Trim switch issues

[xcrmckenna]

My yoke mounted electric trim switch stopped working Saturday. I verified I have power to the switch from the on off switch on the panel. My avionics guy won't be able to look at it till next month. But he tasked me with doing some more trouble shooting on it. He wants me to ohm the wire harness at the servo to make sure it isn't bad before changing the switch. I have an Stec 30 w/alt hold. By the looks of it the trim servo is on the left and I'm guessing the unit on the right is the alt hold servo?

 

My next question I can't seem to get the blue plug pulled from the servo. It feels pretty brittle and there is a metal clasp of some sort holding it in. How does it come apart? I really really don't want to break anything.

 

 

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[xcrmckenna]

I did poke around enough and figured out the plug on the back of the servo. I don't have a working diagram of the stec. Does anyone know what each pin is? Thanks for any help....


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[KSMooniac]

STEC doesn't share much...best to call your shop and see if they'll give you the pin assignments. Good luck!

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[xcrmckenna]

STEC doesn't share much...best to call your shop and see if they'll give you the pin assignments. Good luck!

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I've been texting him while on another job so don't want to be to much of a pain right now.


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[bradp]

Sure you don’t have a century electronic trim (left) and a s-tec altitude hold (right)? 

If it’s a century the most likely culprit are the solder points on your trim switch leads or a chafed wire as it exits the yoke and tracks toward the relay box typically mounted on the firewall.  Look there first before you go tearing into the motor harness.  Also remember that the motor harness goes to a relay and not to the switch directly.  Another point of failure is that there is typically a klixon or other switch between the relay and the motor (ie switch in OFF position you should still hear the relay clicking when you depress the yoke switch).  Finally (or maybe firstly depending on the symptoms) check the trim circuit breaker.  They get old and the can be worn. 

Back up a sec and first questions 1) does the CB pop? 2) does the trim wheel move freely and 3) can you hear clicking in the relay box no matter whether the klixon switch is turned on or off? This will help narrow down where in the system you should be looking. 

An aside - I thought the Stec alt holds would have been push rod rather than cable capstan connections to the elevator control rod.  Does it connect at a bell crank or along a rod or something.  Have always been curious how pitch autopilots worked in the Mooney as I don’t have one in my bird. 

[xcrmckenna]

Sure you don’t have a century electronic trim (left) and a s-tec altitude hold (right)? 
If it’s a century the most likely culprit are the solder points on your trim switch leads or a chafed wire as it exits the yoke and tracks toward the relay box typically mounted on the firewall.  Look there first before you go tearing into the motor harness.  Also remember that the motor harness goes to a relay and not to the switch directly.  Another point of failure is that there is typically a klixon or other switch between the relay and the motor (ie switch in OFF position you should still hear the relay clicking when you depress the yoke switch).  Finally (or maybe firstly depending on the symptoms) check the trim circuit breaker.  They get old and the can be worn. 
Back up a sec and first questions 1) does the CB pop? 2) does the trim wheel move freely and 3) can you hear clicking in the relay box no matter whether the klixon switch is turned on or off? This will help narrow down where in the system you should be looking. 
An aside - I thought the Stec alt holds would have been push rod rather than cable capstan connections to the elevator control rod.  Does it connect at a bell crank or along a rod or something.  Have always been curious how pitch autopilots worked in the Mooney as I don’t have one in my bird. 

I sent you a message. It is a century yoke switch. I have power to it and I got the harness pulled out of the servo. It was really easy after I figured it out. Do you know where the relay is typically mounted? The manual trim moved freely, the breaker is not popped. The power goes from the breaker to the on off switch on the panel, to the yoke switch, relay, then to the servo. I have power to the yoke switch. I've pulled the switch out of the yoke at least as far as I could. It doesn't look like a broken solder and the wires are in a thick protected sleeve going through the yoke and yoke shaft. Alan has a switch for me, I just want to do all the checking I can before bite the bullet and buy a new one. When I bought the plane in March you had to make sure you pressed the top safety switch pretty hard but it had freed up and had been working really well over the last few months. I don't hear any clicking now except when I bridged the test light probe across the contacts on the back "or bottom" of the switch.


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[bradp]

Sounds like it’s your switch - you’ve done a bunch of troubleshooting already.  I see why your avionics guy wants you to test the continuity of the harness.

Does the trim work intermittently or is it dead dead? 

Look for a box like this against the firewall: 

If you can get the rely to close consistently (click click) by shorting terminals on the  underside of the yoke switch you know your relay works.  The more difficult thing is testing that there are multiple contacts on the relay the run the motor in forward and reverse directions.  You need to be able to understand the switch wiring to be able to get test the relay in all configurations.  It shouldn’t be too hard to figure out the output from the relay to the motor to figure out which wires are which once you do that relay step. Scout the internet for an installation manual for the trim system or approach Mooney to see if you can get the wiring diagram. 

Back to the yoke switch - I had a little bit of insulation on the wire shorting two contacts on the switch.   The mechanical portion may be worn especially if you had to push real hard for a while  and now it doesn’t work.  Use my previous thread to see a  basic wiring diagram for the switch.  The individual switches are about $5-10 each on mouser or Newark.   You could overhaul your own (with IA supervision) for about $20 total in parts before paying for Alan’s switch.  

