Engine won't run on the Left Magneto

[Adverseyaw0317]

I have run out of ideas. The engine had begun to have large RPM drops during run ups on the ground on the Left magneto and running rough.

It starts perfectly fine with no issues.

Now after 3 months of troubleshooting it won't run at all on the left magneto. It does run fine on the right magneto only.

The problem does not change with the boost pump off or on or the selection of the left or right tank. (probably not a fuel related issue)

No induction leaks that we can find.

Sparks plugs have been checked and replaced, magneto had a new 500 hour inspection, ignition harness replaced, p leads checked and ignition switch checked.

I unfortunately do not have a Engine Monitor with downloadable data and therefore cannot post plots.

After much scouring of the internet I have run out of ideas.

Anyone have any other ideas? The entire ignition system has been looked over at this point.

[hammdo]

Are the exhaust/flameout tubes in good shape?

-Don

[Adverseyaw0317]

24 minutes ago, hammdo said:

Are the exhaust/flameout tubes in good shape?

-Don

Indeed they are.

[MikeOH]

Bad mag switch? (grounding left mag when it should not) Pull P-lead on left mag and see if problem goes away.

Edit: Do the rpms change when going from just right mag to both? If the left is dead you won't see any change.

[EricJ]

What does "won't run" mean?   It runs rough or stops completely or...?

It's gonna be ignition related, so despite your description that you've touched everything, it may need to be touched again.    Does it run if you disconnect the P-lead at the left mag?

[Adverseyaw0317]

11 minutes ago, EricJ said:

What does "won't run" mean?   It runs rough or stops completely or...?

It's gonna be ignition related, so despite your description that you've touched everything, it may need to be touched again.    Does it run if you disconnect the P-lead at the left mag?

It used to run rough now it completely stops. I'll try that next.

[EricJ]

2 minutes ago, Adverseyaw0317 said:

It used to run rough now it completely stops. I'll try that next.

Is the left mag the starting mag?   If it starts, and it's the starting mag then it's working at least during start.   If that's the case the impulse coupler may not be releasing after start.

[Adverseyaw0317]

19 minutes ago, MikeOH said:

Bad mag switch? (grounding left mag when it should not) Pull P-lead on left mag and see if problem goes away.

Edit: Do the rpms change when going from just right mag to both? If the left is dead you won't see any change.

I'll try that next, I havent tried that. I do know it runs rougher on both than on Right alone so I assume the left mag is doing something sometimes.

[Adverseyaw0317]

2 minutes ago, EricJ said:

Is the left mag the starting mag?   If it starts, and it's the starting mag then it's working at least during start.   If that's the case the impulse coupler may not be releasing after start.

I've been trying to figure that out. I thought for the IO-360A3B6D the left was the starting mag. I was hoping someone on here knew.

[EricJ]

44 minutes ago, Adverseyaw0317 said:

I've been trying to figure that out. I thought for the IO-360A3B6D the left was the starting mag. I was hoping someone on here knew.

Ah, sorry, didn't notice that you have a J model.    Never mind about the impulse coupler, it's common to both left and right on the dual mag.    The configuration of the ignition switch determines whether it starts on the left or on both with the dual mag.   I think most are configured for both, since that makes starting easier.

 

[skykrawler]

So are you saying your engine has the dual magneto?  (IO-360 A3B6D).     If so the single impulse coupling would work on both left/right.

Is there an assumption that the "500 hr inspected" magneto can't have a defect?   They're fairly simple to bench check the timing but performance of the coils is a different thing without equipment.   "Inspections" on the D mag often omit the condensers.

When the ignition harness was replaced (with the cover) were new condensers installed?   These are attached to the inside of the magneto cover and are what the P-leads screw into.   The two condensers are connected to the points.   If the condenser is bad (shorted) the mag won't work.   The resistance across the condenser can be checked as well as the capacitance.   Worst case would be to take off the cover and swap the condensers left/right.  If bad the right mag should now have the problem.

If you don't have the D mag, then this doesn't really apply.

[Jim F]

On your dual mag, do you have 2 P-leads on the left side(shower of sparks) and one on the right?  Or when you pull the prop through in the normal direction do you hear a loud clunk like a deadbolt lock(Impulse coupling)?  My guess is you have a shower of sparks and that kind of limits your troubleshooting options.  From your symptoms I would bet that your left starting points(shower of sparks) are working and your left mag points are not set correctly.  When you turn your key to energize your starter, it grounds both LH and RH points and only the LH start points operate.  When it kicks and you go to both then both the LH and RH points operate as normal.  This is so the normal points don't fire the spark plugs during start at 20 degrees BTDC and kick back. If you have a impulse coupling your left mag is most likely bad and your starting on your right. 

To prove the Mag/start switch is working correctly: Caution you are moving the prop with the mags HOT if there is fuel in the cylinder the prop can swing through and hurt you. 

1) You can test your ignition switch by putting a Mag timing buzz box on the P leads(if studs) mag switch to both, move the prop to get both buzz box lights to come one.  Then sit in the pilot seat, master off, wiggle the switch in the off position to see if the buzz box light change, then right and the right light should change, wiggle the key, then go to left, wiggle the key, move fully clock wise to the start circuit(mast off) and wiggle again.  If you have P-lead caps you can remove both normal caps and connect the buzz box clips to the p-leads and run the same test above.

