The Cessna 120-140 Association Forum

Hey everyone, the title basically says it. Have a 120 with a Continental 85 with slick mags. A couple of months ago, they didn’t ground. No RPM drop during run up, didn’t cut off when I flipped the toggle switches. Tightened the P lead connections a little and it worked fine the next day. Didn’t think much more about it.

Last week, same thing. No RPM drop, won’t cut off with the switches. Had a mechanic look at it yesterday, it worked fine (of course!) and he said it all functioned as it should.

I’ve checked the wiring behind the panel (it’s old, but all connected) and it looks ok. It’s grounded through the wire insulation; picture attached. P leads are old but look ok.

Thinking I need new P leads but want to make sure it’s not something like the mags themselves? They have around 325 hours on them.

Any thoughts? An engine continuing to run doesn’t bother me as much as the thought of it becoming “grounded” during flight.

Where is the other end of the shielding attached? Ideally it should be connected to the ground screw on each magneto.

Hey John, It’s not connected anywhere I can tell. Stops short of the mag.

So, how is the p-lead grounded when you close the switches, or is there a ground at the switch that I'm missing?

I’m assuming it’s grounding at the switch. Here’s another picture if that’s helpful.

superpilotjustin wrote: ↑Mon Mar 31, 2025 8:12 am I’m assuming it’s grounding at the switch. Here’s another picture if that’s helpful.

That's my point: It appears the switch is not grounded.

The shields should be grounded, ideally at only one place, i.e. the magneto(s).

Sure, what you’re saying makes sense to me. I don’t understand how it’s working properly the way it’s setup right now though so it’s confusing.

The shields are touching the airframe, randomly, is my guess.

Hmm, I’ll look around. Maybe as they pass through the firewall?

Loose connection (or haphazzard connection) somewhere........... Some time spent with a good Ohm meter ought to solve your issue.