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mag not wired - PLead grounded?

P

prkaye

Well Known Member
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?
 
Don't be confused.

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.
 
I don't understand.
what you are seeing is a grounding through the points
Click to expand...
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.
 
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.
 
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)?
 
prkaye said:
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)?
Click to expand...
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.
 
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Mel said:
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.
Click to expand...

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.
 
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.
 
Mag Wiring

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
 
jetjok said:
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.
Click to expand...

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.
 
grounding to keyswitch

I need some help.

One lightspeed plasma 2, one slickmag run to the acs keyswitch.

Reading here, reading z11 notes from aeroelectric etc I am one again slightly confused about the shielding and grounding routes for this config.

Can someone clarify the shielding connections and the proper grounding connections for this type of config? Reading these older posts, seems I am not the only struggling with this.

THanks.
 
Rick,
I have the same setup but I can not remember the details. I will go to the hangar at lunch tomorrow and make a copy of my wire diagram for you.
 
The ACS keyswitch supplies a ground to either the Plasma II or the magneto or both when they are off. It is best if this ground comes from the magneto case, not the airframe, through a shielded wire grounded at the magneto. There is a coaxial ferrule available for the magneto end of the shielded wire to make this connection. The reason for doing this is to return the high voltage, high frequency, P-lead signal back to its origin at the magneto so as not to give rise to RFI and EMI. The Plasma ignitions have a protective circuit in the ON-OFF input that eliminates any possibility of damage from the P-lead signal or a difference in ground potential between the magneto case and the Plasma chassis.
 
Please clarify

Mel says:
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.
Click to expand...

Jetjok says:
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.
Click to expand...

So, on the switch side, does the shield ground to the airframe, to the ground position on the ACS switch, or somewhere else?

Also, if I am connecting the ACS ground post to the common ground block on the firewall, is that not the same as grounding that to the airframe?
 
Here's a wiring diagram from ACS for their key switch.

A-510-2-INSTALL-DIA-no-M.jpg


Aerolectric Connection also has great information on this.
 
Thanks, but

Thanks for the prompt reply. Why does Mel not recommend grounding the mag at the switch to the airframe? Or am I miss-reading his comment?

Edit - I read the Aeroelectric article at http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf, which was quite informative as usual. What I learned from this is that the shields from both P-Leads, at the switch end, are connected to the GRD terminal at the center of the switch. No other ground wire should be connected to this terminal. The shields at the magneto side are then grounded to the ground studs on the Slick magnetos. This however does not match the ACS wiring diagram, but it makes sense. It seems the goal is to keep the noise collected by the shielding transmitted back at the engine, and not into the common ground bus for all the other systems

Did I understand the article correctly?
 
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It's not necessarily "bad" to ground mags to the airframe. It's just not recommended to use the airframe as the "primary" ground.
The primary ground should be through the shield directly back to the ground terminal on the mag.
Grounding the mags to the airframe can conceivably cause ignition noise in the audio systems.
 
Which is the correct way? Both?

If I have interpreted the instructions correctly:

ACS switch instructions have you grounding the center ground lug (#5) on the switch to the airframe locally, with no P-lead shields connected at the switch end.

AeroElectric instructs to connect the P-lead shields to the ground lug (#5) and no local grounding.

Which is correct? Perhaps both? Advantages of either over the other?
 
mikehoover said:
If I have interpreted the instructions correctly:

ACS switch instructions have you grounding the center ground lug (#5) on the switch to the airframe locally, with no P-lead shields connected at the switch end.

AeroElectric instructs to connect the P-lead shields to the ground lug (#5) and no local grounding.

Which is correct? Perhaps both? Advantages of either over the other?
Click to expand...

Mike-I don't think there is an answer to this question that is going to put you at ease. Here are three threads I happened to have bookmarked from a long time ago. I found them (sort of) useful when struggling with this question. In the end, you will find that each side believes its way best. FWIW, I used Aeroelectric Bob's.
Hope this helps.

http://www.vansairforce.com/community/showthread.php?t=36272&highlight=mag+wiring
http://www.vansairforce.com/community/showthread.php?t=53009
http://www.vansairforce.com/community/showthread.php?t=36726&highlight=grounding+shield+magneto
 
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