Removing stall warning switch mystery

[Jer]

Folks:

My aircraft is a 1989, S/N 3079.  The switch function was intermittent and then eventually total failure.  I want to take the unit out, inspect it and probably replace the microswitch.  I have the maintenance manual (see photo) which only use the verbiage: "Install two attach screws".  I don't know if they are from inside the wing or outside on the leading edge.  The manual shows two configurations, and my serial number points to figure 27-21A.  The 4 outboard screws seem to match the pattern shown, however the inside is not visually the same.  The manual shows two screws inboard, which don't exist on my aircraft (see photo).  Figure 27-21A indicates 2 of the 4 outboard screws are for adjustment, so I loosened the other two, expecting the vane assembly to loosen up.  It did not.  So I tightened those, loosened the two "adjustment" screws (why not), and the vane assembly still not loose.  I loosened all 4, still not freeing the vane assembly.  None of the figures identify the "two attach screws".  Can anyone offer any direction on this problem?  Thank you.

Jerry

[N201MKTurbo]

It is probably painted in place. You can remove the cover plate under the wing and look up in there and see how it is put together. 
 

Don’t worry about adjustment. There will be enough witness marks to get it back the same way.

[Jer]

Thank you rich, the painted in place is something I did not consider.  With regards to checking out through the access plate, it is about 14 inches back from the leading edge, as evidenced by 14 inches of cuts on my forearm as measured from my fingertips.  I tried looked at it through fiber optic bore scope and holding i phone in hand, with little luck.  I'll give it another go tomorrow with the paint in place in mind.  Still I wonder why the service manual picture no way represents the actual aircraft.

 

Jer

[Bartman]

If I remember correctly, mine had some sort of locking nuts on the end of those screws and I had to hold them from the inside. It was definitely not an enjoyable project, and I am thankful that the cuts and abrasions did not leave scars on my forearm. 

[EricJ]

4 hours ago, Bartman said:

If I remember correctly, mine had some sort of locking nuts on the end of those screws and I had to hold them from the inside. It was definitely not an enjoyable project, and I am thankful that the cuts and abrasions did not leave scars on my forearm. 

That was the case with mine as well.   Something that helped me a lot for this task was a kevlar mechanic's sleeve and taping up the edges of the inspection hole.

https://www.amazon.com/dp/B009A5BPVE

[Will.iam]

That looks identical to my stall switch in my M20K 1986 model. The top right and lower left screws (closest to the stall plate opening) are the adjustment screws and they are also in a aluminum 1/2 wide bar that goes over the stall flange on the top inside  to the other screw and another bar over the bottom part of the stall flange to the other bottom screw. Loosen the top left screw and lower right screw as they will hold the bars from going anywhere in the wing. Completely take out the top right and lower left screws and the stall vane assembly will be able to remove. The grounding wire will be attached to one of those screws and the other wire is the power wire for the assembly. Take a picture before you remove the wires to remember which one goes in which hole as there are three holes you could put the wires in and only 2 will work the right way. There is a post here on mooneyspace of taking the stall vane apart and cleaning it. Mine had a blade of grass wrapped around the plunger effecting the movement of the vane to micro switch. Once i cleaned the debris out of the plunger hole and put it back together it’s worked as good as new. Hopefully you will be lucky here too. Had they designed the plunger upside down instead of right side up less debris could get in but it is what it is. 

[Jer]

Folks:

I appreciate all the help, particularly the detailed instructions of Will; however, I have been barking up the wrong tree.  I haven't even found the correct tree yet.  I looked up the schematic last night, and realized if the "hot" wire coming in to the switch was grounded out (bypassing the switch of unknown reliability), the horn should sound,  I took a voltmeter to that wire (showed 23 volts), grounded it out and no horn.  As long as I was doing the exploratory surgery, I took an ohm meter and checked the function of the switch, which showed proper functioning.  I closed and stitched up the patient's wound.  Meanwhile, my tie-down neighbor, a fellow Mooney driver dropped by and I disclosed my suspicion the the actual horn may be the issue.  He said he hears the warning in his headphones, so if so, the lack of audible horn really wouldn't be a problem if it played in my headphones.  I turned master and radio switches on, put on the headphones and triggered the stall vane.  Just silence.

