Mooney M20J Gear Unsafe / Down WCAs & Floor Mounted Gear Position Indicator Lights Not Working

[Tester22]

 

• 1978 Mooney M20J (Missile STC)
• TTA:  3718.8
• Over the last 4 flights, when reducing power from a high throttle setting, the gear up warning horn would sound, sometimes when power was above 20 inHg
  • As power was further reduced, the gear up warning honr would silence, usually around 18 in Hg of manifold pressure
  • With further power reduction, the gear up warning horn would sound, usually around 15-16 inHg of manifold pressure
    • The gear warning horn sounded twice (horn alert on, off, then on) on two of the last four flights
• Immediately prior to the gear inidciation issue manifesting, A&P installed a manual tachometer and engine manifold pressure gauge that I had had removed during an avionics upgrade two years ago
  • Post maintenance check flight after the reinstallation of the gauges was nominal (all gear lights operated normally)
    • Gear Unsafe WCA (Warning/Caution/Advisory) illuminated on the panel when gear were in transit
    • Gear Down WCA illuminated once the gear were fully down and the Gear Unsafe WCA extinguished at the same time
    • The floor mounted gear position indicator (FMGPI) indicated gear down and the backlight under the plexiglass was illuminated
• During the next flight (2.0 hours) none of the lights illuminated when the gear was lowered
  • No gear unsafe WCA, no gear down WCA, no backlight in the FMGPI
    • The actual gear position off the FMGPI indicated gear was down (two black lines aligned as normal)
      • Waived off, entered the overhead delta, retracted the gear
        • No gear unsafe WCA, the FMGPI indicated the gear were up
          • All CBs were in
          • Emergency gear handle was stowed and locked
          • Instrument panel dimmer switch was off
      • In the overhead delta, I once again lowered the gear
        • Runtime of gear motor and mechanical thunk were consistent with gear down and locked
          • However, the Gear Unsafe WCA did not illuminate while the gear were in transit and the Gear Down WCA did not illuminate once the gear were down nor did the FMGPI backlight illuminate
            • FMGPI indicated gear were down (two black lines aligned)
              • Landing was uneventful
  • A&P and I inspected area around the engine tachometer and manifold pressure (installed just to the left of the CB panel)
    • No loose wires
    • Noted 7 of 8 round wires attached in the vicinity of the upper glare shield cross brace (just to the right of the center panel mounted upper avionics stack)
      • Same as when we commenced the installation of the gauges
  • Put the aircraft on jacks
    • Lowered the gear
    • Noted no power to FMGPI backlight
  • With the gear down and locked, A&P wiggled one of the wires attached to the throttle mounted microswitch
    • Gear Down WCA illuminated and FMGPI backlight illuminated
    • Raised the gear
      • Gear Down WCA extinguished
      • Gear Unsafe WCA posted while gear was in transit
      • FMGPI light extinguished
      • Lowered the gear; all three lights worked as they should
        • Wiggled the wire again and the Gear Down WCA and FMGPI backlight extinguished
          • • • A&P thought the connector was loose inside the throttle mounted microswitch
                • Replaced throttle mounted microswitch with PART: 480-2877-ND
                • DESC: SWITCH SNAP ACTION SPDT 15A 125V MFG : HONEYWELL SENSING AND PRODUCTIVITY SOLUTIONS / V3-1 from DigiKey.com
                • All gear lights were inop regardless of gear position
                • It was quitting time so we couldn’t even wiggle some wires

 

Does anyone have any idea what may be happening?

 

Does anyone have the wiring diagram for the circuit that includes the throttle position microswitch, gear WCAs and FMGPI backlight?

 

Any thoughts and insights would be appreciated.

Edited January 8 by Tester22
Modified for clarity

[EricJ]

Volume 2 of the M20J Service Manual has the wiring diagrams organized by applicable serial numbers.   I think it's available in the download section here, and if not, they are available elsewhere online for free.

[PT20J]

The power for the gear down, gear unsafe, and floor gear position light comes from the Gear Warning circuit breaker and the lights are controlled by the gear up and gear down limit switches. This circuit also supplies power to the gear warning sonalert. With the plane on jacks and the gear up, closing the throttle should cause the sonalert to sound if the throttle switch is working/adjusted properly and this is a check that the gear warning circuit has power. If not, you’ll need to trace power through all the connections to find the issue. 

The throttle switch should only control the sonalert, so it’s curious that wiggling its wires affected the lights. Since the switch has been replaced and wiggling the wires no longer has an effect, I would ignore this for the moment and check that the gear warning circuit has power first.

