Bizarre bendix magneto internal timing situation

today I was trouble shooting my bendix s4ln-20 right mag that was not working. I suspected a bad coil or bad capacitor but figured i would first take the mag apart and clean everything and then retest; I did this and still had a dead right mag. next up I decided to swap the capacitor and coil from my good left mag and install them into the right mag; this also left me with a dead right mag. so I decided to disassemble the right mag one last time and reinstall the original coil and capacitor and send the mag off to be inspected. i reassembled the right mag with the original capacitor and coil and decided to give it one more go out of frustration. low and behold i now had a working right mag and a working left mag. i was so happy until i went to sit down at my work table where i had done all the disassembly today on the mags and noticed i forgot to install the carbon brush onto the right mag that goes on the distributor gear; this carbon brush touches the high terminal on the coil. this got me thinking how the mag could possibly operate without this carbon brush in place. i reinstalled the carbon brush on the right mag and low and behold the mag is totally dead again. i have been able to replicate this twice today. any ideas what is wrong with my carbon brush that could cause this. sorry in advance, my shift key and cap lock keys are dead on my computer; hence the lack of proper punctuation.
I've attached the manual for this mag for anyone interested in seeing the part in question. It is part# 72. 
http://hhh.gavilan.edu/hspenner/Main...-20Magneto.pdf

thanks,
bill

I can't offer any suggestions at all, but I will say that you are a brave man, both to pull your mags down and work on them and even more so to pull parts out of your left mag to try to fix your right mag. I can't imagine how many hours you have spent doing all of that testing. I run a shop, so I would much sooner send the bad mag out for a service/overhaul rather than spend all of that time testing. My hat's off to you.

Define working

Bill, out of curiosity, can you define working? Do you mean that you were cranking the engine and the mag does not fire, or you were unable to TIME the mag?

vic

I'm not at all a mag expert so feel free to scoff at this suggestion.

It sounds a bit like there's a short involving that one brush. Somehow, the installation of it shorts the mag out maybe at the p-lead. Or maybe somewhere else.

The good news is you get to take it apart again, very carefully, looking intently at everything.

Dave

Check the black insulators in very good light with good vision. If there are any little carbon tracks on the surface that is your energy dissipation path.

Are you going to repair or send for service?

vic syracuse said:

Bill, out of curiosity, can you define working? Do you mean that you were cranking the engine and the mag does not fire, or you were unable to TIME the mag?

vic

Click to expand...

Hi Vic the right mag worked during idle. I could turn from both to right mag and see a rpm drop. Now as soon as the carbon brush is installed and I do a mag check from both to right mag the engine quits. Im at a loss at this point. I think I'm going to tear the right mag apart and look very closely at the carbon brush with 10x magnifying. Perhaps the carbon brush has a crack which is causing something funky to transpire.

Bill

BillL said:

Check the black insulators in very good light with good vision. If there are any little carbon tracks on the surface that is your energy dissipation path.

Are you going to repair or send for service?

Click to expand...

Hi Bill Im hoping to service them myself. I originally cleaned those insulators when I first took the mag apart. There was a little carbon but nothing that I would expect to prevent spark. I scraped it off carefully. The mag will work without the carbon brush on the underside of the distributor gear. This is what I don't understand. Why this is causing a direct short?

Thanks
Bill

David Paule said:

I'm not at all a mag expert so feel free to scoff at this suggestion.

It sounds a bit like there's a short involving that one brush. Somehow, the installation of it shorts the mag out maybe at the p-lead. Or maybe somewhere else.

The good news is you get to take it apart again, very carefully, looking intently at everything.

Dave

Click to expand...

Thanks Dave for your thoughts. I think you're exactly right on the direct short, million dollar question is why. I've attached the manual for this mag for anyone interested in seeing the part in question. It is part# 72.
http://hhh.gavilan.edu/hspenner/Main/Manuals/BendixS-20Magneto.pdf

Thanks
Bill

Jesse said:

I can't offer any suggestions at all, but I will say that you are a brave man, both to pull your mags down and work on them and even more so to pull parts out of your left mag to try to fix your right mag. I can't imagine how many hours you have spent doing all of that testing. I run a shop, so I would much sooner send the bad mag out for a service/overhaul rather than spend all of that time testing. My hat's off to you.

Click to expand...

Hi. Jesse I really enjoy understanding how things work, but more so too stubborn to loose this challenge. I definately have a new found understanding of how these bendix mags are assembled. You were actually the guy I was thinking about calling when I decided to box the màgs up and send them out for inspection. Decided I would try one last time and atleast now I know it's the carbon brush causing this, but can't figure out why? Who do you send your mags out to when needed?



Thanks
Bill

Mags

I made a simple mount out of some spare steel scrap. Mounted the mag on the workbench. Reversible drill motor connected to drive end with hoes and hose clamps. Spare harness with ends held close to a grounding point. The mag should be turned counterclockwise in your case.
Now you can test the mags with ease without installing them on the airplane.
OH manual cautions not to test the mags with the oil seal installed, but I think a squirt of oil on the seal is good for a short test.
Check the contact on the coil, make sure it is clean.
I made a set of tools to overhaul these mags. Time consuming but I wanted to educate myself on these mags.

jrs14855 said:

I made a simple mount out of some spare steel scrap. Mounted the mag on the workbench. Reversible drill motor connected to drive end with hoes and hose clamps. Spare harness with ends held close to a grounding point. The mag should be turned counterclockwise in your case.
Now you can test the mags with ease without installing them on the airplane.
OH manual cautions not to test the mags with the oil seal installed, but I think a squirt of oil on the seal is good for a short test.
Check the contact on the coil, make sure it is clean.
I made a set of tools to overhaul these mags. Time consuming but I wanted to educate myself on these mags.

Click to expand...

I like your solution to use a drill with a rubber hose and clamp to spin the mags for testing. . I'll have to give that a go.

Thanks
Bill

William said:

Hi Bill Im hoping to service them myself. I originally cleaned those insulators when I first took the mag apart. There was a little carbon but nothing that I would expect to prevent spark. I scraped it off carefully. The mag will work without the carbon brush on the underside of the distributor gear. This is what I don't understand. Why this is causing a direct short?

Thanks
Bill

Click to expand...

IF the mag is sparking without the carbon conductor in it, then the electrical parts are functioning. A tiny carbon track on the insulator will create a short to ground, or dissipate the energy to another location. I have seen hundreds of these in automotive, and was never able to remove them and have it work. Cleaning, sand, grinding, none would render the part usable. I suggest that you might check rotating insulator and the cover where the plug wires attach.Take the ones that work and check on the good mag against the ones that don't. This will confirm the working of the mag and failure of the insulators. A mag voltage is higher with a larger gap to ground. The missing carbon button may be allowing it to reach a higher potential due to the missing conductor, and allowing it to fire.

Do you have an overhaul manual as your guide?