Stall horn switch and wiring

[milotron]

Hi,

 

It would seem that my stall horn has decided to stay on permanently. Only pulling the breaker turns it off.

 

The stall switch changes state and appears to be functioning correctly suggesting that the wiring between the switch and horn is grounded to the frame somewhere. ( the stall switch completes the circuit to the frame/ground )

What route does this wiring go from the wing to the overhead panel? I am assuming through the B pillar just behind/beside the pilot, but I would like to avoid taking half of the interior apart to find this.

Thanks,

iain

[Yetti]

Get wet recently.... tried contact cleaner on the switch to dry things out?

 

[milotron]

Yes, TKS and rain. The switch works fine. I put a meter on it and the contact shows as open, then closes when operated, so seems to be good. I gave it a shot of contact cleaner any ways as I had it out to clean the headphone jacks.

[carusoam]

Your POH should show it in an electrical diagram.  

That and the gear up, throttle out, combination of switches.

something is powering your stall horn without going through the stall switch...?

Or that's not the stall horn you are hearing..?  Make sure your gear is down and locked... 

PP ideas, not a mechanic...

Best regards,

-a-

Edited March 17, 2017 by carusoam

[milotron]

Thanks. That is what I need to sort out now. Switch works fine; gear horn works fine; No other issues.

I'll pull the side panel on the weekend and poke around some more. Ws thinking that maybe an interior panel screw jagged the wire or something, or maybe it is shorting to ground at the horn itself.

Just glad I am not buying a crazy expensive life detector or trying to get that thing out to replace the microswitch. With the TKS on the wings, the two philips screws holding it in place are not accessible and there are two nuts bodged in the back with straps.

 

[carusoam]

Great photo, Milo!

check the wiring diagram to see where the power can come from.  

1) If power is being delivered through the stall switch (possibly), it is going to be hard to get power to the sound maker accidently...

2) if power is delivered to the sound maker first (most likely), and drained out through the stall switch to activate the sound.... the accidental ground can be anywhere from the sound maker to the stall switch...

Makes sense to put the blow dryer in where you got the nice photo from...

3) have you tried it lately on the ground? After a couple of days of drying out did it make any difference.

I recall somebody else having this challenge.  It was kind of odd that this can happen since we fly in some really high moisture envronments....  a search for the right terms might reveal their experience.

PP thoughts, still not a mechanic...

Best regards,

-a-

Edited March 17, 2017 by carusoam

[milotron]

Switch contact is doing as it should. It grounds the circuit under a stall event causing horn to sound. At least that is what the schematic shows. Assume that it is wired correctly.

I will be chasing down 2) next. Just trying to figure out what routing it takes to minimize chances of breaking more interior plastics...

 

 

iain

[dcastor]

It comes through the wing root, then forward to a bundle under the instrument panel then up what you would call the A pillar.

Wiring diagram can give you the number Mooney printed on the insulation of the wire every so often so you might be able to find it in that bundle before removing any plastics.  If I had to guess though its shorting behind the panel to the pilots left...lots of sheet metal there and screws there, and it has to be between the horn and switch.

Be glad its not the switch, safeflight charged me 2 AMUs to overhaul.

Dave C

[milotron]

Fantastic info Dave! Thanks, that is the direction I was looking for. Wiring diagram suggests a connector or terminal there too. Hopefully I can isolate it and verify which end has the ground.

Also glad I don't have to service the switch...

 

iain

Edited March 18, 2017 by milotron
cause I can't type good

[dcastor]

There are a couple of amphenol connectors in that area.  I don't know if that wire passes through there, but probably it does.  If you're lucky it's a loose connection or corrosion at that connector.

[Tommy]

Your stall switch wires had shorted / grounded somewhere. I had exactly the same symptom two years ago. I traced - using a multimeter to check the integrity of the wire - from the switch, to wing root, to the cabin panel and found an exposed wire and replaced. Now good as gold. There is really unfortunately no other way around it. The wires are poorly marked so a schematics with wire marking number isnt going to help you at all. The switch can be taken down but make sure you mark a line on both the wing and the stall switch so you can line them up when putting it back. Otherwise you will need to realign the stall switch with flight tests! To make it easier to put it back, loop wires thru the holes around the stall switch and pull it back to the position. 

Good luck!

And trust me this will save you 1 AMU (too me 10-12 hours) if you can do it yourself!