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Magneto P-leads

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dean.psiropoulos(at)veriz
Guest

Posted: Sun Nov 11, 2007 12:01 am Post subject: Magneto P-leads

I did a test on my magneto P-leads (one mag on left side, electronic on

right side)with an ohm meter.  Thinking that when the key switch is in the

off position, the resistance across the p-leads should measure close to dead

short and, it does.  By the same logic, I thought that when the mag was

ungrounded the resistance across the p-leads (I used a shielded wire with

the shield grounded at the magneto through the key switch, no local

grounding).  But when I measured across the p-leads at the magneto the

resistance only went from 0.2 ohms to 0.4 ohms when I turned the key.  Not

an appreciable increase in resistance but it was consistent (happens every

time) and measurable. I thought maybe I had a faulty key switch but not so,

I removed one p-lead from the mag and made the same measurement and the

resistance with the key in that mag's position read infinite.  I suspect

that there is some thing about magnetos I don't know and that this is not a

problem but would like to be sure. Anybody ever done this test?  Did you get

the same results? Thanks.



Dean

RV-6A N197DM

closer to 1st flight.

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apilot2(at)gmail.com
Guest

Posted: Sun Nov 11, 2007 6:36 am Post subject: Magneto P-leads

Was the P lead connected to the magneto when you measured resistance to ground? If so, and the points were closed, you were measuring resistance of the points. You need the P lead off the mag, or the points open to get a good resistance reading.  I've seen a couple instances, (with old P-leads) of the internal insulation cracking and grounding to shield, but not too likely with relatively new wiring. 



On Nov 11, 2007 12:59 AM, DEAN PSIROPOULOS <dean.psiropoulos(at)verizon.net (dean.psiropoulos(at)verizon.net)> wrote:

[quote] --> Engines-List message posted by: "DEAN PSIROPOULOS" <dean.psiropoulos(at)verizon.net (dean.psiropoulos(at)verizon.net)>

I did a test on my magneto P-leads (one mag on left side, electronic on 

right side)with an ohm meter.  Thinking that when the key switch is in the

off position, the resistance across the p-leads should measure close to dead

short and, it does.  By the same logic, I thought that when the mag was 

ungrounded the resistance across the p-leads (I used a shielded wire with

the shield grounded at the magneto through the key switch, no local

grounding).  But when I measured across the p-leads at the magneto the 

resistance only went from 0.2 ohms to 0.4 ohms when I turned the key.  Not

an appreciable increase in resistance but it was consistent (happens every

time) and measurable. I thought maybe I had a faulty key switch but not so, 

I removed one p-lead from the mag and made the same measurement and the

resistance with the key in that mag's position read infinite.  I suspect

that there is some thing about magnetos I don't know and that this is not a 



[b]

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glcasey

Joined: 01 Jul 2007
Posts: 23

Posted: Mon Nov 12, 2007 5:32 am Post subject: Magneto P-leads

Dean,

I'm a little confused about which resistance you are concerned about,  

but here is what you can expect to measure:  If the switch is  

disconnected from the mag, a reading of the resistance from the P- 

lead terminal to ground will be either (with the points closed)  

essentially zero, or (with the points open) almost zero.  The circuit  

inside the mag is from ground through the primary ignition coil  

winding to the points and back to ground.  The P-lead is connected to  

the connection between the coil and points.  If the mag is rotated so  

the points are closed only the point resistance is in the circuit,  

which should measure as zero.  When the mag is turned so the points  

are open the only connection to ground is through the coil primary  

winding, which will measure at something very low, but not zero.  It  

will probably be less than 1 ohm.  I use this difference to set the  

timing, eliminating the need for the "buzz box" that mechanics  

normally use.  You can, if you like, check the connected ignition  

switch by turning the mag until the points are open (ignition switch  

has to be in the "on" position to know this)  and watching the  

resistance of the P-lead to ground while turning the switch on and  

off - there should be a small but noticeable change in resistance.   

Oh, and when the switch is in the "R" position the right mag is on  

(switch is open) and the left mag is off (shorted to ground).



Hope I didn't confuse things more.



Gary

 	  Quote: 	
		  

 Time: 12:01:57 AM PST US

 From: "DEAN PSIROPOULOS" <dean.psiropoulos(at)verizon.net>

 Subject: Magneto P-leads



 I did a test on my magneto P-leads (one mag on left side,  

 electronic on

 right side)with an ohm meter.  Thinking that when the key switch is  

 in the

 off position, the resistance across the p-leads should measure  

 close to dead

 short and, it does.  By the same logic, I thought that when the mag  

 was

 ungrounded the resistance across the p-leads (I used a shielded  

 wire with

 the shield grounded at the magneto through the key switch, no local

 grounding).  But when I measured across the p-leads at the magneto the

 resistance only went from 0.2 ohms to 0.4 ohms when I turned the  

 key.  Not

 an appreciable increase in resistance but it was consistent  

 (happens every

 time) and measurable. I thought maybe I had a faulty key switch but  

 not so,

 I removed one p-lead from the mag and made the same measurement and  

 the

 resistance with the key in that mag's position read infinite.  I  

 suspect

 that there is some thing about magnetos I don't know and that this  

 is not a

 problem but would like to be sure. Anybody ever done this test?   

 Did you get

 the same results? Thanks.



 Dean

 RV-6A N197DM

 closer to 1st flight.

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_________________
Gary Casey

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