I am getting ready to build my two P-leads to my Bendix/TCM magnetos S4LN-1227 series on my IO-360 A1B6. I am having trouble figuring out where to ground the P-lead shielding in the engine room. My mags have a simple stud where the main P-lead wire is connected and not the more common P-lead cap. They are probably 10 year old or more magnetos that have never been used. I can't seem to find a location on the magneto itself to ground the shielding pig-tail. Do I make a longer shielding pig tail and ground to firewall? Or do I ground the shielding on one of the four ignition harness cap screws? Thanks in advance for any help I can get.
Van's Air Force
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P-Lead Shield Grounding
- Thread starter Boomer506
- Start date
You should ground the shield at the magneto body.
You can use the shield at the switch end for the ground that turns the magneto off. (most direct possible circuit)
If your magneto is 10 years old, perhaps you should have it checked by a mag shop. The grease in it may have dried out, and there might be parts inside that have Airworthiness Directives and should be replaced.
I had a 20 year old very low service hours mag that caused so much noise on my radio that it was auto squelching most of the time. I sent the mag to Magneto Services, and Dick found the coil (which has an A/D) had grounded, and the impluse coupling was also superseded.
I met Joe Loggie, the Slick Mag service guru at the Reno Air Races this year. Naturally we discussed his passion, and my Slick mags. He warned me about dried out grease from years of sitting, I think he said 10 years is about the max.
You can use the shield at the switch end for the ground that turns the magneto off. (most direct possible circuit)
If your magneto is 10 years old, perhaps you should have it checked by a mag shop. The grease in it may have dried out, and there might be parts inside that have Airworthiness Directives and should be replaced.
I had a 20 year old very low service hours mag that caused so much noise on my radio that it was auto squelching most of the time. I sent the mag to Magneto Services, and Dick found the coil (which has an A/D) had grounded, and the impluse coupling was also superseded.
I met Joe Loggie, the Slick Mag service guru at the Reno Air Races this year. Naturally we discussed his passion, and my Slick mags. He warned me about dried out grease from years of sitting, I think he said 10 years is about the max.
Good advice Scott, thanks. I'll have the mag internals checked out. I can't find a good location to ground the shielding at the magneto, so I think I will ground the shielding at the switch as you indicated in your second sentence. I think there may be a problem if you ground the shielding at both ends. I heard you could get ground loop noise, whatever that is. On the left mag, with the retard breaker, I will also have the jumper at the start switch to disable the left mag during cranking, but that is independent from the shield grounding question.
You misunderstood. Scott was talking about a standard arrangement in which one pole of the switch is connected to the core wire, and the other pole is connected to the shield. At the other end, the core is connected to the mag primary, while the shield is connected to the mag case or engine block. With the switch closed, the current path is then mag to switch, and back to ground at the mag or engine block.
Ah, interesting. I get it now. I think that would work well if you had individual SPST type switche for each magneto. What I have is a standard left/right/both/start type switch that you would find in a Cessna. In my case, I'm pretty sure you only ground the shielding at one end or the other and leave the ungrounded end open. I think in my case (left/right/both/start type switch) it looks like the consensus is to ground the shielding at the switch in the cockpit, and not at the magneto. Unless I am missing something.
Thanks Ray, yes I would prefer to ground at the magneto, I just can find a location on the magneto or the engine where I can do that. The only thing I can think of is maybe the on studs that are used to secure the ignition harness cap, but I wasn't sure if those were good grounds or not. Seems like they are isolated from the engine block by the mag gasket. The other location I was considering near the mags would be to make the shielding pigtail very long, and reach to either the firewall, or an oil pan bolt. But that just seems too long and fragile to me.
Raymo
Well Known Member
Sam,
A common way to ground a shield is to use a solder splice. You can get them from ACS and most people that are or have built have some laying around.
Here is a link to the ACS site.
You'd run a 18 or 20 gauge wire under the splice to solder it to the shielding.
A common way to ground a shield is to use a solder splice. You can get them from ACS and most people that are or have built have some laying around.
Here is a link to the ACS site.
You'd run a 18 or 20 gauge wire under the splice to solder it to the shielding.
rv7charlie
Well Known Member
Sam,
The p lead shield is (should be) doing double duty.
