Interesting Mag fail

[Plummit]

So a couple of months ago I taxied out for a flight and the Right mag was a dead soldier on run-up. The Mags had approximately 370 hours since their 500 hour inspection and service. Back to the hangar and I dropped the ignition switch as that was the easiest way to check the P leads on my Bendix 1200 series mags.

I found several of the wire terminals on the switch were loose. Don't know how or why but they were very loose. I tightened them up, congratulated myself on my find, and did another mag check. Everything checked out perfect.

A week later I flew up to Big Bear (L35) for a bite to eat. Upon leaving, the right Mag was again dead. It was a Saturday so I bought a new switch from Spruce and installed it. Another Mag check and everything was OK.

The following week I flew to Hemet Ryan (KHMT) and upon leaving, the right Mag was again dead. I checked the P leads and figured out that the wires were reversed on the switch! I had put everything back the way it came off when I replaced the switch so I know it was that way since I purchased the plane.

To top it off, I have been starting the engine on the right Mag which doesn't have an impulse coupling! (Yes I occasionally got kick-back) I finally pulled the left mag and had it overhauled. It seems that when you have a "500 hour inspection" done they only replace the points but not the condenser. The overhaul fixed the problem, and my best guess is that the 10 year old condenser was going "open" with heat. Something to watch out for when you have the 500 hour inspection done.

-Marc

 

[Desert Rat]

Thats an interesting set of circumstances.

I'm going to suggest that perhaps the loose terminals didn't have anything to do with your initial dead mag.

Key on or key off, conventional magnetos are making electricity any time they are rotating. The p-lead is a ground wire that allows electricity to take a path of least resistance back to ground without jumping the gap at the spark plug.

If the switch fails open, a terminal comes loose, or the P-lead wire breaks, that mag should be hot all the time i.e. you would see no mag drop on runup because you can't shut it off by grounding it through the P-lead and switch.

I dunno- I've only had one cup of coffee so far so maybe I'm not thinking about this the right way, but thats my initial thought.

 

[lr172]

Thats an interesting set of circumstances.

I'm going to suggest that perhaps the loose terminals didn't have anything to do with your initial dead mag.

Key on or key off, conventional magnetos are making electricity any time they are rotating. The p-lead is a ground wire that allows electricity to take a path of least resistance back to ground without jumping the gap at the spark plug.

If the switch fails open, a terminal comes loose, or the P-lead wire breaks, that mag should be hot all the time i.e. you would see no mag drop on runup because you can't shut it off by grounding it through the P-lead and switch.

I dunno- I've only had one cup of coffee so far so maybe I'm not thinking about this the right way, but thats my initial thought.

I agree. Also, the condenser would need to fail as a short, not an open to cause the mag to not fire.

 

[N804RV]

I don't know about Bendix mags. But, I'm pretty sure the 500hr inspection on Slick 4300 series mags includes replacing the condenser, as well as the points, carbon brush and gear.

 

[Plummit]

Thats an interesting set of circumstances.

I'm going to suggest that perhaps the loose terminals didn't have anything to do with your initial dead mag.

I agree. It was just the first thing I found when I started trouble shooting the problem.

-Marc

 

[Plummit]

I agree. Also, the condenser would need to fail as a short, not an open to cause the mag to not fire.

Correct. I imagined that it was a dead short to ground which would make it like the points weren't breaking.

When I used to do tune ups in cars we always replaced the points and condenser together, and maybe added a rotor and Dist cap. I couldn't believe that they didn't do that on a 500 hour inspection. BTW, that info came from Aero Accessories in Van Nuys. I've been using them for mag work for over a decade. I will for sure specify condenser replacement in the future!

-Marc

 

[rmarshall234]

I send all my magnetos to Aircraft Magneto Service in MT. It’s not inexpensive but worth it in my opinion, I don’t like problems with magnetos. When they come back they look and operate like new magnetos with a quart sized ziplock bag full of all the parts they replaced. Pretty much everything except the housing, magnet, and sometimes the coil. I often have informative and educational discussions with them during the process, those guys are phenomenal.

 

[lr172]

Correct. I imagined that it was a dead short to ground which would make it like the points weren't breaking.

When I used to do tune ups in cars we always replaced the points and condenser together, and maybe added a rotor and Dist cap. I couldn't believe that they didn't do that on a 500 hour inspection. BTW, that info came from Aero Accessories in Van Nuys. I've been using them for mag work for over a decade. I will for sure specify condenser replacement in the future!

-Marc

Generally speaking, the condensor is either shorted or isn't and not likely to fail intermittently. The general failure mode is for them to change capacitance over time. This diminishes their energy absorption capability and causes the points to arc and pit. My guess is something other than the condensor is causing the intermittent no spark. The consdensors we use are a bit different, as they have a center conductor to pass current and therefore more of a shorting possibility than most, however, a shorted center conductor can't cause a no spark if the P lead is not grounded. If it always works cold and always fails hot, then I would think that a condensor is a possible culprit. Given the proximity to recent service, my first guess would be that they assembled something incorrectly.

Larry