Interesting stall warning horn behaviour

[Tommy]

Hi guys! Here is another one for you intelligent good looking Mooniacs out there!

 

My stall warning went completely silent recently so I replaced the sonalert (found one wire was detached from the sonalert!) and now it's blaring when with Master Switch (and continues even with CB pulled) and silent as soon as the Radio Power On? 

 

What does it mean?

 

 

[Marauder]

Hi guys! Here is another one for you intelligent good looking Mooniacs out there!

My stall warning went completely silent recently so I replaced the sonalert (found one wire was detached from the sonalert!) and now it's blaring when with Master Switch (and continues even with CB pulled) and silent as soon as the Radio Power On?

What does it mean?

Can it be wired backwards? Positive/negative reversed? What is confusing is the radio master influence. I would check for polarity, no chafing wires and also that you are using the correct type Sonalert.

[MB65E]

I replaced mine with the new sonolert from lasar. For some reason, Turns out there was too much of an Amprage draw in the old switch in the wing to really make it work. I went back to the old boat anchor and it worked fine. (Mine had an issue with the gear warning portion)You might have some corrosion in the switch that's keeping it on. The new sonolert will take less power to run most likely.

Also, the new gear laser sonolert is really week to hear in the cockpit...but Better than an INOP one from 1964.

Matt

[Tommy]

Well it does the same thing with both new and old sonalert.

[Marauder]

Well that is an interesting twist of events. I am still troubled by the role of the radio master in this. If you take this out of the equation, it sounds like a short in the wiring. Since you were working with the wiring for the Sonalert, I would start there.

[N201MKTurbo]

Did you hook up the correct wire? Does it work OK after the avionics master is turned on? Have you put a meter on it to see what voltage and polarity you have in the various conditions?

 

With  a digital meter measure with the black wire on the airframe and the red wire on each of the sonalert terminals.

 

With the master on and the stall switch on and off

 

with the master and avionics on and the stall switch on and off.

 

report back and we can give you a much better assessment of the problem.

[Tommy]

Ok the polarity was correct (stall warning SC628)

The voltage was 9 to 10 v when master is on - blaring

Then dropped to 1 or 2 when avionics is on - silent

I also replaced my autopilot one with a new sonalert and it too is blaring (my head hurts!!) when master is on then went reasonably quiet (but can still hear the horn warning) with radio master on.

I haven't filed the bird yet so not sure how it all pans out...

[Marauder]

Which Sonalerts did you use? The stall one I thought was a SC-628.

[Tommy]

Yes sc628

[Marauder]

You certainly are a strong runner for the "Weird Problem of the Year" award. If it wasn't for the fact that the old ones are doing the same, I would have said you had a couple of defective Sonalerts.

When you replaced the new ones with the old ones, did you replace both new ones with the old ones at the same time? If not, I could some sort of electrical short in one of the new ones.

Your voltage is really low as well with the master on. I would see if the same thing happens when you fly it.

[N201MKTurbo]

Ok, did you do the measurements from ground to the individual terminals? I can't tell from your reply exactly how you did the measurement. I assume you were measuring across the terminals of the sonalert. That just verifies your observation but doesn't help with the diagnosis. Also how does it behave when the stall switch is activated?

[Tommy]

I measured the voltage across the wire - all of which are clearly labelled. If I swap the polarity then the horn went completely silent.

The stall vane doesn't seem to be doing anything at all with or without avionics switch on.

It all kind of started after a bit of rain two weeks ago. The stall warning started blaring pre flight then gone completely silent which was unusual. Before that the stall warning is working intermittently.

I am just surprised to find one of the wire came off the stall warning.

Forgot to mention the autopilot warning now is working!

I am just hoping that after drying the stall warning vane with a bit of flying. Everything will be working again!

[N9405V]

How do you get to the switch on the wing? I tried by taking a panel off the bottom, but my god, you'd have to be a friggin scarecrow to get an arm up there...

[MB65E]

Yes Sir! I think mooney installed it before the wing was rivet together. Lol. Long extensions with some tight seal and fuel lube on your finger to get them threaded.

[Tommy]

Hi guys! So today I revisited my stall warning issue.

 

I have taken down the cabin speaker section and these are my obervations

 

1) the stall warning AND autopilot warning will both come on when only Master Swtich is on and remain on EVEN when stall warning breaker is pulled. (pulling gear warning breaker has no effect at all)

2) the autopilot warning will extinguish as soon as the avionics switch is on but the stall warning will continue to sound. It can, however, be silenced when stall warning breaker is pulled.

3) when I roll my J down the runway the stall warning sound gets quieter and evetually it will not be audible. However it remained that way for the rest of the flight - even when I am pretty convinced that I am about to stall the plane on landing - and no sound after landing.

 

Attached are photos of sonalert attachment and stall warning switch.

 

So do I have a stuck stall switch? Or was it a bent horn?

 

Help please!

[Marauder]

If the stall horn comes on with the master on and won't shut off when the breaker is pulled, something is providing power to it. I would pull all of your breakers if you hadn't already and see if you can get it shut off. Very possible that the stall warning is wired to another breaker.

As for it coming on during ground ops and turning off when under motion, it sure sounds like it is wired backwards at the stall vane. Did anyone work on it before this problem started?

With the stall vane closed, the horn should be off. If you pull gently up on the vane, it should start coming on. If it is not working this way, something is wired incorrectly. I doubt it work this way if it was broken.

[Tommy]

It was working intermittently before. The problem only surfaced recently after having the aircraft covered in morning dew a few times. No one has worked on it beforehand.

