Right Magneto issues on a 1967 M20F

[all4thekidz]

Details:  When the Aircraft is cranked it starts with no issues.  During the Mag check when placed to Left there is NO change in RPM or engine sound.  When I go to Right the aircraft DIES! The first time I did this at test RPM and did get a slight backfire but subsequent test have been at IDLE!

>>The Right Mag was removed thinking the points needed cleaning and to test the capacitor which tested out fine. 

>>The Key switch has been tested to the Magnetos and appears to function properly. 

>>Right Magneto was removed cleaned and tested and appears to be firing properly. 

>>When installed it was timed to 25 degrees and both mags time properly. 

>>I also have to lean aggressively to ensure smooth operation of what appears to be an engine operating on ONE Magneto! 

 

We are at a lose to why this is happening!  Any ideas we will accept.

VR

TODD

[ShuRugal]

Plugs and wires good? Condenser in the mag good? You can have good set of points, good timing, good coil, but still no spark if the condenser is bad. (I'm assuming aircraft mags have condensers, and don't just operate off of sheer "big stinking magnet goes brrr" to get a good spark)

Sent from my Pixel 3a using Tapatalk

[EricJ]

Check the p-lead from the ignition switch to the mag.   If it is shorted or the shield shorted to it, the mag won't fire.   Another, sketchy, way to test this is to disconnect the p-lead from the mag and then try a run-up.   If you can't turn the R mag OFF with it disconnected, and it won't run with it connected, then the problem is the p-lead.

The R mag will be hot as long as the p-lead is disconnected, so treat the prop accordingly.

 

Edited December 24, 2021 by EricJ

[Prior owner]

If you checked switch function, then it may be an intermittent bad P-lead- conductor wire is grounding out to the shield wire. 

[N9434V]

Make sure p-leads have not been reversed between mags.    If they have been reversed you may be chasing the problem on the wrong mag.  Just a thought 

[1964-M20E]

to check P leads with ohm meter.

1. disconnect cable to the starter
2. remove P leads from both mags
3. with switch in off position verify each p lead is grounded
4. with switch in both position verify each p lead is open, not grounded
5. with switch in left mode verify right p lead is grounded
6. with switch in right mode verify left p lead is gounded

if this checks out then the ignition switch and p leads are OK.

 

[Prior owner]

This is a pretty good article explaining what we’re talking about regarding P-lead issues with the Bendix mag on the right side in the Mooney:

 

Aged P-leads and spark issues

[Ragsf15e]

If it’s starting ok, the left mag is working fine.  They start on just the one mag.  If you try to start with a bad left mag, it can start when you let off the key (push/turn) and it goes back to both, but it’s usually rough and apparent.  I would definitely check out that right p lead.

[all4thekidz]

I believe this Mag was gone into during the complete rewire phase of this rebuild.  My thoughts are that the GEAR was installed 180 degrees out and although it test fine on the bench it is firing on the exhaust stroke therefore not resulting in a spark while fuel is in the cylinders.  I am at a loss and reaching for straws any thoughts?  I am going into the the right mag tomorrow!

Thanks

TODD

[Andy95W]

Occam’s Razor:  the simplest explanation is usually correct.

Everyone keeps trying to steer you toward the right magneto p-lead, which is a simple explanation and common point of failure.  But instead, you’re going to tear into the magneto.  Good luck.

[all4thekidz]

I have tested the right P Lead several times and every way and it TESTS fine.  I see no need to replace or repair what test out with no issues.  Should I rewire the Right p lead again?  Is there another way to test the P lead?  I have followed every recommendation on how to test the P lead and every way appears to function normally.

 

VR

TODD

[carusoam]

56 minutes ago, all4thekidz said:

I believe this Mag was gone into during the complete rewire phase of this rebuild.  My thoughts are that the GEAR was installed 180 degrees out and although it test fine on the bench it is firing on the exhaust stroke therefore not resulting in a spark while fuel is in the cylinders.  I am at a loss and reaching for straws any thoughts?  I am going into the the right mag tomorrow!

Thanks

TODD

wait a sec….

When was the last time this mag was known to run?