[1980Mooney]

On 1/6/2025 at 3:24 PM, Tester22 said:

 

• 1978 Mooney M20J (Missile STC)
• TTA:  3718.8
• Over the last 4 flights, when reducing power from a high throttle setting, the gear up warning horn would sound, sometimes when power was above 20 inHg
  • As power was further reduced, the gear up warning honr would silence, usually around 18 in Hg of manifold pressure
  • With further power reduction, the gear up warning horn would sound, usually around 15-16 inHg of manifold pressure
    • The gear warning horn sounded twice (horn alert on, off, then on) on two of the last four flights
• After the fourth flight with an amorous gear warning horn activation, my A&P installed a manual tachometer and engine manifold pressure gauge that I had had removed during an avionics upgrade two years ago
  • Post maintenance check flight after the reinstallation of the gauges was nominal (all gear lights operated normally)
    • Gear Unsafe WCA (Warning/Caution/Advisory) illuminated on the panel when gear were in transit
    • Gear Down WCA illuminated once the gear were fully down and the Gear Unsafe WCA extinguished at the same time
    • The floor mounted gear position indicator (FMGPI) indicated gear down and the backlight under the plexiglass was illuminated
• During the next flight (2.0 hours) none of the lights illuminated when the gear was lowered
  • No gear unsafe WCA, no gear down WCA, no backlight in the FMGPI
    • The actual gear position off the FMGPI indicated gear was down (two black lines aligned as normal)
      • Waived off, entered the overhead delta, retracted the gear
        • No gear unsafe WCA, the FMGPI indicated the gear were up
          • All CBs were in
          • Emergency gear handle was stowed and locked
          • Instrument panel dimmer switch was off
      • In the overhead delta, I once again lowered the gear
        • Runtime of gear motor and mechanical thunk were consistent with gear down and locked
          • However, the Gear Unsafe WCA did not illuminate while the gear were in transit and the Gear Down WCA did not illuminate once the gear were down nor did the FMGPI backlight illuminate
            • FMGPI indicated gear were down (two black lines aligned)
              • Landing was uneventful
  • A&P and I inspected area around the engine tachometer and manifold pressure (installed just to the left of the CB panel)
    • No loose wires
    • Noted 7 of 8 round wires attached in the vicinity of the upper glare shield cross brace (just to the right of the center panel mounted upper avionics stack)
      • Same as when we commenced the installation of the gauges
  • Put the aircraft on jacks
    • Lowered the gear
    • Noted no power to FMGPI backlight
  • With the gear down and locked, A&P wiggled one of the wires attached to the throttle mounted microswitch
    • Gear Down WCA illuminated and FMGPI backlight illuminated
    • Raised the gear
      • Gear Down WCA extinguished
      • Gear Unsafe WCA posted while gear was in transit
      • FMGPI light extinguished
      • Lowered the gear; all three lights worked as they should
        • Wiggled the wire again and the Gear Down WCA and FMGPI backlight extinguished
          • • • A&P thought the connector was loose inside the throttle mounted microswitch
                • Replaced throttle mounted microswitch with PART: 480-2877-ND
                • DESC: SWITCH SNAP ACTION SPDT 15A 125V MFG : HONEYWELL SENSING AND PRODUCTIVITY SOLUTIONS / V3-1 from DigiKey.com
                • All gear lights were inop regardless of gear position
                • It was quitting time so we couldn’t even wiggle some wires

 

Does anyone have any idea what may be happening?

 

Does anyone have the wiring diagram for the circuit that includes the throttle position microswitch, gear WCAs and FMGPI backlight?

 

Any thoughts and insights would be appreciated.

Per your question on wiring diagrams:

• All Electrical Components and Wiring are shown in the Service Manual
  • All the Electrical components are listed at the end of Volume 1 of the Service Manual.
  • The wiring schematics are in Volume 2 of the Service Manual.
• Below is a link with 2 downloads.

Since you have a 1978 model, then you need to look at Schematic 800330 (E1) which is for S/N 24-0378 thru 24-0757

I also have a J converted to a Missile.  There are a few things that your A&P has done that make no sense to me.

• "After the fourth flight with an amorous gear warning horn activation, my A&P installed a manual tachometer and engine manifold pressure gauge that I had had removed during an avionics upgrade two years ago"
• "A&P and I inspected area around the engine tachometer and manifold pressure (installed just to the left of the CB panel)"

The Tach and Engine Manifold Pressure have nothing to do with the Gear Warning System.  It is only throttle position.  However, the complicated thing is that the throttle micro switch works in conjunction with the main gear lever/switch, and up/down gear limit/position sensing microswitches (in the belly). The micro switch on the throttle is the primary switch.  It simply senses a cut out in the throttle shaft.  When raising the gear, any intermittent problems with the airspeed switch and gear over-ride switch can also make diagnosis more complicated.

