Slick mag ground question

flyboy1963 · #1 06-08-2009, 11:24 AM

.......an easy one for some of you......
I've searched every thread I could think, and can't find this in any online manual.
just replaced Bendix with a Slick mag, and see there's a threaded hole in the top of the mag case labelled 'GND'.
What is this for????? Do I need to connect it to.......?
(....Haven't managed to connect with my mechanic, who timed the mag, )

thanks

Mel · #2 06-08-2009, 11:29 AM

This is the "ground" that "kills" the magneto by connecting it to the "P" lead by way of the mag switch. Connect the shield of your mag harness to this point and the other end of the shield to the ground terminal of your mag switch at the panel. This way when your switch connects the "P" lead to the shield, the mag will be off.
Without this connection, the mag may or may not be grounded through the engine, but you shouldn't depend on it.

flyboy1963 · #3 06-08-2009, 12:09 PM

Mel,
wow, great fast response.........
it looks like I'm out of my knowledge zone though. Much as I need to learn all this, I'll get the mechanic to look at it. ..or check with the AME /builder...who obviously knows a ton about wiring .

I read thru a dozen posts about this, and I'll I don't see any obvious way of connecting the 'shield' of the harness to ground.

My Bendix right mag has nothing but the P-lead to the ignition, so you can appreciate my confusion.
( ignition shuts off engine as expected.)

thanks again Mel,
(can't wait to install my Electroair in place of the right mag!)

Ironflight · #4 06-08-2009, 12:24 PM

One note of caution Perry (if you don't already know) - if the Mag is not fully/properly connected, it could very well be "hot", so stay clear of the prop!

Chino Tom · #5 06-08-2009, 06:13 PM

Quote:

Originally Posted by Mel

This is the "ground" that "kills" the magneto. Connect the shield of your mag harness to this point and the other end of the shield to the ground terminal of your mag switch at the panel.

I was under the assumption that you only grounded the shield at one end,
either the mag or the switch and is the way I did it on my -6 and now my
-8A. I have the shield grounded to the switch terminal ground. I would be
interested in your opinion (or anyone elses) and why I'm in error. The truth
being I don't remember why I was told to do it that way. Thanks in advance.

vic syracuse · #6 06-08-2009, 07:03 PM

Tom, you are right for most electrical wiring. For the mags, I've always grounded them at both ends with no problems. The GND on the mag is not exactly the "kill" terminal. That terminal usually has a 10-32 nut on it, and a fiber washer to insulate it from the case ground. It is that wire that goes back to your switch, and when connected to ground through the switch it effectively shorts the mag. Hence, no spark. A break in this wire will leave the mag "hot." A break in the shield wire (gnd) may or may not cause radio noise, but it will not affect the operation of the mag.

Vic

Chino Tom · #7 06-08-2009, 08:02 PM

Vic,

Thanks for the response. The original poster was referring to the case
ground as you pointed out, not the P-lead itself. I'm sure that was what
Mel was posting about. I've never had any radio noise associated with
the shield (around the p-lead) not being grounded at both ends. This thread
was timely as I'm going to install a LSI Plasma II + in place of the right mag
this weekend. I had called Klaus with a ground question concerning the
wire to my mag/starter switch and he was miffed that I hadn't grounded the
shield (around the p-lead) at both ends. Is it standard practice to do so?

Mel · #8 06-08-2009, 08:06 PM

Quote:

Originally Posted by Chino Tom

Vic,

Thanks for the response. The original poster was referring to the case
ground as you pointed out, not the P-lead itself. I'm sure that was what
Mel was posting about. I've never had any radio noise associated with
the shield (around the p-lead) not being grounded at both ends. This thread
was timely as I'm going to install a LSI Plasma II + in place of the right mag
this weekend. I had called Klaus with a ground question concerning the
wire to my mag/starter switch and he was miffed that I hadn't grounded the
shield (around the p-lead) at both ends. Is it standard practice to do so?

Yes, it is standard practice; at least for the 35+ years that I've been an A&P.

Chino Tom · #9 06-08-2009, 09:01 PM

Thanks....I'll change mine.

alpinelakespilot2000 · #10 06-08-2009, 09:50 PM

Isn't this the big debate we had a year or so ago? Those who recommend grounding the shield at both ends (like Bob Nuckoll's) and the other "standard" practice of only grounding the shield at the mag end (Vans OP-10 drawing)? Or am I misinterpreting something?

http://www.vansairforce.com/communit...d+aeroelectric

I still don't entirely understand this. I wired mine per Nuckoll's (grounded at both ends--at mag and at ignition key switch) but my EAA tech advisor believes it is only proper (or "standard") to ground the shield at the mag end. Go figure.