M20J Landing Gear Warning Horn Sounding in Flight Tech Help

[martinboyd]

Hi guys,

     I wanted to reach out and see if any one has ran across this issue before. I have put some time into diagnosing and repairing the issue but have not found the correct fix yet. 

1993 M20J serial number 24-3xxx range area after flying 5 minutes the landing gear warning horn sounds in flight with normal flight speeds way above stall. Pulling the gear warning breaker quiets the alarm. It is not the stall horn coming over the overhead speaker, as it has a different tone. We can duplicate this on the ground on jacks at the same outside ambient temperature and the alarm will sound after 5 minutes with ground power applied and landing gear retracted.  I believe it is temperature sensitive. The flight was at 72 degrees F and it took 5 minutes for the alarm to sound.  We later tested again on a cold day 50 degrees F and it took 13 minutes to sound the alarm. Then again on a 40 degree day it took 23 minutes for the alarm to sound. The alarm starts out with a slow growl that gets louder and then grows into the full alternating tone.  If you retard the throttle or cycle the breaker the alarm will stop for a few moments then start back again. This seems like either something is getting hot from the avionics , power being on and warming up, or electricity bleeding off from another source. We have disconnected the pitot pressure switch to eliminate that input. We have replaced and adjusted the throttle switch. We have replaced the alarm audio control box above the pilots rudder petals. The only other limit switches I can seem to find in the system are the up and down limit switches that tell the gear motor when to turn off and not over extend. I am scared to disconnect one of those for test purposes and to damage from over extension. The serial number 24-3xxx range area TBA manuals and schematics are a bit hard to navigate around this concern. Any of you guys have any suggestions? Thanks

[Slick Nick]

I was going to say your throttle switch, but since you've replaced it already, maybe you need to start looking for a fault in it's wiring.

[PT20J]

I’d check this diode for leakage.

[martinboyd]

46 minutes ago, PT20J said:

I’d check this diode for leakage.

I seen the diode in the schematic but have not tested it yet. That is a good idea. 

[martinboyd]

I have time to work on this project again today. I am having trouble locating the diodes on the aircraft? Does anyone have a good idea of the location and part number? I checked the gear switch, warning control box, gear relays under the belly, and gear motor. Thanks

[PT20J]

Probably at the connector shown in the schematic. I’d remove the back shell and look in there. Part number will be in the electrical wiring diagrams section at the end of the service manual for the appropriate serial number.

[martinboyd]

The connectors listed in the schematic are Gear control PL09A CL09A and Electric Gear PL14A. I do not have any information on those connection locations in the harness. 

[martinboyd]

The names gear control and electric gear are not actual components in the system but names describing the harness plugs the best I can tell. I have not found any documentation stating station numbers of the locations etc.

[PT20J]

If the connectors are not located in the belly, they are likely located on the left sidewall behind the carpeted side panel where most of the wiring runs.

[martinboyd]

On 12/6/2024 at 3:34 PM, PT20J said:

If the connectors are not located in the belly, they are likely located on the left sidewall behind the carpeted side panel where most of the wiring runs.

One connector and diode ended up being in the left sidewall to the left of the pilots seat. The other connection with diode is the actual plug that connects to the gear switch at top center of instrument panel. That one was very difficult to dig out. I lost count of the amount of zip ties. 

 

[martinboyd]

On 12/4/2024 at 12:30 PM, martinboyd said:

I seen the diode in the schematic but have not tested it yet. That is a good idea. 

I installed the 2 new diodes and tested the system for 40 minutes yesterday without the gear alarm sounding. I believe that fixed the problem. We will do more operational checks Monday and then turn it loose for a flight test. I did find a nice mouse nest while I was behind the pilots side wall panel. 

Edited December 14, 2024 by martinboyd

[martinboyd]

2 hours ago, martinboyd said:

I installed the 2 new diodes and tested the system for 40 minutes yesterday without the gear alarm sounding. I believe that fixed the problem. We will do more operational checks Monday and then turn it loose for a flight test. I did find a nice mouse nest while I was behind the pilots side wall panel.