Anyone want to point me where to go next?  Please pardon my mixing sylvan and medical metaphors.

Jer

[Jer]

What is a sonalert, and where is it?

[MikeOH]

In my F, it is in the headliner.

$17 from DigiKey:

https://www.digikey.com/en/products/detail/mallory-sonalert-products-inc/SC628/528117?gclsrc=aw.ds&gad_source=1&gad_campaignid=120565755&gbraid=0AAAAADrbLlit1t_cdCF17FKAaGWeiA0_S&gclid=Cj0KCQjw-4XFBhCBARIsAAdNOkvHv19kfm6slsPwSpvnE730c_jKFrVanQUJYXM_hnIZtAdKFYtM3tUaAlkOEALw_wcB

 

[Jer]

Thank you.  I'll try an find it and test it.  Does anyone have any comment on hearing the stall warning through headphones.  I don't really recall that myself.

[MikeOH]

34 minutes ago, Jer said:

Thank you.  I'll try a find it and test it.  Does anyone have any comment on hearing the stall warning through headphones.  I don't really recall that myself.

On my F the ONLY sound comes from the Sonalert; nothing to do with headphone/speaker audio.

No idea if it's different for a J.

[dkkim73]

,

12 hours ago, Jer said:

Please pardon my mixing sylvan and medical metaphors.

We understand. You're not out of the woods yet. 

Going straight to the noisemaker makes sense. But if that doesn't work, you might consider testing the wire. I hear stories around here of things wearing where they cross metal, especially if not secured with a clamp. Just a thought. 

[Jer]

dkkim:

Thanks for the sylvan metaphor.  I don't suspect the wire as where I tested at the end of the line, I had voltage, and when grounding that out, juice was flowing in the circuit.  If the wiring shorted out between the battery and the sounding device, the breaker would have tripped.  If the wire shorted out after the sound device and the stall vane switch, I wouldn't have voltage at the switch, which I did.  I have great hope for the noisemaker being the issue.  If that fails to be the issue, I think the next logical step would be to check my hearing, which unfortunately would then open the door to medical metaphors.

Jer

[RoundTwo]

15 hours ago, MikeOH said:

On my F the ONLY sound comes from the Sonalert; nothing to do with headphone/speaker audio.

No idea if it's different for a J.

No stall warning through headset on my J, just the Sonalert.

[Bartman]

3 hours ago, RoundTwo said:

No stall warning through headset on my J, just the Sonalert.

Same.  PS Engineering 7000 series.

[Paul Thomas]

4 hours ago, Jer said:

dkkim:

Thanks for the sylvan metaphor.  I don't suspect the wire as where I tested at the end of the line, I had voltage, and when grounding that out, juice was flowing in the circuit.  If the wiring shorted out between the battery and the sounding device, the breaker would have tripped.  If the wire shorted out after the sound device and the stall vane switch, I wouldn't have voltage at the switch, which I did.  I have great hope for the noisemaker being the issue.  If that fails to be the issue, I think the next logical step would be to check my hearing, which unfortunately would then open the door to medical metaphors.

Jer

That should be easy to test, there should be two of them right new to each other: one of the gear and one of the stall. You should be able to switch the wires.

[MikeOH]

17 hours ago, Paul Thomas said:

That should be easy to test, there should be two of them right new to each other: one of the gear and one of the stall. You should be able to switch the wires.

Keep in mind that the gear Sonalert is intermittent NOT continuous like the stall. If you just touched the wire to the terminal to test and did not immediately hear a tone you might come to the wrong conclusion!

[Paul Thomas]

7 minutes ago, MikeOH said:

Keep in mind that the gear Sonalert is intermittent NOT continuous like the stall. If you just touched the wire to the terminal to test and did not immediately a tone you might come to the wrong conclusion!

I believe in the J, the gear alert is continuous, just a different pitch. I'll have to test that at some point.

[EricJ]

10 hours ago, Paul Thomas said:

I believe in the J, the gear alert is continuous, just a different pitch. I'll have to test that at some point.