It looks like something important may not have been stated clearly…
 

Todd,

Start at the top, and read through again…

Or, start at the bottom and work your way up… (shortest way to the answer…)

 

Then, enjoy the video by the famous maintenance tech about how magnetos work… know that spark plugs fire twice… called the wasted spark…

Try to avoid re-designing mags, rebuilding the mags, or swapping mags… until after the usual simple failures have been gone through…

 

In your first post you described the affects of a common broken ground wire…

 

Of course something could have been cross wired R & L…

Or confused which mag is R or L…

Testing the wire is nice… but testing that it grounds the mag out is what you want… see how well it is attached at both ends… cleaned and solid connection… (how many connections are there?)

Stay focussed… it’s a very simple system… and when a P-lead goes bad, it can be a very dangerous system… (live mag, able to start without using the key and starter…

+1 for Occam and his fancy razor…

+1 for explaining this failure to your mechanic as you have above…

+1 for the Left mag being the one that adjusts timing for start…

+1 If the engine starts easily, the Left mag is the one working…

+1 if the engine croaks when selecting R mag only… Rmag isn’t working….

+1 for following how the Rmag gets wired all the way from start to finish… which includes the ignition switch…

+1 if you don’t see two sparks… one 180° out of phase (wasted spark)… and the one you expect to see…  when was this magneto last known to run?

+1 if you have an engine monitor… a sure visible sign of what is working and what isn’t…   
 

+1 for it is or isn’t working… “appears to be” means you are unsure if it is… and your mechanic will send you back to find out if it is or isn’t working…   

Its really a complex simple failure…

Unless this mag is a complete unknown… and it hasn’t been proven to run in a long time….(?)

-a-

[carusoam]

This is where things get complex…

1) “>Right Magneto was removed cleaned and tested and appears to be firing properly. ”

2) “I believe this Mag was gone into during the complete rewire phase of this rebuild.  My thoughts are that the GEAR was installed 180degrees out and although it test fine on the bench it is firing on the exhaust stroke therefore not resulting in a spark while fuel is in the cylinders.”

appears and I believe…. Indicate you are unsure of the status of this mag…

You sound positive that everything is wired properly…

but no spark occurs when it is supposed to be there…

You may be distracted by the wasted spark, that is normal…

 

Tell us about the mag?

How old…

How many hours on it…?

Any OH history?

By who, when?

 

Sounds a lot like…

1) the mag isn’t working properly at its core… or…

2) you accidentally checked the p-lead on the left mag and found it good, but missed the Rmag…

PP thoughts only, not a mechanic…

Supply some more detail, and the fine gents above can adjust their input….

and check the other mag… not the one you proved is wired correctly… (check both in case labels or wires have been swappped) easy to do at the back of the ignition switch…

Your mechanic should be able to see both sparks… if you only get one… seek a mechanic with more mag experience…

It is not normal to swap things around inside a mag to get them to work…

There are procedures and manuals and people with the proper skills to get this done the way you expect it to be done…

Many people send their mags out every 500 hours for reliability… don’t be surprised if this mag is ready for OH…

Best regards,

 

-a-

[N201MKTurbo]

You can do a quick and dirty test of a mag by putting a socket on the drive gear nut and spinning it with a drill driver. You will see the sparks jump from distributor terminals to the frame. 
 

If you removed the mag, you may have installed it 180 degrees out of phase. You can check this by removing all the spark plug leads, the top plugs and connecting the #1 top plug lead to a plug and lay it on top of the cylinders. Pull the prop through the timing mark and the plug should fire. If it is a SOS, you will need someone to turn on the SOS. If it fires the #3 instead of #1 you installed the mag out of phase. Do the same thing with the bottom plug lead. This will validate the mags, SOS and P leads are all working properly.

This can be a dangerous operation, so be careful. Be sure to remove all the top plugs and all the plug leads before doing this. The engine should not have any compression on any cylinder. If you don’t remove all the wires, the bottom plug can fire residual fuel in a cylinder. It will scare the hell out of you and everyone else at the airport. (Ask me how I know).

Edited December 26, 2021 by N201MKTurbo

[Shiny moose]

You installed the mag, timed the mag( without the p lead ) then you connect the P lead and mag doesnt work !!!! Look at the shielding on the p lead, it cannot be crimped to the p lead terminal end

[Andy95W]

12 hours ago, all4thekidz said:

I have tested the right P Lead several times and every way and it TESTS fine.  I see no need to replace or repair what test out with no issues.  Should I rewire the Right p lead again?  Is there another way to test the P lead?  I have followed every recommendation on how to test the P lead and every way appears to function normally.

