Intermittently Failed Left Mag Check...Now It Won't Pass

[pilot_jb]

About to start really digging in...

Went through a period of time when I would Intermittently fail a left mag check.  The issue started happening progressively more often, so as the first step in diagnosing the problem, we swapped plugs from the right to the left.  The expected result was for the problem to move to the right if it was the plugs, and to stay on the left if it was the wires or magneto.  Oddly enough, the issue corrected itself.  Temporarily, and always on the ground, both mags passed with flying colors under warm and hot scenarios.  I let the plane cool and then the problem returned.  We then found the timing to be, at most, a couple degrees off.  After this was corrected, I once again could not get it to fail a check on either side.  I let the plane sit for a couple days, and now the issue is as bad as it's been...can't get it to pass on the left.  

We also rebuilt the P-leads, because we found the left side wiring to be old, brittle, and only connected by a few strands of wire.  

Both mags have 512 hours.

IO360-A1A

Bendix S4LN-200 shower of sparks

Left dropping between 200-250 RPM with the last round of runup's showing a 300 RPM drop once.  

Bottom number 4 plug is slightly wet with oil, but is not totally uncommon and should not be enough to cause the drop

 

My planned next step is to pull the mags, which is going to be a total PitA.  Is there anything else I/we should look at prior to pulling them?  If pulling them is the only option, who is the recommended mag shop for Bendix?  

Thanks!

[DXB]

17 minutes ago, pilot_jb said:

About to start really digging in...

Went through a period of time when I would Intermittently fail a left mag check.  The issue started happening progressively more often, so as the first step in diagnosing the problem, we swapped plugs from the right to the left.  The expected result was for the problem to move to the right if it was the plugs, and to stay on the left if it was the wires or magneto.  Oddly enough, the issue corrected itself.  Temporarily, and always on the ground, both mags passed with flying colors under warm and hot scenarios.  I let the plane cool and then the problem returned.  We then found the timing to be, at most, a couple degrees off.  After this was corrected, I once again could not get it to fail a check on either side.  I let the plane sit for a couple days, and now the issue is as bad as it's been...can't get it to pass on the left.  

We also rebuilt the P-leads, because we found the left side wiring to be old, brittle, and only connected by a few strands of wire.  

Both mags have 512 hours.

IO360-A1A

Bendix S4LN-200 shower of sparks

Left dropping between 200-250 RPM with the last round of runup's showing a 300 RPM drop once.  

Bottom number 4 plug is slightly wet with oil, but is not totally uncommon and should not be enough to cause the drop

 

My planned next step is to pull the mags, which is going to be a total PitA.  Is there anything else I/we should look at prior to pulling them?  If pulling them is the only option, who is the recommended mag shop for Bendix?  

Thanks!

I can't preclude a harness issue, but if you're past 500 hours, pulling both mags for routine IRAN right now seems like a no brainer.  This is one place that has a good reputation:

http://crossfiremagneto.com/Crossfire_Magneto_Service/Welcome.html

You could also consider overhaul exhange to save time and/or replace the left with a Surefly.

[PT20J]

When you get excessive mag drop, is the engine running smoothly or rough? Rough indicates a misfire likely a plug or harness issue. Smooth indicates a mag or timing issue. Did you clean and gap the plugs when you rotated them? Mag guys tell me that the engine actually prefers a gap on the wide side of the allowable range because it allows the voltage to build higher before the gap arcs resulting in a hotter spark and more efficient ignition. Also, as the points wear, the internal timing of the mag (E-gap) can change. The quick fix for this is to "bump" the mag buy rotating it slightly to bring the mag-engine timing back to spec. If successive bumping accumulates to more than 5 degrees (that's Slick's spec, not sure what Bendix recommends but probably similar) the internal timing needs to be reset. When the internal timing is off, it results in a weaker spark even though the mag-engine timing correct. As @DXB suggested, they're due for the 500 hour inspection anyway and as part of that, the shop will reset the E-gap.

I use these guys: https://www.aircraftmagnetoservice.net/

Skip

 

 

[DXB]

Also an engine monitor if you have one should give you clear indication of whether it is mag/timing vs. a plug/harness component.  

