Hot Mag question

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Ltdriser

Filing Flight Plan
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Ltdriser
Chasing down a hot mag issue. 62 M20C, 0-360-A1D (I think) w slick magnetos ~250 hours on them.

The mags check out fine during initial run up, both give me about 100rpm drop. Issue is during shutdown mag check or midday run up, after fueling up or something. When on L I get 100 rpm drop, R 0 rpm drop, OFF engine continues to run. So this issue only presents itself when the engine has been running for a period of time then goes away during a cold start and initial run up until warm again.

So to me, instantly, I’m thinking p-lead. Pull off the cowls and check the L p-lead and find nothing out of the ordinary. Same for the R p-lead. Hmmm?? They’re old but really for the most part seem fine.

I do a continuity check from frame(Gnd) to L p-lead stud on mag. I get continuity in OFF and R. Which is how it should work. And how Id expect it to work, at least in my case, since during initial run up everything seems fine. So I’m guessing the switch is fine (Bendix w the AD).

Anyone experience similar or have some pointers? I’d rather not toss parts at this thing. Not sure what stone to turn. P-Leads seem fine, switch seems fine, mags are 250 hours old.
 
Hopefully Mr. Dan Thomas will be along shortly. This sounds like it would be on the first floor of his wheelhouse ... :)
 
My thought would be to make a temporary p-lead with test clips and wire.

Connect to p-lead stud and take through vent window to ground out when running.

If there is still no drop with LEFT grounded you have a mag issue.

I’m unsure if an overtorqued and “ pulled “ capacitor would act as described.
 
Ltdriser said:
Anyone experience similar or have some pointers?
Click to expand...
Did you shake/wiggle the P wires while performing your continuity checks? However, if the internal magneto capacitor fails open then no way to ground primary coil. Its possible the heat is degrading the capacitor (which can be check also) but I would ensure its not a wire problem with a wiggle test.
 
I did not wiggle/tug the wires while doing my check. Good call, I should have done that. I do recall checking continuity between (-) on the battery and the frame ground and not getting a tone. That seems odd to me?
 
Sounds to me like the condenser is failing. Slick uses a feed-thru condenser, with the P-lead stud being common to the wire on its other end inside the mag that goes to the points. The wire end must be OK or the mag wouldn't fire, but that stud end might be opening up inside the condenser when it gets hot. Can't short the mags off if that P-lead terminal is dead.
upload_2021-9-5_19-44-56.jpeg

Yellow wire goes to the points. Stud at left is the terminal for the P-lead. They are connected inside the condenser. The case itself is the other pole of the condenser (capacitor).

Checking it? Get it hot, shut it off. Connect a multimeter to that stud and ground, a multimeter with capacitance function. If it reads nothing, move the prop backwards and see if it starts to say anything. That's to open the points, which could be closed and shorting the condenser. Best to disconnect ALL the sparkplug leads before messing with that prop, though. We already know it's hot. Don't go asking for more a lot trouble than you already have. Multimeter should show around 0.34 microfarads. If it doesn't, let it cool off and check again. If it's OK cold but not hot, that stud is disconnecting inside when that aluminum case expands.

Oh, and the P-lead must be disconnected.

From the Slick manual:

Capacitance value should be measured at
room temperature and charged to 400 ± 10
VDC. The service limit of the capacitor is .315
to .385 microfarad.

Sorry for all the edits to add stuff. Sometimes I get carried away with advice.
 
Last edited:
Dan Thomas said:
Sounds to me like the condenser is failing. Slick uses a feed-thru condenser, with the P-lead stud being common to the wire on its other end inside the mag that goes to the points. The wire end must be OK or the mag wouldn't fire, but that stud end might be opening up inside the condenser when it gets hot. Can't short the mags off if that P-lead terminal is dead.
View attachment 99823

Yellow wire goes to the points. Stud at left is the terminal for the P-lead. They are connected inside the condenser. The case itself is the other pole of the condenser (capacitor).

Checking it? Get it hot, shut it off. Connect a multimeter to that stud and ground, a multimeter with capacitance function. If it reads nothing, move the prop backwards and see if it starts to say anything. That's to open the points, which could be closed and shorting the condenser. Best to disconnect ALL the sparkplug leads before messing with that prop, though. We already know it's hot. Don't go asking for more a lot trouble than you already have. Multimeter should show around 0.34 microfarads. If it doesn't, let it cool off and check again. If it's OK cold but not hot, that stud is disconnecting inside when that aluminum case expands.

Oh, and the P-lead must be disconnected.

From the Slick manual:

Capacitance value should be measured at
room temperature and charged to 400 ± 10
VDC. The service limit of the capacitor is .315
to .385 microfarad.

Sorry for all the edits to add stuff. Sometimes I get carried away with advice.
Click to expand...
So much knowledge the advice pours out all at once. I'm not complaining
 
Thank you for the insight. I was really hoping to find something obvious and ask the mechanic team(maybe hangar fairies) to repair. This sounds more like the mag needs to come out and by the time the culprit is found I could have the mag rebuilt for similar costs? Which stinks since they are at half of their rebuild life.
 
