R

Rolly9

Active Member
In the last month or so, every time I key the mike several of the engine instruments (CHT, Oil Tmp, MP) will swing toward full scale, and then go back to normal when the mike is released. The autopilot will disconnect also, if engaged...What is happening? Thx. Rolly9.
 
It is an RF feedback problem. I am not an expert but had a similar problem.

My comm antennae was a simple wire bent whip located ahead of the spar on the cabin floor centerline. It was connected via ring connectors (center pole to ant, shield to ground). I changed it out for a "real" Comant antennae with a BNC connection. Problem is now minimal (only fuel pressure effected).

Look for bad or poor connections in your transmit coax that might "leak" RF energy back into the cabin. Good luck
 
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That definitely sounds like an EMC (Electromagnetic Compatibility) problem. There are probably at least as many causes and cures as there are airplanes flying, but as has been suggested checking your antenna and feed line is probably a good starting point. Not just the grounding of the feed line (co-ax) at both ends, but the resonance and impedance of the antenna as well. (Find a local Ham Radio Operator and borrow a VHF SWR meter.) It would be good to monitor the 12 Volt system as well, because it is possible that when you key the radio it is causing a voltage drop to the instruments and AP. Check all of the grounds of the affected instruments. Do they, by chance, ground at a common point? It is also possible, though not likely, that an RF filter might be needed for each of the instruments. Just keep checking until you find the cause. The fix is usually easy once the cause is uncovered. I make my living preventing and curing problems like this, and believe me there is no magic fix. It is an iterative process unless you get very lucky!
 
inadvertent ground

when you key the mic you are grounding one of the connections at the jack, the autopilot likely has a switch that will disconnect on ground... the sensors are resistance based. First thing to check would be the other push to talk switch... if it is not installed you can use a jumper wire to test. Get eyeballs on the ptt and mic wiring and check what kind of resistance to ground you have and what sort of voltage when when the ptt is grounded. Assuming that everything was working fine before and the problem is repeatable a bit of probing should uncover the source failure.
 
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