Gear Warning Circuit Breaker Tripping

[MikeOH]

1970 M20F

Just had the static system/transponder/encoder check done.  Panel was pulled apart looking for static system leak (found & fixed).  Short flight home, gear and gear down light worked fine.  Next flight when I put the gear down the green gear down did NOT illuminate.  I checked the CB and found it tripped; I reset and it immediately tripped again.  (NOTE: I checked it at pre-flight and it was NOT tripped; when the master was turned on the green gear down light was illuminated).

Tower said gear appeared down, floor indicator showed down, and I landed without problem. After landing I tried resetting the CB and again it tripped immediately.

While I'm an electrical engineer, the Mooney schematic (page 9-102) is less than user friendly.

Anyone had this happen, any ideas where the short could be?

I'm afraid this could get time consuming....

Thanks!

Mike

[carusoam]

Often, a gear CB tripper is the back-up manual gear system, partially deployed...

Your procedure before start-up probably details making sure the manual gear system is properly locked...

Often these systems get unlocked by back seaters and people moving about the cabin, not knowing...

Let us know... that could be a quick fix... Also be aware of what happens when the electric system and the manual system clash... a brass gear gets trashed making both gear systems unuseable...

I’m not sure if this applies to the F.  It is experience I remember from Andrew on his M20J... @Hyett6420 has pics...

For the gear position sensors for the F...  I think @Marauder May have the pics...?

Best regards,

-a-

[MikeOH]

I'm probably not understanding, but it is NOT the landing gear actuator breaker tripping, it is the gear warning system breaker.

[carusoam]

Mike,

I was slow to finish all that typing...

See if Marauder has some pics of the gear position sensors...  the system itself isn’t very complex.  But a bare wire is all it would take...

Best regards,

-a-

[MikeOH]

8 minutes ago, carusoam said:

Mike,

I was slow to finish all that typing...

See if Marauder has some pics of the gear position sensors...  the system itself isn’t very complex.  But a bare wire is all it would take...

Best regards,

-a-

Thanks!

I suspect a bare wire somewhere; it's a pretty solid short given the breaker trips as fast as I take my finger off of it.  I was thinking it was something behind the panel since the shop had just had the panel apart...but, that may well be an unwarranted assumption.  I just don't believe in coincidences, I guess!

[carusoam]

Do you have the yellow and green, push to test, twist iris, type of lights?  Start there and follow the wires...

The manual gear used a single position sensor on the hole where the handle secures... a total of about four ft of wire to chase...

it is a simple two output switch.  Sending power in either direction based on if the switch was open or closed.....

• Power
• switch
• lights
• Grounds

This is done from ancient memory of a PP...

Around here... some people are still learning that the fancy ancient lights are push to test and twist to dim... Some lights are so dimmed the light is completely blocked....

Happens all the time... 

Now go find that switch with its bare wire...?

Early on, See if testing the lights causes the CB to trip...? This could really simplify the challenge.

I’m a ChE, so my electrical skills aren’t very well developed...

Best regards,

-a-

[MikeOH]

3 minutes ago, carusoam said:

Do you have the yellow and green, push to test, twist iris, type of lights?  Start there and follow the wires...

The manual gear used a single position sensor on the hole where the handle secures... a total of about four ft of wire to chase...

it is a simple two output switch.  Sending power in either direction based on if the switch was open or closed.....

• Power
• switch
• lights
• Grounds

This is done from ancient memory of a PP...

Around here... some people are still learning that the fancy ancient lights are push to test and twist to dim... Some lights are so dimmed the light is completely blocked....

Happens all the time... 

Now go find that switch with its bare wire...?

Best regards,

-a-

No. Unfortunately, the F's electrical gear has these kind of odd rectangular indicator lights arranged in a vertical stack.  The light above the gear selector switch is amber illuminated when the gear is up, and the light just below the gear selector switch is the green gear down light.  They are not at all like the twistable 'iris' type in the earlier planes.

Yeah, time to start tracing wires....

[carusoam]

Often, Marauder is around after midnight east coast time... but I haven’t seen him here tonight...  yet. 

Best regards,

-a-

[MikeOH]

HAH!

It's getting to be my bedtime even here on the west coast.

Thanks, again!

[Stephen]

I had a similar problem after Avionics work in my 69 F.  A nutplate had fallen from behind panel and landed in the throttle posn sensor switch (visible by taking off the  throttle quadrant plastic cover)

[MikeOH]

I spent some time this afternoon troubleshooting.  The breaker tripping goes away when I disconnect the connectors (20LP04A) to the "control box" (20LP05A).  So, I'm pretty sure the issue is within the 'control box.'

Further, when I took off and brought the gear up, I remember hearing a weird buzzing sound for a few seconds.  Examination of the top of the 'control module' finds two 2N3055 transistors (TO-3 case) mounted.  The collector (case) wires are loose!  And, one has evidence of arcing!  I'm thinking that was the buzzing I heard and, if so, it is likely that the associated 2N3055 is now shorted (collector to emitter) as the case is isolated from ground by a mica insulating washer.