Because of the interconnected design of switches and limit switches it can be hard to diagnose.  It sounds like by trial & error wiggling, your A&P determined that the throttle micro switch had intermittent problems.  Clearly you have multiple intermittent problems.  He needs to be checking the continuity of the wiring per the Schematic.

Did he check the connections and continuity of the Up/Down limit microswitches in the belly?  Also did he adjust the microswitch on the throttle per the Service Manual?

Here is a general article on the gear control system by KNR:  201901 Electric gear safety system

 

 

 

Edited January 8 by 1980Mooney

[Tester22]

Thanks for the input, everyone. The reference to the re-installation of the tach and manifold pressure gauges was proivded only to document the most recent maintenance that was performed. And, the work was performed BEFORE the gear issue manifested; not after, as I originally wrote. (N.B. the original post was editted to increase clarity.) The fact that the guages were installed next to the CB panel was of interest in that, perhaps, a ground or power wire had been dislodged. A Mooney tech rep provided both the gear wiring diagram as well as a simplified schematic, attached. Troubleshoting is ongoing. I’ll report back with the results. Thanks again. 

2 GEAR DN LOCKED.PDF 800330_E1_email1.pdf

Edited January 8 by Tester22
Improve clarity

[Tester22]

We made some headway today.

-The throttle mounted microswitch indicates 12V recevied whenever the gear are up; no power when the gear are down

-The Gear Warn CB is showing 12V on both sides

 

Both A&Ps are starting to supsect the belly-mounted gear down limit switch. There seem to be 5 wires that terminate at the switch. It’s not clear which wires coorespond to the Gear Down and Gear Unsafe WCAs as well as the FMGPI. Any insight would be appreciated. 

 

If the gear down limit switch needs to be replaced, do you have the wiring diagram and wire numbe codes? We cannot find a cannon plug or similar quick disconnect that would facilitate a quick remove and replace. Also, can you verify that the nute for the mounting bracket is in inside the fuselage and beneath the carpets? They’re guessing it may be located under the rear seat.

 

N.B. While both A&P/AIs at my home field are very experienced, this is the first time that either of them has had to troubleshoot a Mooney landing gear issue. 

 

Thanks everyone. 

[PT20J]

5 hours ago, Tester22 said:

The throttle mounted microswitch indicates 12V recevied whenever the gear are up; no power when the gear are down

Per the simplified training schematic, this indicates that the gear down limit switch is operating properly. 

[Tester22]

Can anyone send a pic of their throttle microswitch? The switch that was mounted in mine is attached (two pics, red on one side, black on the other) Honeywell 1SM1-T p/n 480-2877-ND. This also corresponded to a cross-reference parts compatibility list on the Textron webpage. However, the Mooney tech rep advised that the microswitch is Honeywell 1SM1-T, (stock photo attached). The switch that was removed and replaced was rated at 15A@125V whereas the Mooney recommended switch is rated at 5A@125V as well the actuation force was higher on the installed switch vice the Mooney recommended switch, though I don’t see how either difference could impact continuity and operation. 

If the throttle mounted microswitch is showing 12V with the gear up and no voltage when the gear are down, and the Gear Warn CB shows power on both sides, does anyone have any idea what the next troubleshooting step should be?
 

Edited January 10 by Tester22

[Fly Boomer]

18 hours ago, PT20J said:

Per the simplified training schematic, this indicates that the gear down limit switch is operating properly. 

I'm trying to imagine what the throttle microswitch looks like when mounted.  Is the microswitch activated by a thinning of the throttle shaft on the last inch or so as you pull it out?

[EricJ]

36 minutes ago, Fly Boomer said:

I'm trying to imagine what the throttle microswitch looks like when mounted.  Is the microswitch activated by a thinning of the throttle shaft on the last inch or so as you pull it out?

Yes, there's a spot on the throttle cable specifically made with a switch detent to activate a switch on low throttle.

[Tester22]

Attached image is the old switch as installed. You can see the slot in the throttle cable housing where the throttle cable is able to actuate the switch. 

[PT20J]

I don’t have a picture of my throttle switch, but I’m pretty sure it is a Honeywell V3-1. 

Does the gear horn sound under the following conditions?

1. Gear up, throttle closed.

2. Gear down, no airspeed, gear switch up (bypass red button should also illuminate)

Are you still not getting any Gear Down, Gear Unsafe, floor gear position lights inder any condition of gear up or down?

