Trim CB popping

[bradp]

Hi all

Trim circuit breaker keeps popping.

I have a 77J with electric trim and no autopilot servo for pitch.  

The recent mx history is that the windshield was replaced and the glare shield was off for that.  It started happening soon thereafter.  Thinking about debris that may have fallen behind the CB panel but difficult to visualize from above or below .  

I brought the plane to the MSC and had them look at it.  The trim jack screw was well lubed and not dirty.  The wheel moves freely on the ground. It does have some resistance in the air, so I am considering doing the M20KTurbo method with my IA to make sure everything is cleaned and lubed.  I haven't had the CB pop while the trim motor is turning, however. 

The word from the MSC was that they were trying to look for a short to ground as CB had popped on the ground without trim being commanded to move.  They thought that there might be a short near the CB panel, but as they were searching they could no longer reproduce (? Something wiggled and temporarily fixed?).  So they told me that it was likely a shorting wire behind the CB panel and to bring it back if it recurred.  It was happy for a while, but now it is back with a vengeance. The symptoms are as follows:

1.if I reset the circuit breaker with the master off, it stays set when I turn on the master.

2. When I am at idle, the CB stays set. 

3.  When I apply power ~1500 rpm or greater, the CB pops.   I can't tell if this is due to vibration or alternator output related with electrons jumping an air gap with voltage across the bus.  It happens as I'm power up for run up.

4.  Attempt to reset in flight is about 20% successful.  Most of the time it will instantaneously pop.   When I do get to reset it will randomly pop - not necessarily when the switch is engaged or the trim motor is actuated. Again voltage or vibration??

5. CB pops whether I have the trim disconnect toggle switch in the ON or OFF position.

So ... 

Should we a) replace the CB empirically? Search for a chaffed wire or other type of short to ground?  c) you are barking up the wrong tree brad - look elsewhere entirely.

I'm unfamiliar with the wiring diagram for this system and am having trouble locating it.   I'm really wondering how the system is configured in order to better localize the issue (most interesting to me is that it still happens when the disconnect / trim master switch is OFF).  

If it's near to the CB panel it seems like a huge pain to access the wiring behind it.   With 40 yr old wiring, I'd be hesitant for us to torque or move something that will cause additional problems.  My local IA I think would want to minimize any crawling behind the panel (as do I honestly).  

As always your wisdom is appreciated.  

-B

[N201MKTurbo]

I would put an amp meter in the circuit and see what the current draw is so you can tell if it is the circuit breaker or something else.

[Yetti]

here are the J wiring diagrams.  They are by model number

http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 2 of 2.pdf

 

I vote chafed wire. The vibration is causing the wire to hit against something and popping the breaker

 

 

[Yetti]

here is the thread on replacing circuit breakers.  the later Fs and Js had the same panel.

 

[bradp]

Update:

-Hunted around the back of the panel with my mark 1 eyeballs and the little endoscope I use to examine my valves / exhaust.   I could clearly identify the circuit breaker for the trim and nothing looked chafed / markedly abnormal.  I was able to visualize the trim switch and the wires exiting the yoke with no signs of wear.  

- Flew today.   On to something.  My wife took a video which I'll post here.  Here's the what I think will be the diagnosis.  Chafed lead crossing cowl flap cable.  Here's why I think this might be it:

-- Cowl open (takeoff), trim CB in (working)

-- Cowl flaps in trail.  CB is in. 

-- Cowl flaps closed, breaker trips and can't be reset.  

-- Cowl flaps open, trim once again functional.  

-- it's not vibration per se- there's a heck of a lot more when cowl flaps are in trail and that when the circuit isn't shorting.  

-- it may be due to cable position with airspeed - on landing the breaker tripped with the cowl flaps open   

- hopefully will be able to confirm something tomorrow.  

 

-Brad

 

[bradp]

Here the video :

 

[Joe Larussa]

I had a similar issue. Chaffed wire coming out of the yolk shaft. Repaired wire and all is good.

[Marauder]

That is an interesting one Brad. Sure looks like it is related to cowl flap movement. I thought the cowl cable was shielded. Wonder if it is getting pinched when the cable flexes.

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[bradp]

So I traced the leads around the cowl flap cable in both positions and neither the leads cross nor the cable sheath moves too much when the cowl flap position handle actuated.  

Next is to examine the switch wires where the exit the yoke as Joe suggested.  At least that is on the general vicinity of the cowl flap handle.  

Time to whip out the ammeter and start looking ...

(Btw thanks carusoam)

[Marauder]

So I traced the leads around the cowl flap cable in both positions and neither the leads cross nor the cable sheath moves too much when the cowl flap position handle actuated.  

Next is to examine the switch wires where the exit the yoke as Joe suggested.  At least that is on the general vicinity of the cowl flap handle.  

Time to whip out the ammeter and start looking ...

(Btw thanks carusoam)

Brad -- keep in the back of your mind that the breaker itself can be bad. I have replaced a half dozen over the years because they pop unexpectedly.

The cowl flap twist is different but when it comes to strange problems with even stranger solutions, Mooney leads the pack.

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[OR75]

I believe there is an FAA guidance that if the CB pops, you can reset it once. If it pops again , then the cause needs to be investigated (a bad CB of course can be the culprit) 

[Yetti]

Still voting for direct short.  

Pulling the circuit breaker and taking the lead wire off and VOM it down the path till you find the short.   May want to check up around the end of the yoke shaft first.

 

[bradp]

Follow up to this old post.  It was the wiring to the trim switch itself.  There was a place where the solder was arcing across to another lead.  This wouldn't become apparent until some vibrational change from opening and closing the cowl flaps.  Here's a diagram I made for the wiring contacts from the old switch.  Cleaned up, rewired and now it looks and works great. 

 

 updated below

[carusoam]

Great follow-up, Brad!

Best regards,

-a-

[bradp]

Further follow up for this thread;

Here are the correct color codes for the Mitchell wires to the switch (gray = white wire = ground).

[carusoam]

Looks familiar....

Is that the wiring diagram for the Jarvik-7?

I hope that is funny...

I’m surprised I could remember that one...

Best regards,

-a-

[bradp]

I corrected some mistakes and clarified by using PPT 

We don't need solidworks electrical schematics here!