In mine the stall is continuous and the gear is intermittent/pulsed. 

[PT20J]

The stall warning is continuous and the gear warning is intermittent even in the later models that have a tone generator and speaker rather than sonalerts. 

[Jer]

Today's investigation led to some perplexing results.  Removed the Sonalert, and put it across a 28v APU.  Sounded fine.  I removed the negative lead (which connects to the stall vane switch and then to ground), and checked that lead for continuity with the airframe ground while the vane was triggered.  Checked zero OHMs, as one would expect.  However, connecting this back to the sonalert negative, did not sound the alarm.  However, just connecting a wire shunt between the negative terminal on the sonalert and the airframe, and the thing does sound.  The conclusion of my electrical engineer tie-down neighbor was that while the negative side of the circuit, did show continuity to ground, there must be excessive resistance in that portion of the circuit.

Tomorrow I intend to wire it up correctly, turn on the master, trigger the vane, and check the voltage drop across the sonalert.  I believe I did this earlier but did not recognize the importance of this test.  I believe the plus side checked 24 v to ground and the negative side checked 16v to ground.  I did not directly check voltage across the unit, but if what I think I recall is true, there would be 8 volt drop across the device, with the other 16 volt drop eaten up by excessive resistance in the negative side of the circuit.  If this is so, I will check quite locally for a voltage drop across the stall vane switch.  It may have continuity, but also excessive resistance, in which case I will have the opportunity to try laparascopic removal of the switch from that access panel.

Thanks for all your help.

Jer

[MikeOH]

@Jer

I think your plan is solid.

You may have had the ohmmeter set to a too-high resistance scale; it would read zero when, if fact, resistance could be excessive. Successful operation when you grounded locally, plus your previous voltage measurements, strongly suggests high resistance in the switch circuit, as you suspect.  I'd check for a good ground from the switch to the airframe.  Worst case will be a high on-resistance switch

[Bartman]

There is a thread on here somewhere that details switch removal, opening it up, internal contact cleaning, and the results. If you determine it’s the switch, that thread may be helpful. 

[PT20J]

If you remove the stall warning assembly, given that it is difficult to remove, I would replace the microswitch rather than spray it with contact cleaner. Electronic switches have contacts plated with gold or silver to reduce oxidation and they open and close with a wiping action to remove any oxidation that may occur. Eventually, the plating wears through and the base metal will be exposed and oxidize. Spraying with contact cleaner (if you can get any to the contacts -- most of these switches are sealed pretty well to prevent contamination) may temporarily remove the oxidation, but it will come back eventually. I have a zero tolerance for failed electrical components -- if it fails once, I replace it.

Also, be sure to mark the position of the switch unit before removing it so you don't have to make a lot of test flights readjusting it.

[N201MKTurbo]

1 hour ago, PT20J said:

If you remove the stall warning assembly, given that it is difficult to remove, I would replace the microswitch rather than spray it with contact cleaner. Electronic switches have contacts plated with gold or silver to reduce oxidation and they open and close with a wiping action to remove any oxidation that may occur. Eventually, the plating wears through and the base metal will be exposed and oxidize. Spraying with contact cleaner (if you can get any to the contacts -- most of these switches are sealed pretty well to prevent contamination) may temporarily remove the oxidation, but it will come back eventually. I have a zero tolerance for failed electrical components -- if it fails once, I replace it.

Also, be sure to mark the position of the switch unit before removing it so you don't have to make a lot of test flights readjusting it.

I take a different view of the failure mode of these switches. The switches have internal mechanisms that rock a contact arm between two stable positions. I think the predominant failure mode is the mechanism getting hung up because any lubrication added at the factory has dried up years ago and because of dirt getting inside and mucking up the works. They are in a bad place with regard to getting stuff blown in. Spraying lube in doesn’t help the contacts, it loosens up the mechanism. 
 

These switches are sealed, but the seals don’t last forever. They are rubber and it gets hard and looses its compliance after years in the elements.

lubing the switch, will probably give it a few years before it needs more service, but the best plan is to replace the switch. They are somewhat hard to come by.