flyboy1963

flyboy1963

Well Known Member
.......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
 
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.
 
Last edited:
slick mag ground

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!)
 
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!
 
Mel said:
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.
Click to expand...

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.
 
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
 
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?
 
Chino Tom said:
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?
Click to expand...
Yes, it is standard practice; at least for the 35+ years that I've been an A&P.
 
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/communi...4950&highlight=mag+ground+shield+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.
 
Last edited:
Standard practice with MAG p-leads is to ground both ends. Guarantee's that the mag will have a path to ground. All other shielded wiring only gets one end grounded.

I had a nice experience with an un-grounded mag a week or so ago on the racer... got my attention, as these things usually do.
 
flyboy1963 said:
Mel,

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!)
Click to expand...

Your Bendix grounds inside the case where the ferrell is inserted and the nut screwed on. You just can't see it without unscrewing the nut.

My Slicks, before I converted to Bendix were grounded with the shield on both sides, as Mel recommends.
 
One more question for Mel and/or Vic. Do you run a ground wire from the center ACS switch ground terminal to the airframe (per ACS drawing, which i have always done) or not per Nuckolls? Thanks again.
 
Chino Tom said:
One more question for Mel and/or Vic. Do you run a ground wire from the center ACS switch ground terminal to the airframe (per ACS drawing, which i have always done) or not per Nuckolls? Thanks again.
Click to expand...
I personally do not. But then again, I don't use key type ignition switches on aircraft. They fail! And because you typically kill the engine with the mixture, you never know when they fail, unless you routinely test the "OFF" position of the switch.
I use toggle switches. With toggle switches, you test the "off" position every time you do a mag check.
It shouldn't hurt anything to ground the terminal. It will give you another ground path for the mag. Grounding the terminal "could" introduce noise into your audio system, but probably not.
 
Thanks Mel. The noise/ground issue has always been a tad confusing. My
engine, battery and panel ground bus all go to the same bolt on the firewall.
I don't have any ignition noise in the radio. I do have noise when the strobes and landing light flashers (both local grounds) are on but only when I play XM or iPod music through the intercom. I had the same set up in my -6 with whelen strobes and no noise. The Aeroflash stobes in the -8A make noise. But again only when playing music.
 
grounds for confusion

ok,hahahahaaaaaaa,
didn't know I'd stir the pot so much!
........but after THIS thread, and all the previous ones about mags ...the big un-answered question is.....

HOW!...HOW,HOW....

..do you ground one or both ends of the ignition lead 'shield', harness, whatever one chooses to call it!?!?!?
( if in fact this 'shield' is what I'm thinking of.)

I thought the shielded cover became grounded when you screwed it to the mag or a spark plug.
What's all this talk about the ignition switch?
that's gotta be the P-lead, right?
 
We have actually been talking about two seperate items. the P-lead wire and the shielding around the p-lead wire. The p-lead wire connects the mag at the p-lead terminal to the mag switch at the appropriate switch terminal for that mag. Most of this discussion has been about how to ground the shielding around the p-lead wire. The wire used for the p-lead should be a shielded wire, in other words two wires in one. The center wire is surrounded by a insulation and then a braided wire surrounds it and then more insulation. The center wire is the p-lead and the shielding is used as a seperate ground to eliminate ignition noise. The shielding cannot touch the center p-lead wire or it will short the mag out.

My questions have concerned whether to ground the shielding at both ends of the wire. At the mag (as you asked in you original post) or at the switch. The knowlege base (that is what is so cool about this forum) suggests to do it at both ends. But part of the reason to do this is that the mag switch (or switches) if
it is not grounded at the switch (mine is, I have a ground wire from the center GRD post on the ACS to
my ground bus) needs the shield wire connected at both ends to enable the switch to operate properly
and ground out the mag when in the off position.
 
Last edited:
To answer your question, yes I run a wire from the ACS switch to ground. Mel is right in that the switches can fail. However, I don't think we actually see enough use in our airplanes for them to fail. I've been in electronics my whole life, and I have seen the toggle switches fail, too. You are actually checking the "off" function in both the keyed switch and the toggle switches every time you do a mag check, so unless there is a different practice that I am not aware of, the toggles don't get used any more than the keyed switch to verify they are in fact working. That's presuming you shut down the engine with the mixture (not like on Rotax 912's which use the toggles or a keyed switch to shut down, as there is no mixture control).
I still use the keyed switch, as there are too many poking hands into our cockpits at fly ins for me to get comfortable with the toggles. I like having the key in my pocket. :)
On a good safety note, everyone should make it a practice to check the grounding of the mags at the flyins prior to shutdown. Ever watch how many people are adjusting their props to hold the EAA folder on the prop? I had one light off on me one day, and I will never forget it as long as I live. I am very fortunate to still be here.

Vic
 
vic syracuse said:
You are actually checking the "off" function in both the keyed switch and the toggle switches every time you do a mag check, so unless there is a different practice that I am not aware of, the toggles don't get used any more than the keyed switch to verify they are in fact working.
Click to expand...

Vic, The "OFF" position is not checked when you do a mag check with a key switch. You go to "left" and/or "right", but not to "off". I have seen more than a few key switches fail in the "off" position and still work normally in the "left" and "right" positions. I still recommend that the "off" position be checked periodically to be sure.
 
I agree, Mel. One difference I have is that I wire the Light Speed igintion to the key switch, and I can see that the off postion works each time as the power to the LS goes off (seen via the optional display).

Vic
 
vic syracuse said:
I agree, Mel. One difference I have is that I wire the Light Speed igintion to the key switch, and I can see that the off postion works each time as the power to the LS goes off (seen via the optional display).

Vic
Click to expand...

I understand that. I was speaking strictly from the "mag" standpoint. The only way of checking that the "off" position is truly grounding the mag is by actually killing the engine with the switch. You can't measure with an Ohmmeter because you will get continuity through the mag points.
 
You must log in or register to reply here.