 

[EricJ]

43 minutes ago, Jim F said:

On your dual mag, do you have 2 P-leads on the left side(shower of sparks) and one on the right?  Or when you pull the prop through in the normal direction do you hear a loud clunk like a deadbolt lock(Impulse coupling)?  My guess is you have a shower of sparks and that kind of limits your troubleshooting options.  From your symptoms I would bet that your left starting points(shower of sparks) are working and your left mag points are not set correctly.  When you turn your key to energize your starter, it grounds both LH and RH points and only the LH start points operate.  When it kicks and you go to both then both the LH and RH points operate as normal.  This is so the normal points don't fire the spark plugs during start at 20 degrees BTDC and kick back. If you have a impulse coupling your left mag is most likely bad and your starting on your right. 

To prove the Mag/start switch is working correctly: Caution you are moving the prop with the mags HOT if there is fuel in the cylinder the prop can swing through and hurt you. 

1) You can test your ignition switch by putting a Mag timing buzz box on the P leads(if studs) mag switch to both, move the prop to get both buzz box lights to come one.  Then sit in the pilot seat, master off, wiggle the switch in the off position to see if the buzz box light change, then right and the right light should change, wiggle the key, then go to left, wiggle the key, move fully clock wise to the start circuit(mast off) and wiggle again.  If you have P-lead caps you can remove both normal caps and connect the buzz box clips to the p-leads and run the same test above.

 

I don't think any dual-mag airplanes had a shower of sparks.   They all have impulse couplers.

[chriscalandro]

I’ve seen a number of mags come back from overhaul with the cover on the back grounding out the cap. 
 

Probably going to be the mag, but disconnect the PLead and try it to rule out the switch. 

[OR75]

The magneto needs to be removed , visually inspected or bench tested . Check the capacitors as well 

[PT20J]

2 hours ago, Adverseyaw0317 said:

I've been trying to figure that out. I thought for the IO-360A3B6D the left was the starting mag. I was hoping someone on here knew.

The dual mag has one impulse coupling for both mags so it can start on the left mag or both mags depending on how the ignition switch is wired. See Mooney SIM20-59A.

[MatthiasArnold]

I had a similar situation with one of my (not dual) magnetos:

App. 90hours after overhaul the magneto failed.
The root case was a completely worn "cam follower" of the contact assembly which was replaced by new.
I personally attended the follow up overhaul in the renowned repair shop (in Germany), where the magneto was completely disassembled, cleaned, capacitors replaced (although the passed the tests) and yet another new contact assembly utilized during assembly. The test run on the Magentor tester looked perfect.
After reinstalling the magnet on the A/C the freshly (double) overhauled magneto started performing a little bit strange again (no real rpm drop during mag check although the p-leads & ignitions switch are ok) after only 40h.
Talking to my AP and IA we choose another renowned repair shop (in Belgium this time) and I once again personally attended the overhaul number #3..
Once again the (Nylon) "cam follower" of the contact assembly was pretty much worn (thus the points were close to not opening again).
We checked every part and detected some very light corrosion on the magneto's cam shaft. After replacing the cam shaft with new and of course yet another new contact assembly the magneto finally seems to work as expected..

Long story short: many reasons for a magneto to fail - this is just one of them..
 

[Adverseyaw0317]

New capacitors/condensers
checked the p leads with a buzz box and all good.

Still no smoking gun.


Sent from my iPad using Tapatalk

[chriscalandro]

42 minutes ago, Adverseyaw0317 said:

New capacitors/condensers
checked the p leads with a buzz box and all good.

Still no smoking gun.


Sent from my iPad using Tapatalk

Have you tried any of the suggestions given to you here?

[Adverseyaw0317]

38 minutes ago, chriscalandro said:

Have you tried any of the suggestions given to you here?

Yup that’s what the above comments were in relation to. 

[N201MKTurbo]

My dual mag did that once. It turned out to be the distributor wheel. the plastic gear had rotated on the metal center. This put the distributor arm out of sync with the plug posts. You have to disassemble it a bit to see this. I can't remember if you can see the arms without taking off the distributor blocks or not. The overhauler said he had seen this quite a few times lately. 

You should also check the points. If you put a mag timer on it you should see activity on both mags as you rotate the engine. I assume you have troubleshoot the P-Leads already. Just checking the timing will tell the tail of the points.

[N201MKTurbo]

Just to get this straight, You have sent the mag out for inspection since this dead mag problem happened? If so ask the shop that inspected it if they ran it on a spark gap tester.

[MikeOH]

3 hours ago, Adverseyaw0317 said:

Yup that’s what the above comments were in relation to. 

@Adverseyaw0317

Just to confirm, you TRIED to RUN on the left mag with the left mag's P-lead DISCONNECTED? (not just a buzz-box check)

[PT20J]

If you’ve methodically eliminated everything else, it must be the mag. There are only a few shops that work on dual mags anymore. I would call Top Gun or Maxwell and ask where they send mags and use a shop they recommend.

[Adverseyaw0317]

2 hours ago, MikeOH said:

@Adverseyaw0317

Just to confirm, you TRIED to RUN on the left mag with the left mag's P-lead DISCONNECTED? (not just a buzz-box check)

That’s correct previous that worked. I plan on trying that again later this week.