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Master Switch and Flap Malfunction




During a recent preflight, the master switch on my 1975 M20F would not turn off. In addition, the flaps stuck at 1/2 and would not retract after takeoff. A similar thing happened last year, and the local mechanic replaced a relay which seemed to take care of the problem for a while. Now he's telling me that there is a system upgrade retro fit to replace all of the relays. Does anyone have info on this issue? Thx.



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The circuit for the master switch is quite simple the master switch controls a large relay similar to an automotive start relay. The master relay is in fact the same part as your start relay and these relays can sometimes get stuck closed. If your F is below S/N 670539 the flaps are entirely hydraulic so any failure to retract is either the retract spring or what the M/M refers to as the control valve. The control valve is just a needle valve that is adjusted to control the retraction rate. If the control valve were not there in a go around situation too rapid a flap retraction could result in a stall. The valve is located next to the pump under the floor.

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I could be wrong (as my wife always says), but the Master relay should be different than the starter relay. The starter relay is desigend for short term power inputs, where as the Master relay is designed for long term power. I'm an A&P and although my Mooney maint. experience is limited, all the other A/C I work on have this set up. If your relay is a Cuttler & Hammer, you can disassemble it, clean out the old grease, and regrease with dieelectric grease. This may fix the problem, or your relay may just be going. Strange electric things occur with a relay that is going, but not fully gone. Failure of electric flaps to fully extend or retract, especially under a load are one of those symptoms.

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