Does anyone have the schematic for the 20LP05A 'control box?'  This is the item that has the panel light dimmer and light sensor right under the glare shield above the pilot's instruments.

[Marauder]

Mike -- I have a hand drawn schematic of the gear system in my 1975 F. Not sure how much changed from a 1970 to 1975.

 

 

 

[MikeOH]

Thanks, Marauder, but that's somewhat different than my '70.

Most significantly, your gear warning lights (gear up, gear down) are directly driven.  In my '70 the lights are driven by the 'control box' whose schematic I seek.

[Marauder]

9 hours ago, MikeOH said:

Thanks, Marauder, but that's somewhat different than my '70.

Most significantly, your gear warning lights (gear up, gear down) are directly driven.  In my '70 the lights are driven by the 'control box' whose schematic I seek.

I suspect there were changes made between when Butler and Republic owned Mooney and this is why you are seeing the differences. Sorry i could not help.

[carusoam]

Since we know a lot more about the challenge...

Lets ask the pro... @M20Doc

Looking for guidance on a ‘70 M20F Gear ‘annunciator Panel’  electrical Short....  problem occurs when the gear is put in the up position.

The ‘70 Panel seems to be unique compared to earlier and later panels...

Best regards,

-a-

[MikeOH]

7 hours ago, carusoam said:

Since we know a lot more about the challenge...

Lets ask the pro... @M20Doc

Looking for guidance on a ‘70 M20F Gear ‘annunciator Panel’  electrical Short....  problem occurs when the gear is put in the up position.

The ‘70 Panel seems to be unique compared to earlier and later panels...

Best regards,

-a-

Thanks!

But, to be accurate the Gear Warning CB trips when the gear is UP or DOWN; doesn't matter.

[carusoam]

Hopefully Clarence knows the details at this level too...

Best regards,

-a-

[MikeOH]

Today's progress: I removed the control box from the panel; not without some minor issues...the overlay has to be removed to access the mounting screws and the knob comes off with two set screws, and the push to test button unscrews.  That still wasn't enough!  The 1/4 turn panel fasteners had to be released and then the panel tipped finally provided clearance to remove the box.

I also ohmed every pin of the mating connector (plane side, NOT the box side) to ground and wrote down the pins that showed close to zero ohms.

Now, to the plane schematic to see if all of those pins should have a path to ground.

Edited June 16, 2018 by MikeOH

[MikeOH]

Looks like all the pins that are 'grounded' are connections to bulbs; so, the low resistance is expected.

Tracing the gear warning circuit from the circuit breaker (20PB03E) on schematic WE-3 (page 9-102) ends up going to pin U on the control box.  Next step is to see if that pin has a low resistance to ground.  If so, that's likely the issue.

Edited June 16, 2018 by MikeOH

[MikeOH]

I believe the problem is solved.  Pin U on the control box connector had low resistance to the chassis.  Per the control box schemati that Marauder kindly supplied (Thank you!!) that should NOT be the case.  In fact, if I made sure the control box did NOT touch the aircraft frame the CB no longer tripped.

While coincidental to the shop working on the plane, the proble was unrelated to anything they did.  It was a manufacturing defect that took nearly 50 years to manifest itself!  As you can see (somewhat) in the attached photos, there is a rubber insulated Adel clamp holding a bundle of wires.  Careful examination revelealed that a single wire, the one going to pin U, had been clamped between the metal portion of the Adel clamp and the control box' metal chassis rather than having been properly routed inside the insulated portion of the clamp.  Vibration eventually wore through the insulation and caused pin U to be shorted to ground.... on the second flight after the shop worked on the plane!

Thanks for everyone's input!

[carusoam]

The power of MS can be amazing some days!

Great follow-up with pics, Mike!

Great MS puzzle... find the Adele clamp in Mike’s pics... (it’s in there) 

Best regards,

-a-

[Stephen]

Nice troubleshooting Mike and team MS!

[Stephen]

@carusoam

Anthony, do you ever sleep? Ive watched you at it every night Ive had insimnia and you seem to read / post 24x7!! Wow!

... or maybe you are the MS AI ... hmmmm. 

[carusoam]

I used to keep up with every post...

I wasn’t alone...

On MS, you are never alone...

Some of us start early in the morning in Eastern Europe time... then Hyett comes in an hour later...

Some of us are working through the night In an office environment... working the Raptor life style...

The west coast seems to be up late, often... last night JohnB was discussing Bravo engine oil pressure...

I’m just working on some old memory skills, a little reading, some writing... that kind of thing...

Nothing like re-learning to read, write, and spell, with a side benefit of refreshing Mooney skills....

Go Late-nite on MS!   

Best regards,

-a-

[carusoam]

Good Morning Andrew!

It’s amazing that spaghetti bowl lasts 50years...

Best regards,

-a-