 

VR

TODD

I feel your frustration, but this is the first time you’ve mentioned p-lead work.  A lot of us here are A&Ps, some of us are IAs, and we want to help you.  We just needed more information.

Rich- N201MKTurbo- is an A&P/IA who gave some great advice.

[Prior owner]

If you had the adapter for the right magneto that is used when timing the magneto, you could run a long wire to the cockpit from that adapter- ground the lead inside the cockpit for engine start, and then disconnect it from ground after the engine is started. Turn the ignition switch to “off” and see if the engine still runs. You have then ruled out the magneto, harness, plugs. All that remains is the switch, correct wiring, and the p-lead.

[Guest]

If you have Bendix magnetos with the tall cover with knurled nuts instead of short covers with a 10-32 stud, the internal spring on the capacitor and the P lead solder button can wear out causing weird ignition and running issues.  

Clarence
 

 

[Ragsf15e]

For us non a&p folks, to test something like this could you just disconnect the right P lead and run the engine?  Yes, I understand the prop is hot and the only way to shut off the engine is mixture (or fuel shutoff).  But that would allow it to run on both mags and on just the right (left p lead still connected there).  That would at least tell him if it really the right p lead or the right mag itself?

[EricJ]

5 minutes ago, Ragsf15e said:

For us non a&p folks, to test something like this could you just disconnect the right P lead and run the engine?  Yes, I understand the prop is hot and the only way to shut off the engine is mixture (or fuel shutoff).  But that would allow it to run on both mags and on just the right (left p lead still connected there).  That would at least tell him if it really the right p lead or the right mag itself?

Yes.   It might be a little tricky to start if the R mag works and tries to kick back.   Or like @PilotCoyote said, run a different lead through the window to ground it during start and then open it up to verify the mag works okay once it is running.    In any case, it provides a means to see whether the R mag is working properly or not.

There are some Bendix mags that have an internal safety that grounds it if the p-lead is disconnected, in which case you have to have the adapter, which many/most A&Ps have anyway to set timing.

 

Edited December 27, 2021 by EricJ

[Ragsf15e]

32 minutes ago, EricJ said:

Yes.   It might be a little tricky to start if the R mag works and tries to kick back.   Or like @PilotCoyote said, run a different lead through the window to ground it during start and then open it up to verify the mag works okay once it is running.    In any case, it provides a means to see whether the R mag is working properly or not.

There are some Bendix mags that have an internal safety that grounds it if the p-lead is disconnected, in which case you have to have the adapter, which many/most A&Ps have anyway to set timing.

 

Makes sense.  I thought it would still start ok on the left mag if you left that p lead connected?

[EricJ]

3 minutes ago, Ragsf15e said:

Makes sense.  I thought it would still start ok on the left mag if you left that p lead connected?

Yes, the left mag is the starting mag, but if the R mag is enabled during start the engine can kick back since the R mag timing is still way too advanced for starting.

[Ragsf15e]

11 minutes ago, EricJ said:

Yes, the left mag is the starting mag, but if the R mag is enabled during start the engine can kick back since the R mag timing is still way too advanced for starting.

Ahh, got it, thanks!

[all4thekidz]

We installed the P lead adapter for doing the timing.  This takes the ground OPTION out of the mix.  The engine starts fine with no kick back.  It runs really rich and after some leaning it runs fine indicating it is running on one Mag.  Because the P lead is disconnected when you go to RIGHT on the key switch it should continue to run, grounding out the left MAG.  In this case the engine dies indicating the Right Mag is not functioning correctly when installed on the Aircraft.  I am going to remove the Mag and conduct another bench test.  I appriciate all the comments and continue to work through this problem.  If the bench test is not good this time I will do a rebuild using a rebuild kit for both Mags.  Next challenge will be the installation of the FLAP/TRIM panel.

[carusoam]

Good Mag expertise is getting harder to find…

There may be a few recommendations around here.

Or you can ask…

People like to have their mags OH’d out of sync with each other… on 500hr service schedules…

This keeps OH challenges from occurring at the same time, and wear issues occurring at the same time later down the road…

The capacitors and plastic gears caused a few common challenges…

-a-