It's helping keep mag noise contained while the mag is running, and that requires a good bond to the mag case. Grounding it elsewhere has the potential to drive noise into other components, since any noise riding in it will have to travel along the airframe back to the mag.
The 2nd job is to provide a grounding path to short out the mag's points when the ignition switch is 'off' (meaning the contacts are closed, in this case). In this case, if you ground the shield elsewhere, it can't do its job at all. It's been a while since I played with an actual ignition switch (I use toggles), but I'd check on whether the 'ground' terminal in the switch is actually grounded to anything. If you tie that terminal to local ground, you're depending on an uncounted number of junctions through the airframe to get the ground path back to the mag. That can be a safety issue.
To terminate the shield, you can use a solder splice, as others mentioned, or just solder a short pigtail of wire to the shield & cover with shrink tubing. Terminate the other end with a ring terminal to fit. The tubing just serves as a strain relief on either side of the actual joint; a cheap/quick way to make your own solder splice. Even a PIDG crimp terminal could be used, but they tend to be a lot bulkier and heavier than a simple soldered joint with heat shrink.
edit: If the harness cap screws are long enough to add a ring terminal under one, that will work fine as the ground point. Yes, there's a gasket under the mag, but the bolts that secure it to the engine also electrically bond it to the engine. Even if they didn't, your goal is to tie the shield back to the mag, anyway. The mag is a generator, and its case is its ground. If you don't complete the p lead circuit back to the mag's ground, you can't turn off the mag. Using the airframe as the path from p lead center wire back to the mag case, is, as I mentioned earlier, a needlessly more complicated, less reliable way to do it.
Charlie
The p lead shield is (should be) doing double duty.
It's helping keep mag noise contained while the mag is running, and that requires a good bond to the mag case. Grounding it elsewhere has the potential to drive noise into other components, since any noise riding in it will have to travel along the airframe back to the mag.
The 2nd job is to provide a grounding path to short out the mag's points when the ignition switch is 'off' (meaning the contacts are closed, in this case). In this case, if you ground the shield elsewhere, it can't do its job at all. It's been a while since I played with an actual ignition switch (I use toggles), but I'd check on whether the 'ground' terminal in the switch is actually grounded to anything. If you tie that terminal to local ground, you're depending on an uncounted number of junctions through the airframe to get the ground path back to the mag. That can be a safety issue.
To terminate the shield, you can use a solder splice, as others mentioned, or just solder a short pigtail of wire to the shield & cover with shrink tubing. Terminate the other end with a ring terminal to fit. The tubing just serves as a strain relief on either side of the actual joint; a cheap/quick way to make your own solder splice. Even a PIDG crimp terminal could be used, but they tend to be a lot bulkier and heavier than a simple soldered joint with heat shrink.
edit: If the harness cap screws are long enough to add a ring terminal under one, that will work fine as the ground point. Yes, there's a gasket under the mag, but the bolts that secure it to the engine also electrically bond it to the engine. Even if they didn't, your goal is to tie the shield back to the mag, anyway. The mag is a generator, and its case is its ground. If you don't complete the p lead circuit back to the mag's ground, you can't turn off the mag. Using the airframe as the path from p lead center wire back to the mag case, is, as I mentioned earlier, a needlessly more complicated, less reliable way to do it.
Charlie
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Man, that is a great explanation Charlie, thanks. I will do exactly that, ground my shielding on one of the studs that hold my ignition harness cap to the magneto.
I have been testing my left/right/both/start key switch though, and if I simply connect a single unshielded wire from a mag to the corresponding mag terminal on my key switch, when I am in either the 'both' position, or the position corresponding to that particular mag (left or right), my key switch will un-ground that mag and make it hot. The system would work and the engine would run just fine using single wire unshielded wire, you would just have a ton of noise.
The left/right/both/start key switch also has a terminal that you connect to airframe ground, and that is where your unshielded wire would ground to internally in the key switch. No shielding required.
But I did want to minimize noise with shielding that is properly utilized, and your explanation is perfect. Thank you for your help.
I have been testing my left/right/both/start key switch though, and if I simply connect a single unshielded wire from a mag to the corresponding mag terminal on my key switch, when I am in either the 'both' position, or the position corresponding to that particular mag (left or right), my key switch will un-ground that mag and make it hot. The system would work and the engine would run just fine using single wire unshielded wire, you would just have a ton of noise.