[Tommy]

Ok looks like I was wrong! The horn came on today whilst I am taxiing to the hangar after flight! Love the idea of a backward wiring, Maraude, but if that's the case, wouldn't the horn stays on all the time (on the ground or in flight) and extinguish with stalling? Am I missing something here?

[Marauder]

Ok looks like I was wrong! The horn came on today whilst I am taxiing to the hangar after flight! Love the idea of a backward wiring, Maraude, but if that's the case, wouldn't the horn stays on all the time (on the ground or in flight) and extinguish with stalling? Am I missing something here?

You're right. Since we are grasping at straws to figure this out, I threw out the possibility that it got miswired at some point. It would at least explain it being on during ground ops. If the stall vane was able to move at the wrong angle of attack during motion, it would explain why it would go off under motion.

It could be that you just have a bad stall warning switch. Maybe internal to the switch something has broken loose and is shorting out the switch.

At a minimum, your mechanic should be able to put a voltmeter on the horn side and see that it activates correctly when the switch is moved up and down. I suspect it isn't and I'm afraid the next step is getting to the stall switch in the wing itself and checking it out.

You have some sort of electrical gremlin. The stall switch is not an overly complicated mechanism and it should be eliminated as a cause. If the switch is good, the only other possibility is some sort of short -- but a weird one...

[Tommy]

It sure is weird! If it is shorted or stuck on, I would've thought it will be blaring flight or ground. And the mystery remains why would both Autopilot and Stall warnings be sounding when and only master switch is turned on and neither can be silenced with pulling of the breakers.

 

And why is it such a pain to get to the stall switch wiring! Also, nayone knows the P/N for a J's stall switch? Thanks!

 

 

[Marauder]

It sure is weird! If it is shorted or stuck on, I would've thought it will be blaring flight or ground. And the mystery remains why would both Autopilot and Stall warnings be sounding when and only master switch is turned on and neither can be silenced with pulling of the breakers.

And why is it such a pain to get to the stall switch wiring! Also, nayone knows the P/N for a J's stall switch? Thanks!

I forgot about the autopilot horn involvement. The circuitry should be straightforward and simple. With master off, the ground wire for the horns should be common to ground. Has anyone checked to see if the ground resistance is good? When the master is off, the power line to the horns should not be grounded. If you run an ohm meter from the disconnected power lead to the horn and touch it to ground, it should be an open circuit.

I know these are basic steps and hopefully someone checked them.

[larryb]

So lets look at this one point at a time. First, the stall horn sonalert has it's positive terminal connected to a circuit breaker. So it gets +12 v all the time when the master is on. The negative terminal is grounded by the stall warning switch.

1. So the wire was disconnected. I'm thinking a previous owner had this same problem and disconnected the sonalert rather than fixing it.
2. Sonalert sounds when master switch on. This indicates a short on the negative connection. That short can be anywhere between sonalert and the stall switch. The wire could be chafing against a sharp edge. Could be the switch, but generally switches fail open and not closed.
3. Post #15. Stall warning not affected by stall warning CB, and is affected by radio. To me this means that the positive terminal of the sonlaert is NOT connected to the stall warning CB, but instead connected to something else. This is confirmed in post #7 where the voltage is slightly too low at 9 to 10 v.

I'm thinking that somebody did some wiring in the sonalert area and got the wires mixed up. Here is what I would do going forward:

1. Positively identify the stall warning switch wire. That means disconnect it, and measure the resistance of that wire to GND while you have a helper manually activate the switch on the ground. You should see an open circuit when the switch is at rest, and close to 0 ohms resistance to GND when the switch is activated.
2. Positively identify the power source. Disconnect the positive wire from the sonalert, and measure the voltage compared to GND. This should be controlled by the master switch and the stall warning CB. If the voltage is not controlled by the stall warning CB, try to identify the wire in the sonalert area that is.

I'd like to elaborate one point made in a previous post. If you measure resistance to GND of a power lead, with power off, you will likely see 0 ohms, or close to it. If your's were the only load, as is the case with the stall warning circuit, you would see an open circuit. But generally your load is not the only load, and the side-loads will look like a short to ground. As an illustration, just measure the resistance of a regular light bulb.

 

Larry

[Tommy]

Big thanks to you Larry!! It gives me something to work on. I need to point out, however, the mistake that I have made.

It was actually the AP warning that the previous owner disconnected not the Stall warning. And it is also the AP warning that goes quiet when radio switch is on.

The Stall warning remains on at all time except in flight.

The whole system was working - albeit intermittently - the first few months that I had the aircraft but now it's like this after drenching it with morning dew.

[Tommy]

Ok I finally had a look at my stuck stall warning with my A&P. We found that there is an earthed connection between the sonalert and the stall switch. The switch is working fine. The logical step is to trace the negative wire all the way back to the stall switch but it's it such a pain! My question is where does this wire normally fail (chafing)?

Should I go from stall switch to the sonalert or the other way round?

Thanks

[larryb]

HI Tommy:

 

     I haven't had the pleasure of tracing the stall warning wire back to the switch. So I don't have any specific ideas on where it would chafe. But I'd look at places where the wire bundle makes sharp turns and goes through bulkheads. So I'm thinking the wing root area. Positively identifying one white wire in a sea of white wires is a pain. Sometimes however the wires are printed with a circuit number in black. My plane is. I think I'd start at the stall switch and work backwards, since that is the easy access area. Just pull off all the under-wing inspection plates and get in there with a strong flashlight and a mirror. 

 

     I was tempted to say just run a new wire and abandon the old one. However, if one wire is chafed, others might be too. And I might want to know that, and fix that. So, if it were my plane, I'd just start tracing.... Good luck.

 

Larry