LDG down light not working

[Guitarmaster]

Hey guys (and gals)....

I very recently bought a 75 M20F. It is very low time but has sat for a while. Obviously there will be little issues with a 40 year old plane, but I don't care. She is the fulfillment of a lifelong dream.

Anyway... I need your help.

Went flying and wrung her out a little, testing the low speed envelope, did about five touch and go's and returned her to the hanger. This morning, I pulled her out and found the green gear down light not working. It would not test with the press to test and the gear horn sounded. The "unsafe" light operates normally. Everything operated normally except the gear down light. Of course, the gear horn sounded with throttle retardation.

The manual indicator in the floor showed down and locked and there was no problems with the landing.

I am pretty sharp with electronics and have a maintenance manual coming. I have learned that the indicator half of the down limit switch will fail on occasion. Does sound like a likely culprit?

The plane sat for 14 years after the owner died and I am sure there is some grime in the switch. Is it possible to clean the contacts with contact cleaner?

Thanks in advance for your ideas!

[Super Dave]

If the indicator did not light with the press to test, it means the bulb is likely burned out.

[Guitarmaster]

Dave, thanks for the quick reply. I did think about that and swapped the bulbs from the unsafe indicator with luck.

If that was the case, why would the gear horn sound with the test?

I think there is probably a continuity test of sorts with that system. Thoughts?

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[Marauder]

I have a 75F. The annunciation panel test button will trigger the gear horn when at idle power. If you have it at run-up RPM the lights will come on but the horn will not sound. And if you have a burned out bulb, it will still sound. I will look in my service manual to see if I can figure out anything unusual to look for. It most likely is the down limit switch but I thought there was some sort of fault indication - like the gear down light blinking or the gear horn coming on.

[Guitarmaster]

Marauder... Thanks for the information! I did not know it would trip the horn at idle power. I don't remember it doing that when I bought it... Of course the gear down light was working then.

Come to think of it, I don't think I swapped both bulbs, just one. It would be odd, but maybe the light itself completes the warning circuit.

Somebody mentioned something about the nav lights having an effect on that light. I believe I had them on this morning while doing the test. I am now wondering if that nav relay is somehow affecting the system.

Thanks again for your help!

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[Marauder]

Bulb and down limit switch...

[Guitarmaster]

Excellent! Thanks!

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[Guitarmaster]

Do you know if there is a way to clean the contacts in this switch?

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[Marauder]

Marauder... Thanks for the information! I did not know it would trip the horn at idle power. I don't remember it doing that when I bought it... Of course the gear down light was working then.

Come to think of it, I don't think I swapped both bulbs, just one. It would be odd, but maybe the light itself completes the warning circuit.

Somebody mentioned something about the nav lights having an effect on that light. I believe I had them on this morning while doing the test. I am now wondering if that nav relay is somehow affecting the system.

Thanks again for your help!

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I don't remember where the cut-off rpm is when the horn stops working, but if you are at idle it will sound regardless if the green gear down light is working.

And yes, if the Nav lights are on, it dims the gear down light.

You got the part numbers for the bulbs? I think they are 330s. I do not think you need to have both bulbs working to complete the circuit. As long as the floor indicator is showing gear down, you're good.

[Guitarmaster]

Ok. Thanks for the quick reply and the information. I will let you know how it turns out. It would be nice if it was just bulbs, but I am rarely that lucky.

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[Marauder]

If you need copies of the schematics, let me know. I will make a copy and PM it for you. Will need your SN since there is a break-over point and different versions. Good luck. Just remembered, check for a poor connection on the black (Molex?) connector coming from the gear switch. Sometimes they get corroded. Use a good quality electrical connector cleaner like CRC.

[Guitarmaster]

I would LOVE the schematics. Thank you! I will check that for sure.

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[Guitarmaster]

I will get you the S/N tomorrow evening. Thought I had it with me, but I don't. Sent from my iPad using Tapatalk

[Guitarmaster]

Found it. 22-1286

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[Marauder]

We're a few SNs apart. I will send you over the gear schematics.

[Guitarmaster]

Thanks so much!

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[Marauder]

I just found a hand drawn schematic that I made back in the 90s, much simpler to understand the wiring. I will send this over as well.

[carusoam]

This may be in left field, but....

Do you guys have the independent lights that twist to open/close an iris?

Close the iris, lets no light out...

Open the iris, let's all the light out...

My 65C had the same surprise until I learned the magic. Manual gear leaves few surprises.

Best regards,

-a-

[Guitarmaster]

No. Two lights. "Unsafe". And "gear down". They are not the iris type.

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[Hank]

My landing routing is gear down on downwind, feel the thump and look for green light. Turn base, feel switch, look for green light. Turn final, feel switch, glance and light and look at floor indicator. Throttle to idle when clear of the trees [this should sound the horn if I've missed it on the checks], wind permitting of course. Flare and listen to the stall horn then the tires chirping.

 

Hope you enjoy your new ride. F's are very roomy!

[Guitarmaster]

Ok. I found the problem. With the nav lights on, the gear down light extinguishes. I am assuming with the nav lights on, the gear down light has a resistor added to the circuit to dim it. I am thinking that might be bad.

Anyway, with the nav lights off, it works just fine.

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