Check the switch, too. I had a switch that you could turn to either L or R or even OFF and it worked as expected but remove the key or just jiggle it a bit in the OFF position and it would unground.
 
Dan Thomas said:
Sounds to me like the condenser is failing. Slick uses a feed-thru condenser, with the P-lead stud being common to the wire on its other end inside the mag that goes to the points. The wire end must be OK or the mag wouldn't fire, but that stud end might be opening up inside the condenser when it gets hot. Can't short the mags off if that P-lead terminal is dead.
View attachment 99823

Yellow wire goes to the points. Stud at left is the terminal for the P-lead. They are connected inside the condenser. The case itself is the other pole of the condenser (capacitor).

Checking it? Get it hot, shut it off. Connect a multimeter to that stud and ground, a multimeter with capacitance function. If it reads nothing, move the prop backwards and see if it starts to say anything. That's to open the points, which could be closed and shorting the condenser. Best to disconnect ALL the sparkplug leads before messing with that prop, though. We already know it's hot. Don't go asking for more a lot trouble than you already have. Multimeter should show around 0.34 microfarads. If it doesn't, let it cool off and check again. If it's OK cold but not hot, that stud is disconnecting inside when that aluminum case expands.

Oh, and the P-lead must be disconnected.

From the Slick manual:

Capacitance value should be measured at
room temperature and charged to 400 ± 10
VDC. The service limit of the capacitor is .315
to .385 microfarad.

Sorry for all the edits to add stuff. Sometimes I get carried away with advice.
Click to expand...
How do you expect to get an accurate capacitor reading without removing the part? Basic electronics is that you have to isolate the part you want to test.... Leaving it in the mag and trying to use the engine as a conductor??? Especially a capacitor? Really?

Do you think you can measure microfarads through and engine block???
 
pfarber said:
How do you expect to get an accurate capacitor reading without removing the part? Basic electronics is that you have to isolate the part you want to test.... Leaving it in the mag and trying to use the engine as a conductor??? Especially a capacitor? Really?

Do you think you can measure microfarads through and engine block???
Click to expand...
We're measuring microfarads here, not picofarads. And the ground is at the mag, at the screw where the P-lead shield is grounded. Now, tell me how much different this would make. We're looking for an OPEN, not a difference of two microfarads or something.

Again: what are you aviation maintenance qualifications and experience?
 
Measuring between the P lead and ground puts you at the arrows in this diagram (I'm sure Mr. Thomas will correct me if I am wrong). The points and the mag switch need to be open to measure anything about the capacitor. Not much else in the circuit to mess up your measurement. Note: Of course, there is one big error in the diagram. And even funnier, neither the "ignition on" or "ignition off" diagrams show you if the switch id open or closed - they just change the word "on" to "off". Somebody besides a technical writer really needs to proofread this ****.
https://www.faa.gov/regulations_pol.../media/FAA-H-8083-32-AMT-Powerplant-Vol-1.pdf
upload_2021-9-6_12-40-47.png
 
flyingron said:
Check the switch, too. I had a switch that you could turn to either L or R or even OFF and it worked as expected but remove the key or just jiggle it a bit in the OFF position and it would unground.
Click to expand...
Yes, bad switches can present strange issues, but in the OP's case this repeatedly happens only when the engine is hot. I don't think it's a bad switch. IMHO.

I have found dozens of aircraft logs indicating that AD 76-07-12 Bendix mag switch inspection has been carried out for years and years. Then I get under the panel to inspect the stuff under there and find that it's not a Bendix at all, but an ACS switch, which has a completely different AD against it: 93-05-06. It's been out of compliance so long that the switch is shot and has to be replaced. The AD requires an inspection of the switch and a diode to be installed across the starter contactor coil to prevent burning the switch's contacts, which can affect mag operation.

Or I find that it IS a Bendix switch, but it has a four-digit date code, or a white dot indicating modification, and the AD doesn't even apply to those two types.
 
Capt. Geoffrey Thorpe said:
Measuring between the P lead and ground puts you at the arrows in this diagram (I'm sure Mr. Thomas will correct me if I am wrong). The points and the mag switch need to be open to measure anything about the capacitor. Not much else in the circuit to mess up your measurement. Note: Of course, there is one big error in the diagram. And even funnier, neither the "ignition on" or "ignition off" diagrams show you if the switch id open or closed - they just change the word "on" to "off". Somebody besides a technical writer really needs to proofread this ****.
https://www.faa.gov/regulations_pol.../media/FAA-H-8083-32-AMT-Powerplant-Vol-1.pdf
View attachment 99828
Click to expand...
That's the basic idea, except that in the case of the Slick the ground is the capacitor's case and the leads at each end are joined inside it and are both "hot."

I wanted the P-lead disconnected to eliminate the switch as any source of problems and to get rid of the small capacitance in the P-lead itself, between the core and the shield.
 
Don’t rule out the P-lead wires. Broken strands within wire insulation tend to have different characteristics when warm and cold. Add vibration and weathered connectors.
 
It seems like I forgot to provide an update. Dan hit the nail on the head. Failed capacitor. Replaced it and she’s been running great ever since. Thanks for the help all!
 
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