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Ignition noise

bob turner

Well-Known Member
Jan 7, 2005
Reaction score
Well, this is embarrassing. I have always been quite good at diagnosing and fixing things like this, but I am running out of ideas.

J-3 Cub - no electrical system, but battery power for lights, radios, and GPS. Battery power goes from floor to wing root with a plus 12v wire and a ground wire. I do not use airframe ground.

Mags Bendix S4, shielded harnesses, shielded plugs, capacitors on the P-leads, and shielded P-leads.

Started to get noisy - first big symptom was a "tick . . . .tick . . . .tick" - easy to fix, it was a shield breakdown on one ignition lead. Then it started to get noisy like tickticktick - varying with engine RPM, and obviously ignition noise in the radios.

Replaced both mags with rebuilt. No change. Replaced both harnesses. No change. Replaced capacitors - $140 at Aircraft Spruce, for ten dollar capacitors. No change. Replaced plugs. No change. Replaced left P-lead with shielded wire coming in to cabin over front seat and switch in seat back pocket. No change.

Well, actually some change. Over the six months I have been fighting it it is getting worse.

The left mag is apparently much worse - it comes through the iCom portable that I use for the ATIS. The Grmin SL-40 tolerates the noise a bit better, and seems to get equal noise from either mag.

Today I put another brand new harness on the left mag - it got even worse! After the test flight I disconnected the battery and used the Icom portable with its own internal battery and the whip antenna that attaches directly to the radio. Even worse - inside the cockpit there is a demon of RF proportions!

Tomorrow I shall replace the left mag plugs with known good ones, and completely remove the P-leads. That would at least isolate the left mag noise to the magneto itself or the new harness. I shall report back.

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