 

[Fly Boomer]

6 hours ago, Tester22 said:

Attached image is the old switch as installed. You can see the slot in the throttle cable housing where the throttle cable is able to actuate the switch. 

Thanks.  Words are good, but nothing beats a picture.

[Tester22]

11 hours ago, PT20J said:

I don’t have a picture of my throttle switch, but I’m pretty sure it is a Honeywell V3-1. 

Does the gear horn sound under the following conditions?

1. Gear up, throttle closed.

2. Gear down, no airspeed, gear switch up (bypass red button should also illuminate)

Are you still not getting any Gear Down, Gear Unsafe, floor gear position lights inder any condition of gear up or down?

 

1. Yes

2. Yes

And, correct, no Gear Down, Gear Unsafe nor floor position light regardless of physical gear position (up, down or in transit). 

We are scratching our heads…

[PT20J]

10 hours ago, Tester22 said:

1. Yes

2. Yes

And, correct, no Gear Down, Gear Unsafe nor floor position light regardless of physical gear position (up, down or in transit). 

We are scratching our heads…

The fact that the gear waring sonalert operates when the gear is up and the throttle is closed indicates that you have power from the Gear Warning breaker to the Gear Down Limit Switch NC contact all the way to the throttle switch. The floorboard lamp is connected directly to the NO contact on this switch and should be relatively easy to access. I would check that this light is getting power when the gear is down and that it is not burned out.

The Gear Unsafe light gets power from the C contact on the Gear Up Limit Switch. This switch has contacts that are easier to access than the Down Limit Switch and I would check that there is power to the C contact when the gear is neither completely up or down. Also, check that the lamp lights when the Test button is pressed on the annunciator panel.

The Gear Down lamp dims when the Nav lights are on, so you could cycle the nav lights and see if that makes any difference. Since two of the non-functioning lights are in the annunciator panel, there might be a fault here. There is no lamp test for the Gear Down lamp, so the only way to eliminate a burned out lamp is to replace it or swap it with a known good lamp. You can also remove the connector from the back of the annunciator panel and check voltage on the pins for the lamps to see if the problem might be within the panel. The circuits for these gear lights is pretty simple, but there are series diodes that could have failed.

[1980Mooney]

On 1/8/2025 at 5:56 PM, Tester22 said:

If the gear down limit switch needs to be replaced, do you have the wiring diagram and wire number codes? We cannot find a cannon plug or similar quick disconnect that would facilitate a quick remove and replace.

The wire number codes are clearly labeled and easily identifiable in the wiring diagram that you posted on Wednesday (800330 E1).

 

[Tester22]

11 hours ago, PT20J said:

The fact that the gear waring sonalert operates when the gear is up and the throttle is closed indicates that you have power from the Gear Warning breaker to the Gear Down Limit Switch NC contact all the way to the throttle switch. The floorboard lamp is connected directly to the NO contact on this switch and should be relatively easy to access. I would check that this light is getting power when the gear is down and that it is not burned out.

Skip, thank you for the additional troubleshooting tips. I appreciate it. The floorboard gear indicator light is NOT getting power with the gear down.

I'll report back once we've had a chance to continue troubleshooting. 

[Tester22]

10 hours ago, 1980Mooney said:

The wire number codes are clearly labeled and easily identifiable in the wiring diagram that you posted on Wednesday (800330 E1).

Yep

[PT20J]

1 hour ago, Tester22 said:

Skip, thank you for the additional troubleshooting tips. I appreciate it. The floorboard gear indicator light is NOT getting power with the gear down.

I'll report back once we've had a chance to continue troubleshooting. 

The schematic indicates that there is a connector (probably a knife disconnect) on the power lead to the floor lamp, so it would be good to check that it is secure. Then I would trace power from there back to the down limit switch NO wire. You know you have power on the C and NC contacts (because the down limit switch correctly switches power to the throttle switch depending on gear position), so it is possible that the switch is not making contact between the C and the NO contact. The down limit switch has two poles, one controlling the gear actuator and one controlling the lights. The pole controlling the motor seems to work because the gear actuator stops in the down position (if it didn't stop in the correct place the actuator ball screw would hit the mechanical down stop stalling the motor and tripping the gear actuator breaker). I would also verify that further depressing the plunger on the down limit switch does not activate the lights when in the down position before replacing the switch. If you do determine that the switch is bad, the replacement switch is a  Honeywell 1CH1-6; the original 1CH116-6 is obsolete unless you find some new/old stock somewhere.

A bad pole on the down limit switch would explain the malfunctioning floor and gear downlights, but not the gear unsafe light. So, there may be more than one issue.