in the normal direction until you hear the impulse snap; rotate prop backwards about 10 deg. Repeat your measurements.
Previous post is correct about starting, you have either mag enabling start, which is okay if both mags have an impulse coupling. If the R mag does not, then rerout the enable wire in series thru both mags so the starter is enabled only if L is on and R is off.
Edit: I see you did this while I was typing!
 
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Not sure of the value of the START ENABLE switch. Seems unnecessary with the corrected wiring. The Starter won’t engage if the right mag is hot.
 
That's just a "hidden" switch to add a little security to the system since I removed the key. Probably will never be turned off unless I park outside somewhere I don't trust. Right up there with the hidden switch I have to disable the co-pilot stick buttons.
 
I can't wrap my head around how the open circuit would actually enable the mag. I'll just trust my simple design and proceed with testing it this next week.
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Power flows through closed points and to a ground (p lead is bridged to an area of the primary circuit upstream of the points - specifically the condensor). When the points open, the ground is removed and the field in the coil collapses, causing the spark energy on the secondary winding to release. If the p lead is grounded, the opening of the points no longer removes the ground from the primary circuit and therefore no spark. Grounding the p lead is a way to force the primary circuit from opening, and therefore preventing sparking.

Larry
 
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