Magneto problem?

[azevedoflyer]

Hi to you All,
Along a magneto check (08/05/2023) prior to flying, I found:
- engine (O320 A2B - 150 hp) ran smoothly on BOTH,
- rpm dropped 100 revs on LEFT magneto, which has been the norm since last mag overhaul (~500 tach hours ago).
- NO drop in rpm observed switching to RIGHT mag. Engine ran smoothly. Used to be the same 100 rpm till last flight (07/31/2023).
What is going on?
Magneto / ignition switch / or...
Thanks for helping.
azevedoflyer
N8714D

 

[N2709P]

Good chance the “P” lead to the magneto is disconnected or broken. Mag is probably hot all the time. Be careful!
N2709P

 

[N2709P]

If magnetos are wired normally, when you select right on the ignition switch you are grounding out the left magneto and are running on just the right magneto. Hence the RPM drop. For some reason your left magneto is not dropping out, so no RPM drop. Bad “P” lead, ignition switch or internal magneto issue. I have seen the left and right “P” leads swapped many times. You troubleshoot it to the left magneto and since the “P” leads are swapped, it is actually a right magneto problem.
N2709P

 

[Smokey]

Bendix or Slick? Bendix uses a nut on insulator. The insulator pushes the safety switch off when the nut is threaded on. Slick uses a crimp on ring terminal.

I have seen a lot of damaged internal wires on the noise filters installed on Bendix mags. See attached pic. The nut will be installed on magneto driving the safety switch off allowing the magneto to fire. But it will not allow the P lead to ground it out.

Todd

 

[azevedoflyer]

Thanks Todd and N2709P!
Todd: Bendix S4LN21 Left and S4LN20 Right. Shielding is frayed, maybe completely detached at the left mag end. Must check. Just a potential noise source or are there any implications on the subject discussed?
09P explanation matches my own thinking on the subject. Mags are - as told - about 500h since last overhaul. Maybe best to send them out? Any recommendations?
azevedoflyer

 

[Smokey]

Most times the mags get overhauled and the old filter goes back on. I would remove the filters from the aircraft and use a multimeter to bench check for continuity. As it is a capacitor you will have to use a multimeter with a capacitor function. When you attach the probes, black and red polarity will not matter. As you hold the probes to the opposite ends of the filter you will see the capacitor rise in microfarads. if the capacitor does not rise then there is not continuity in the circuit as it is open. Sometimes there may be one or two strands of wire still attached to the capacitor providing an intermittent path, if you cannot get the capacitor to rise, try wiggling the magneto connection end a little.

The filters are P/N MFA-3 ignition filters, which are not factory parts.

 

[dgapilot]

Just installed Continental Rebuilt mags on the left engine on my TwinComanche. 5 hours later the right mag would not ground out (shut off). P leads are good and the switch is good but have a "hot prop". Called AirPower and got a warranty mag, installed it and all is good now. Sounds like I'm not the only one with this problem.

 

[Steve Pierce]

I would disconnect the P-lead at the magneto and check for proper operation of the switch and for continuity between the center conductor and the shield.

 

[azevedoflyer]

All,
Feedback on assumed magneto problems.
Found P leads frayed and not grounded in one or both ends (left mag.).
A chill ran down my spine when I found that P leads were either ungrounded or about to. How many times have I moved that prop back and forth to the point where the impulse coupling clicked?
Saving grace must have been killing the engine through mixture cut-off, a habit of mine.
Ordered P leads from Bogert.
azevedoflyer

 

[EdH]

Thanks Todd and N2709P!
Todd: Bendix S4LN21 Left and S4LN20 Right. Shielding is frayed, maybe completely detached at the left mag end. Must check. Just a potential noise source or are there any implications on the subject discussed?
09P explanation matches my own thinking on the subject. Mags are - as told - about 500h since last overhaul. Maybe best to send them out? Any recommendations?
azevedoflyer

I have always sent my mags to be overhauled to [FONT=Google Sans, Roboto, arial, sans-serif]Aircraft Magneto Service in Montana, they give a warranty of 500 hrs. never had a problem.[/FONT]

 

[N2709P]

