Right mag quit working

[N201MKTurbo]

I took off Thursday for a short business trip. The run up was just fine. Just as I rotated the engine burped. I was too far along to abort so I continued. I flew a few miles south of the airport and did a mag check. The engine stopped when I switched to the right mag so I returned to the airport. Called my customer and apologized for being late and started driving. 

This has happened a few times before. It has always been a points issue and I assumed it was this time too. I left work a little early on Friday and went over to Frank's and bought a couple of mag gaskets. I had some friends who wanted to go to Lake Havasu yesterday so I got out to the airport early to fix the mag. After pulling the cowl the first thing I did was pull the top plugs, put on the mag tester and check the points. The timing was off about a degree and a half, but both points were working. Not what I was expecting. I figured they might be on the edge of failure, so I pulled the mag. 

Both points looked like I would expect for the time in service. The internal timing was off by the same degree and a half I found the engine timing off, which is expected. I reset the point gap to get the correct internal timing and restore the point gap for the warn points. The mag tester showed both sides working just fine. 

I put the mag back on the engine and fired it up. It started right up and ran smooth I switched to the left mag and smooth as silk, I switched to the right mag and dead engine . So, what else could it be? I ohmed out the P leads and they were correct. Well, maybe it is the condensers. I swapped the two condensers but the failure stayed on the right mag. Oh crap, I'm running out of options. Maybe it is something in the P leads. I put in my test P leads and had a friend start the engine. With the engine running I shorted the right mag to ground, nothing happened, I shorted the left mag to ground and the engine quit. 

So, what do I know? The P leads and switch are good, the points and coil primaries are good or the mag timer wouldn't work. The condensers are good. The only thing left is the coil secondary windings.

I don't have the tools to repair this mag myself so I will have to send it off to Texas. This sucks because I have a couple of trips planned in the next few weeks including a vacation at the end of the month. I hope I get it back in time.

[Dream to fly]

How did you test the P leads?  Did you ohm the secondary coil?  My guess is the P lead is grounding in the right mag when under load.  You can test the P lead all day but when under load it may be grounding.  The wire itself might be fine but internal to the mag the P lead has lost its insulating properties.  Just a guess.  The other scenario is the secondary coil is open but not likely.  

[N201MKTurbo]

2 minutes ago, Dream to fly said:

How did you test the P leads?  Did you ohm the secondary coil?  My guess is the P lead is grounding in the right mag when under load.  You can test the P lead all day but when under load it may be grounding.  The wire itself might be fine but internal to the mag the P lead has lost its insulating properties.  Just a guess.  The other scenario is the secondary coil is open but not likely.  

Read the fourth paragraph.

[N201MKTurbo]

I would have to disassemble the mag to ohm the secondary.

[Dream to fly]

Just now, N201MKTurbo said:

Read the fourth paragraph.

Right nothing happened.  which means you are not making contact in the mag.  you used "test" P leads...   Is it possible that the test leads didn't make contact.  I am guessing the engine continued to run when you grounded your test P leads on the right mag...

[N201MKTurbo]

4 minutes ago, Dream to fly said:

Right nothing happened.  which means you are not making contact in the mag.  you used "test" P leads...   Is it possible that the test leads didn't make contact.  I am guessing the engine continued to run when you grounded your test P leads on the right mag...

Yes.

[Dream to fly]

If the engine runs smoothly with the test P leads the mag is working.  You install a test lead and don't get any response but the engine is running you didn't make a good connection.  You have the planes P lead connected and it grounds.  The problem is in the Planes P lead it is grounding and ohm test won't prove that.  it only proves an open.  Do an ohm test from the plane P lead to engine block key in the run position.  You should not have continuity.

[N201MKTurbo]

Ok, here's how it works.

P leads can only turn a mag off, they are not required to make a mag run. They turn off the mag by shorting the primary coil to ground. Some mags have a shorting bar that shorts the mag if the P leads are not installed.

What I did was put in test P leads instead of the ones from the plane. With them open and hanging in the air the mags will operate. Shorting the test P leads to ground will turn off the mag.

I started the engine. I can't tell if it is running on one mag or both. It is in fact running on only the left mag. When I short the right P lead to ground nothing happens because the right mag is already dead. When I short the left P lead to ground the engine quits because the right mag is not working.

[Dream to fly]

9 minutes ago, N201MKTurbo said:

 

I started the engine. I can't tell if it is running on one mag or both. It is in fact running on only the left mag. When I short the right P lead to ground nothing happens because the right mag is already dead. When I short the left P lead to ground the engine quits because the right mag is not working.

I was reading it as you said the engine runs smooth.  My plane when a mag is out there is a noticeable engine performance loss. 

[Hank]

1 hour ago, Dream to fly said:

I was reading it as you said the engine runs smooth.  My plane when a mag is out there is a noticeable engine performance loss. 

My engine continued to run smoothly when the left mag died at 9500 msl. Performamce suffered only a little, with the key on Both or Right; it shut down when turned to Left, and I completed the flight on Right. It was very noticeable on the ground doing a runup afterwards, and would not start until after I let go of the key. 

[Dream to fly]

3 minutes ago, Hank said:

My engine continued to run smoothly when the left mag died at 9500 msl. Performamce suffered only a little, with the key on Both or Right; it shut down when turned to Left, and I completed the flight on Right. It was very noticeable on the ground doing a runup afterwards, and would not start until after I let go of the key. 

