Stall horn issues M20J

[redbaron1982]

Hey, I'm having an issue with the stall horn in my M20J. A few months ago it start failing intermitently. Now it's not working at all.

I was almost sure it was the sonalert. This not only because I think is the most likely element to fail, but also because when I moved the stall vane I could hear a quick "chirp" and then nothing. So my train of though was, the switch is making contact and the sonalert is dead and it just make a brief noise and then nothing.

So yesterday I went and changed it with a brand new (which I tested before installing it) and nothing, same sympton. As I move the stall vane I hear a "chirp" and then nothing.

I review the schematics and the circuit is dumb simple, basically wires, sonalert, fuse and the switch. So doesn´t seem to be too many things to trouble shoot. Any ideas?

[mike_elliott]

A lot of the stall viens get "bent" by refuelers and made inop. Something to check

[EricJ]

The easy way to test the sonalert in-circuit is just jumper from the ground side of the sonalert to a ground anywhere, including any airframe sheetmetal.   Basically touch a wire to the airframe and then to the sonalert, and it should come on.   The stall vane switch just completes the circuit to ground, so it does exactly the same thing.   That'll verify that everything up to and including the sonalert works.

If the sonalert does pass that test, then it is most likey the switch at the vane, which is a pain to get in and out and kind of a pain to fix, but is doable.   There is a thread here somewhere with multiple examples showing exactly how to take the switch apart and rehab it.   Another possibility is that the connections on the switch are a bit corroded and just need to be cleaned.   Mine was intermittent and I think was just a corroded connection, but I had already taken it out and so disassembled it and cleaned it up.

 

[redbaron1982]

19 minutes ago, EricJ said:

The easy way to test the sonalert in-circuit is just jumper from the ground side of the sonalert to a ground anywhere, including any airframe sheetmetal.   Basically touch a wire to the airframe and then to the sonalert, and it should come on.   The stall vane switch just completes the circuit to ground, so it does exactly the same thing.   That'll verify that everything up to and including the sonalert works.

If the sonalert does pass that test, then it is most likey the switch at the vane, which is a pain to get in and out and kind of a pain to fix, but is doable.   There is a thread here somewhere with multiple examples showing exactly how to take the switch apart and rehab it.   Another possibility is that the connections on the switch are a bit corroded and just need to be cleaned.   Mine was intermittent and I think was just a corroded connection, but I had already taken it out and so disassembled it and cleaned it up.

 

 

Cool, great idea on how to test it. I will do that tomorrow and continue trouble shoorting. I guess having TKS would make any work on the leading edge more difficult... :S

[DCarlton]

I had trouble with my stall warning about six months ago.  Somewhere on Mooneyspace is a thread with schematics and pics discussing where some folks have taken them apart to clean them and get them working.  Mine was stuck with the horn sounding the minute I turned on the power switch.  If you can find that thread it might be helpful.  We were able to clean mine and get it working.  
 

just saw Eric’s same comment 

[Pinecone]

If it got bent it may not be moving far enough to make good contact.

Call Maxwell, they can tell you how to straighten it without removing it.

Mine got bent and was always on.  They fixed it for me at Mooney Max.

[PT20J]

The vane is heat treated and very stiff. It’s most likely a bad connection or a bad switch.

[redbaron1982]

Thanks guys. I do not thing the issue is that the vane is bent. Between the time the stall warning was working ok, and now, the airplane has been always under my control, including refueling. Nothing happened that could result in the vane being bent.

I will try first the troubleshooting that @EricJ suggested (grounding the negative terminal on the stall warning sonalert) and see what I find.

[redbaron1982]

Is this the thread y'all are referencing to? 

 

Edited January 21, 2024 by redbaron1982

[DCarlton]

12 minutes ago, redbaron1982 said:

Is this the thread y'all are referencing to? 

 

That’s not the thread I saw but now I’m wondering if your J switch is different from my F switch… if your switch is like this switch.  I’m not a mechanic.  Perhaps someone knows if there were differences.  

[Danb]

My stall horn is irratic starting at 40 knots plus annoying in the wind. Don’t know what to do about it.

[DCarlton]

Seems like there’s a market gap here for new parts.  

[Pinecone]

1 hour ago, PT20J said:

The vane is heat treated and very stiff. It’s most likely a bad connection or a bad switch.

Except they get bent all the time.

[PT20J]

24 minutes ago, Pinecone said:

Except they get bent all the time.

 Must be something about yours - maybe they forgot to heat treat it.

[mike_elliott]

1 hour ago, Danb said:

My stall horn is irratic starting at 40 knots plus annoying in the wind. Don’t know what to do about it.

Yours is the FIKI stall switch Dan, which is a different bea$t. Don not able to rectify for you?

[Danb]

Thanks 

[EricJ]

Here's the other thread with the multiple accounts of dissecting the lift detector switch.
 

 

[Pinecone]

Considering the number of reports of the vane being bent by people leaning on them when fueling, and that Maxwell has figured out a way to fix a bent one without removing the switch, mine is pretty normal.

[redbaron1982]

I finally got to the root cause of this issue, it was the sonalert. I got confused because I have three sonalerts on the pilot foot well (one for the AP, the other two I'm not sure, and thought they were the gear and stall warnings).

It turns out that the sonalerts for gear and stall warning were in the headliner, as the POH indicates. I replaced the stall one, and problem solved.

Thanks God it was not the switch, because I think the only solution would have been to remove one of the TKS panels... take a look at the picture, no access from outside and the nearest inspection panel is too far to reach from the inside.

[EricJ]

26 minutes ago, redbaron1982 said:

I finally got to the root cause of this issue, it was the sonalert. I got confused because I have three sonalerts on the pilot foot well (one for the AP, the other two I'm not sure, and thought they were the gear and stall warnings).

It turns out that the sonalerts for gear and stall warning were in the headliner, as the POH indicates. I replaced the stall one, and problem solved.

Thanks God it was not the switch, because I think the only solution would have been to remove one of the TKS panels... take a look at the picture, no access from outside and the nearest inspection panel is too far to reach from the inside.

Glad you got it sorted it out!   The switch is reachable from that access panel, it's just not easy.   I suspect that the fasteners for the switch are captive under the TKS so that it can be removed without disturbing the TKS system.