mag not wired - PLead grounded?

prkaye · #1 11-26-2009, 07:37 AM

I haven't put any wires to my mag yet. Out of curiosity, I checked the wiring posts on the mag with my ohmeter. To my surprise, I found that the P-Lead on my mag has a connection to my ground block (through the engine case no-doubt).
Is this to be expected?

Mel · #2 11-26-2009, 07:45 AM

The mag is HOT!
If you are simply measuring with an ohm meter, what you are seeing is a grounding through the points. To make the mag safe, you must add a ground wire from the "P" lead to the engine case.

prkaye · #3 11-26-2009, 07:48 AM

I don't understand.

Quote:

what you are seeing is a grounding through the points

What does this mean?

I was about to run the wire from my PLead to my keyswitch, as per the keyswitch wiring instructions. I would connect the shielding of the PLead wire to the ground post on the mag.

Mel · #4 11-26-2009, 08:11 AM

The points open and close as the mag is turned. When the points are closed, you will see a "ground". At some point you will not see this ground. To make the mag "OFF", the "P" lead post must be grounded externally.

prkaye · #5 11-26-2009, 08:14 AM

Ah, ok. And this grounding is done through the keyswitch when the switch is in the OFF position? This requires that there be a wire from the switch to the GND terminal on the switch, no? From reading through old threads on ignition wiring it seems some people leave that wire off... wouldn't that leave a mag always hot (if the keyswitch had no physical connection to ground)?

Mel · #6 11-26-2009, 08:22 AM

Quote:

Originally Posted by prkaye

Ah, ok. And this grounding is done through the keyswitch when the switch is in the OFF position? This requires that there be a wire from the switch to the GND terminal on the switch, no? From reading through old threads on ignition wiring it seems some people leave that wire off... wouldn't that leave a mag always hot (if the keyswitch had no physical connection to ground)?

Normal wiring is to wire the "P" lead to the switch and the shield from the ground post on the mag to the other side of the switch. The switch connects the "P" lead to ground through the shield. Some people use airframe ground to ground the mag, although it is not recommended.

longranger · #7 11-26-2009, 01:01 PM

Quote:

Originally Posted by Mel

The points open and close as the mag is turned. When the points are closed, you will see a "ground". At some point you will not see this ground. To make the mag "OFF", the "P" lead post must be grounded externally.

Actually, it's doubtful you'll see the points open with an ohmmeter. With the points open the meter will be measuring the resistance of the primary coil, a very low resistance practically indistinguishable from the points' short circuit when they're closed. The "buzz boxes" detect the coil's inductance when the points open.

aerial · #8 07-30-2010, 10:11 AM

This whole "grounding thru the points" threw me for a loop. I had tried testing my mag switches with an ohm meter and could not get them to consistently read resistance in the off position. I ended up flying the mags hot to get home (taking the p-lead's off). Thanks for the post.

jetjok · #9 07-30-2010, 01:50 PM

Phil,
When you have your P-lead wire correctly made up, you will have two ring terminals on each end (total of 4 on each P-lead) There will be a terminal on the wire, and one on the shielding of the wire.
On the engine side, the wire will be connected to the stud on the mag, and the shielding will be connected to the ground screw (assuming that you have Slick mags). The other end of the wire will go to either the left or right (look for the L or R) on the back of your switch. The GND terminal on the switch is to ground the switch for correct ignition operation. This is NOT the place to ground the shielding.
As Mel stated, most folks find an airframe ground for the shielding that is near the switch.
If you test the switch, you will find that the R terminal is grounded when you go to the START position with the key switch. This grounds the non-impulse mag for starting. It is really important for several reasons that you wire the switch correctly.
Sorry to be so verbose, but I wanted to try to explain it all to you.
Good Luck

elippse · #10 07-30-2010, 06:39 PM

Quote:

Originally Posted by jetjok

Phil,
When you have your P-lead wire correctly made up, you will have two ring terminals on each end (total of 4 on each P-lead) There will be a terminal on the wire, and one on the shielding of the wire.
On the engine side, the wire will be connected to the stud on the mag, and the shielding will be connected to the ground screw (assuming that you have Slick mags). The other end of the wire will go to either the left or right (look for the L or R) on the back of your switch. The GND terminal on the switch is to ground the switch for correct ignition operation. This is NOT the place to ground the shielding.
As Mel stated, most folks find an airframe ground for the shielding that is near the switch.

The "P" lead is connected across the points which are connected to the coil primary. There is voltage spike which occurs when the points open and generates the high voltage on the secondary winding through dI/dT and autotransformer action. That spike on the center conductor of the shielded "P" lead couples to the shield through capacitive coupling. That current on the shield needs to return to the magneto from whence it came. If you don't attach the shield to the switch ground terminal and you attach the ground terminal to the airframe, you make any induced current on the "P" lead return to the source, the magneto, through the airframe where it can radiate into a radio receiver. Always attach the ground terminal on the switch ONLY to the shield so that all of the current is returned to the magneto and none through the airframe.