Agreed that is why when I read it ran smooth I was assuming it had secondary output and answered accordingly.   Turns out secondary output was not verified and I overstepped assuming. 

[N201MKTurbo]

One thing that I forgot to post in the title that some might have missed. This is a D4LN-3000 dual mag. It is a real PITA to work with. The condensers are in the harness, the mag is easy enough to get on and off but the harness cap is a pain. Very little room to work, getting the harness cap out of the way is difficult and in my installation there are extra turbo oil lines just to make it more fun. Not one of Lycoming's better ideas.

[Andy95W]

IIRC, the dual mag only came with the knurled knob type p-leads that insert into the case of the mag.  

In that case, since your aircraft p-lead and also your test p-lead don't work, I'd check where the p-lead screws into the case.  That will ensure that something isn't in there shorting the contact or the p-lead to the case.

[N201MKTurbo]

59 minutes ago, Andy95W said:

IIRC, the dual mag only came with the knurled knob type p-leads that insert into the case of the mag.  

In that case, since your aircraft p-lead and also your test p-lead don't work, I'd check where the p-lead screws into the case.  That will ensure that something isn't in there shorting the contact or the p-lead to the case.

I don't get your logic. All my P leads work fine. I was just using the test P leads to confirm that there wasn't anything funky going on with the ships P leads. It is the right mag that doesn't work.

[bradp]

Is Andy suggesting a shorted path to ground at the cap/insert of the p lead to the right mag that’s keeping it dead? 

Sounds like if you’ve ruled out ignition switch, p leads, and have worked through the field serviceable items  you’re at send it time.  

[Andy95W]

1 hour ago, bradp said:

Is Andy suggesting a shorted path to ground at the cap/insert of the p lead to the right mag that’s keeping it dead? 

Sounds like if you’ve ruled out ignition switch, p leads, and have worked through the field serviceable items  you’re at send it time.  

Yes

[Yetti]

seems like you got everything.  like a misaligned safety bar or a strand of wire shorted to the case or a rubbed through wire.

[bradp]

Or something like the secondary has a broken wire and / or is shorting to itself. 

[RLCarter]

Not that this is your issue but last week while chasing down a 450rpm mag drop on a 172 (0-300 Conti) it was determined that somewhere over the past 50 years the "P" leads were reversed, so what started out as a left mag issue ended up as a cracked distributor block in the right mag. So this topic brings up my question, why do A&P's and IA's tend to shy away from things like Mags & Carbs? @N201MKTurbo seems like he would dig deeper into the mag if had the special tools to correctly do the job

[bradp]

Good point @RLCarter I also found that at my L and R mag leads had been reversed after I had them R&Rd at their last overhaul.  I now have them labeled left and right for future reference.  Mine were discovered pretty instantly because of hard starting on an impulse coupled setup so that the ICd mag was getting grounded out during starts- it would barely catch after the starter spooled up past normal starting RPM.  I don’t know much about turbo’s mags and am not qualified to comment but could presumably be a confounding factor in a retard breaker or starting vibrator setup depending on which mag does what during start.     His starts seem to be easy.  Worth noting for the rest of us - especially with impulse coupled mags that we normally start on one mag just fine. 

[Guest]

The early J models had the ignition switch wires the same way as late F models, in that the right P lead was grounded during start.  S/I M20-59 gave instructions for removing the grounding jumper at the switch, thereby allowing both mags to fire all spark plugs.  I’ve encountered several that were not modified.

Clarence

Edited April 9, 2018 by M20Doc

[bradp]

Clarence I’m going to check tomorrow but I bet mine has never been modified.  If I put the switch into the full right spring loaded position while then engine is running (careful to not push to engage the starter- my right mag grounds out ie I’m running one one only). 

This actually helps even if you don’t have two impulse couplings?  I guess theoretically it could result in too advance a spark for starting...? Mind explaining why they went back and forth on the original design.

Thanks professor.    

[N201MKTurbo]

On ‎4‎/‎9‎/‎2018 at 6:38 AM, M20Doc said:

The early J models had the ignition switch wires the same way as late F models, in that the right P lead was grounded during start.  S/I M20-59 gave instructions for removing the grounding jumper at the switch, thereby allowing both mags to fire all spark plugs.  I’ve encountered several that were not modified.

Clarence

I noticed that when I was troubleshooting the system. Not my problem but something I will keep in mind. 

I sent it off to Select yesterday. Hope to have it back next week.

Do you rebuild them yourself or send them out? 

[Guest]

41 minutes ago, N201MKTurbo said:

I noticed that when I was troubleshooting the system. Not my problem but something I will keep in mind. 

I sent it off to Select yesterday. Hope to have it back next week.

Do you rebuild them yourself or send them out? 

We tend to send them out for work.  Although I will do minor tweaking of internal timing if needed between 500 hour inspections.

Clarence

[N201MKTurbo]

BTW Aaron at Select said that you can ohm the secondary without taking it apart. If you measure from the case to the bearing in the center of the distributer it will be the secondary. Both of mine read 16.2 K which is correct. My worst nightmare is if the shop calls and says they bench checked the mag and it works fine.