You most likely have a Bendix ignition switch that has a recurring AD. the AD requires at every 100 hour or annual, the ignition switch it be checked for grounding out the magnetos. The best way to check this is with the engine at normal operation temperature during idle, momentary select off on the ignition switch and make sure the engine quits firing. Immediately go back to both. If you wait too long in the off position, you will get a pretty good pop. Not good for the induction system. If you wait too long in off, just let it die. Also, when checking the mags during run up, as you select “L”, “R” and both, attempt to pull the key out. If the key can be removed in any position other than in off, the ignition switch needs to be replaced. The “P” lead wire is normally a single braided wire in a shield. The shield is to help eliminate electrical noise. Current thinking with most shielded wire is to only ground one end. I grounded my shielding at the ignition switch and the shield at the terminal on the magneto is not grounded. Your loose shielding at the magneto could be that shrink tubing was not placed over the end of the shielding so that it did not fray. The shielding should not affect your ability to turn off the magneto. 500 hours or engine overhaul is about a normal time for magneto overhaul. If you can keep clean fuel and good spark to these engines they are pretty hard to stop.
N2709P

 

[azevedoflyer]

Thanks N2709P.
Will check as told.
Surprised by your "current thinking..." part in the text!
azevedoflyer

 

[N2709P]

As explained to me by my favorite electrical engineer (it takes 2 E’s to spell geek), old school thinking was to ground both ends of the shielding to build a Faraday or “noise shield”. Further experience found that in most cases , especially with GPS equipment, this is detrimental to the shielding’s purpose, as grounding both ends provides a continuous loop for “noise” to enter the system. Leaving one end of the shield not grounded still protects the shielded wire from radiation patterns and limits these radiation patterns from having a path into the system.
N2709P

 

[Gilbert Pierce]

All modern avionics manufacturers installation instructions that I have seen require only one end of the shield to be grounded.

 

[JPerkins]

And I was told to ground one end of the shielding on circuits carrying audio signals and ground both ends on circuits carrying digital data signals. Hmm, there must be a paper somewhere that explains all that and sets it strait.

 

[Steve Pierce]

Has anyone found a bad Bendix mag switch in recent years, I have not. Have had to clean the contacts on the old AN switches. Have found P-leads where the primary wire was contacting the shield on very old p-leads. I do quite a few 500 hour mag inspections and rarely find anything, mainly adjusting the e-gap to make up for the point wear. Most problems I see were created by Champion when they bought Slick and started changing things. I love the SureFly SIM, starts 3rd blade every time and no maintenance, look for leaks, wires aren't rubbing and all is good. No timing adjustments or 500 inspections.

 

[azevedoflyer]

To all that contributed, many thanks.
I stand educated about magneto shielding.
About points, I found that both are from Kelly Aerospace and not, as stated, one of them from Tempest.
Additionally, under a magnifying glass both have the movable arm (the one pushed by the cam shoe) contact crowned. The perceived angle comes from this feature.
However, while one contact is centered, the other is not. On this non-centered, the crown contacts the fixed part towards the edge, explaining the visually "greater" angle.
Difficult to take a picture! To complicate things, I do not know how to post pictures on threads!
Cheers,
Azevedoflyer

 

[Smokey]

Has anyone found a bad Bendix mag switch in recent years, I have not. Have had to clean the contacts on the old AN switches.

Probably because the AD is only applicable to the “twist to start” switches made over 50 years ago. Most short wings still have the big starter contractor and don’t have airframe voltage going through it for the starter solenoid. Some late model Tripacers do use it however.

Bendix now has a coaxial condenser for their mags as a retrofit. The coaxial capacitor which was unique to Slick used an antenna style shielding. Designed by a former Bendix engineer while employed at Slick. The new Piper Tomahawks then had points that would fail in 10 to 15 hours. It was found that Piper installed noise filters inline with the P leads. Electrically speaking capacitors in series, double the capacitance. That is two 10 mf capacitors in series provide 20 mf. Coaxial circuits are parallel. So if you have two capacitors in parallel the capacitor is cut in half. Two 10mf is now 5 mf. The primary purpose of the capacitor is to prevent arcing of the points as they open collapsing the field at E Gap.