The left/right/both/start key switch also has a terminal that you connect to airframe ground, and that is where your unshielded wire would ground to internally in the key switch. No shielding required.
But I did want to minimize noise with shielding that is properly utilized, and your explanation is perfect. Thank you for your help.
rv7charlie
Well Known Member
Glad it helped.
You might want to poll the collective wisdom of those actually using ignition switches, on whether they are grounding the switch locally. If it were mine, I wouldn't, because it gives multiple ground paths to the 'signal', meaning more of a chance to inject the mag noise into other electronics in the plane. Of course, the upside is a redundant ground path if the shield lead breaks on the mag end....
You might want to poll the collective wisdom of those actually using ignition switches, on whether they are grounding the switch locally. If it were mine, I wouldn't, because it gives multiple ground paths to the 'signal', meaning more of a chance to inject the mag noise into other electronics in the plane. Of course, the upside is a redundant ground path if the shield lead breaks on the mag end....
Yes, I would like to hear from others using the left/right/both/start key switch. But it seems to me that when the engine is running, and the switch is in the "both" position, the main wires for both magnetos are disconnected/isolated from the switch ground terminal, which is making them hot. When running, the only part of the p-lead that is connected to anything is the shielding, which remains connected to ground, in my case, through the cap stud on the magneto ignition harness cap, which would be shielding the rest of the avionics from unwanted noise.
After shutdown, and when the key ignition switch goes to the off position, the connection between main p-lead wire and ground terminal is restored internal to the key switch, the magneto is then grounded, and becomes cold/safe. At least that is my current understanding.
After shutdown, and when the key ignition switch goes to the off position, the connection between main p-lead wire and ground terminal is restored internal to the key switch, the magneto is then grounded, and becomes cold/safe. At least that is my current understanding.
rv7charlie
Well Known Member
Just to be sure we're on the same page, in your previous message (#11), it sounded like you're going to ground the shield at the mag, but attach the switch ground terminal to a local airframe ground. When I read it, I assumed that you meant you would tie the shield to both the mag and the ground side of the switch (meaning the shield would be grounded at both ends). If that's done, there is some risk (unquantified level) of feeding noise into the airframe.
If your plan is to ground the shield *only* to the mag, leaving the other end 'floating', then that would be the way to achieve minimal noise during normal operation. (And it's electrically identical to what you'd have during operation, if you tied the shield to the switch's 'ground' terminal.) But with the switch end of the shield 'floating', you're still dependent on all the various connections through the airframe to supply the shutdown path back to the mag. If any joint in the path gets loose, corroded etc, you could lose that shutdown path. Now the odds of that happening are pretty low. But let's play hypotheticals. Say you're doing some maintenance on the plane, and you need to disconnect the battery. Did you run the battery ground cable to a lug on the airframe, and a ground cable from the engine to the same airframe lug, or did you run the engine ground straight to the battery? If it's straight to the battery, and you remove the battery, your mags are now hot. If you disconnect the engine ground cable for any reason, your mags are now hot.
See where this goes? An action totally unrelated to the mags can leave them hot. You might always remember, but if someone else has to work on the plane, will they know?
FWIW....And sorry for the long drawn out response.
Charlie
If your plan is to ground the shield *only* to the mag, leaving the other end 'floating', then that would be the way to achieve minimal noise during normal operation. (And it's electrically identical to what you'd have during operation, if you tied the shield to the switch's 'ground' terminal.) But with the switch end of the shield 'floating', you're still dependent on all the various connections through the airframe to supply the shutdown path back to the mag. If any joint in the path gets loose, corroded etc, you could lose that shutdown path. Now the odds of that happening are pretty low. But let's play hypotheticals. Say you're doing some maintenance on the plane, and you need to disconnect the battery. Did you run the battery ground cable to a lug on the airframe, and a ground cable from the engine to the same airframe lug, or did you run the engine ground straight to the battery? If it's straight to the battery, and you remove the battery, your mags are now hot. If you disconnect the engine ground cable for any reason, your mags are now hot.
See where this goes? An action totally unrelated to the mags can leave them hot. You might always remember, but if someone else has to work on the plane, will they know?
FWIW....And sorry for the long drawn out response.
Charlie