[pilot_jb]

16 minutes ago, DXB said:

Also an engine monitor if you have one should give you clear indication of whether it is mag/timing vs. a plug/harness component.  

Not a formal engine monitor, but a Mixture Mizer.  This is an instrument that shows peak EGT for one cylinder at a time, but not actual EGT.  It will display actual CGT.  Also, it does not record data.  Finally, I do have an EI EP-5L fuel computer, but, again, all of my readings and data recording are manual.  

[pilot_jb]

2 hours ago, PT20J said:

When you get excessive mag drop, is the engine running smoothly or rough? Rough indicates a misfire likely a plug or harness issue. Smooth indicates a mag or timing issue. Did you clean and gap the plugs when you rotated them? Mag guys tell me that the engine actually prefers a gap on the wide side of the allowable range because it allows the voltage to build higher before the gap arcs resulting in a hotter spark and more efficient ignition. Also, as the points wear, the internal timing of the mag (E-gap) can change. The quick fix for this is to "bump" the mag buy rotating it slightly to bring the mag-engine timing back to spec. If successive bumping accumulates to more than 5 degrees (that's Slick's spec, not sure what Bendix recommends but probably similar) the internal timing needs to be reset. When the internal timing is off, it results in a weaker spark even though the mag-engine timing correct. As @DXB suggested, they're due for the 500 hour inspection anyway and as part of that, the shop will reset the E-gap.

I use these guys: https://www.aircraftmagnetoservice.net/

Skip

 

 

A ground mag check produces a 200-300 RPM drop with what I consider to be a rough running engine.  Plug timing was set approximately 20 tech hours ago.  

[PT20J]

1 hour ago, pilot_jb said:

A ground mag check produces a 200-300 RPM drop with what I consider to be a rough running engine.  Plug timing was set approximately 20 tech hours ago.  

Since it’s running rough, and given that it’s been intermittent and got better when you disconnected the harness from the plugs to swap positions, I’d test the harness. Also, I’d check the resistance <5K on the plugs - especially if they are older Champions.  

[Guest]

I’d be starting in order, plugs first, wires then magnetos.  Bendix mags are less likely to fail just because they’re at 500 hours, assuming that they were done reasonably well at the last overhaul.

Clarence

 

[FloridaMan]

I've had two mag failures on my 67F's IO360-A1A. Both times it was the left mag. Go figure. I also ended up AOG three times because of alternator issues, the third time I landed in a field leaving the shop for either something random, or Occam's razor dictates that it was a maintenance induced engine failure. The reason I bring this up is if you use this time to upgrade to a Surefly and if you still have the original wires to your battery in the tail, replace the battery lines when you run the wire for the surefly. 

[86Xray]

On 9/22/2020 at 3:11 PM, pilot_jb said:

A ground mag check produces a 200-300 RPM drop with what I consider to be a rough running engine.  Plug timing was set approximately 20 tech hours ago.  

@pilot_jb I'm having similar issues you had. What did you end up doing to fix your issue? Mine is also on left mag.

[Yetti]

Key switch. It's not really a short since you are grounding out the mags, but the ground wires going to the mag could be shorting.   There are rebuild plates for the bendix ignition switch.

[86Xray]

3 minutes ago, Yetti said:

Key switch. It's not really a short since you are grounding out the mags, but the ground wires going to the mag could be shorting.   There are rebuild plates for the bendix ignition switch.

I had a new key switch (from bendix to ACS) installed last July so maybe something is shorting. We will definitely check that as well. Thank you.

[Guest]

1 hour ago, 86Xray said:

I had a new key switch (from bendix to ACS) installed last July so maybe something is shorting. We will definitely check that as well. Thank you.

The ACS switch isn’t approved for a Mooney according to their application guide.

Clarence

[86Xray]

Just now, M20Doc said:

The ACS switch isn’t approved for a Mooney according to their application guide.

Clarence

Interesting! I will look into it and ask the shop that installed it why they chose to go that route. Thank you. 

[Guest]

This may be helpful 

[pilot_jb]

2 hours ago, 86Xray said:

@pilot_jb I'm having similar issues you had. What did you end up doing to fix your issue? Mine is also on left mag.

We end up pulling the mags. The points and the rotor were shot. Fixed it right up.