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

[Vagabondblues]

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

[Vagabondblues]

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 Aircraft Magneto Service in Montana, they give a warranty of 500 hrs. never had a problem.