 

[xcrmckenna]

The switch is dead. What is weird the switch had been working really well since the first few weeks of owning it and then just stopped working. With the help of@thinwing pushing on the switch and my head stuck in a hole today we verified there was no power or control at the servo. So that's good because I sure didn't want to buy a servo. I will track down the relay, and I will tear into the yoke switch a little more. Maybe try some contact cleaner and re solder the wires, maybe it is a cold solder that I can't see. Thanks for your help!!!!!


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[cnoe]

The switch is dead. What is weird the switch had been working really well since the first few weeks of owning it and then just stopped working. With the help of[mention=7776]thinwing[/mention] pushing on the switch and my head stuck in a hole today we verified there was no power or control at the servo. So that's good because I sure didn't want to buy a servo. I will track down the relay, and I will tear into the yoke switch a little more. Maybe try some contact cleaner and re solder the wires, maybe it is a cold solder that I can't see. Thanks for your help!!!!!


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I’ve been offline a while and just saw this. My setup is very similar to yours and I’ve been through this and resolved it. PM me if you’d like more info and pics. It involves tedious detailed repairs but isn’t expensive.


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[NicoN]

I came to this topic while searching for solutions of my problem.

My electric trim stopped working, but definitely not due to a problem with the switch, relay or motor - they worked fine - but to my own stupid procedure (shame on me).

My task was to replace the MIC-button. while trying to make the hole (for teh button) wider, the drill blocked and turned the assembly of aluminium cover with trim switch violently - all cables were torn off!

Now I have 6 or 7 wires and only partial undertsnading what they do:

red/white    12V

purple/white 12V  both switched with the elev-Trim CB (rocker switch in left panel)

blue/white      Trim motor clutch     (connected to 12V closes the clutch -> cannot move trim wheel by hand)

yellow/white    supposedly GND

black/white      goes to the TRIM INTR- button and comes back to the trim switch via a short blue wire. In idle this wire also has 12V, but sinks to 2-3V when applying load

2 white wires, which I was thinking are the motor supply lines.

 

From the trim switch, I found out that the middle of the  switches inside operates the clutch, while the 2others together seem to reverse the polarity of 2 poles.

Sound for me like a typical H-schematic to reverse the direction of a DC-Motor.

In normal operation you can run the motor by moving the switch in up or down position. But only the motor turns, not the trim wheel. This happens if you also press the switch down.

Pushing the INTR button stops all.

My problem: Now, the motor does not start, not with trim switch nor with temporary cables. I only can opearte the clutch.

 

Can someone check and agree with the functions of my wiring?

Does someone know how the wiring of the switch is exactly?

 

 

[cnoe]

NicoN,

We all make mistakes; I won't mention mine here.

Do you believe that nothing shorted out and that the wires were simply ripped loose (master switch was OFF)? 

I've got a couple of different drawings/schematics from the factory "Edo-Aire Mitchell" trim-switch installations that I'll attach as a .jpg.

It's been a while since I messed with that system, and on my installation I do not have "auto-trim". The trim servo is independent of the "altitude hold" system and does not utilize a "clutch". My "altitude hold" system (STEC) was a later addition that only annunciates an out-of-trim situation; the pilot must trim manually (either electric or with the trim-wheel).

The independent trim-servo should be activated by applying 12V across the motor (and reverses with polarity change). But if you're tied into an A/H unit it could be different. I also believe (but am not certain) that there is a box behind the panel containing a relay for the electric-trim system that isolates the micro-switches from the full amperage draw of the motor. You may hear a faint "click" of the relay under the panel as you compress the trim button. I don't have a diagram of that relay box.

I hope this helps. Let me know if you have any other questions.

CNoe

[bradp]

Awesome.  There is the wiring diagram.  Nico what you’re referencing as the clutch - sure that’s not the trim motor relay?  

[carusoam]

Hey Chuck!
 

-a-

[xcrmckenna]

My problem ended up being the trim switch. One of the micro switches was dead. Used Brad’s diagram he drew from the thread he posted on my thread and rewired three new micro switches together.


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[cnoe]

15 hours ago, carusoam said:

Hey Chuck!
 

-a-

Hey! 

I missed you at Oshkosh this year. But then I missed EVERYBODY at Oshkosh this year. 

CNoe

[CGarrison]

On 7/28/2020 at 11:42 PM, xcrmckenna said:

My problem ended up being the trim switch. One of the micro switches was dead. Used Brad’s diagram he drew from the thread he posted on my thread and rewired three new micro switches together.


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hello, where did u get the micro switches that u rewired? 

[carusoam]

Welcome aboard CG!

got access to electronic device catalogs?

one might be Newark...

but there are others...

The switches discussed here, I believe are commercially available...

it should be easy to search the site for more details...

PP thoughts only, not a mechanic...

best regards,

-a-

[LANCECASPER]

3 hours ago, CGarrison said:

hello, where did u get the micro switches that u rewired? 

Read @bradp 's post of October 6, 2017

[bradp]

@CGarrisonI have my old trim switch available if you’re interested in it.  

[CGarrison]

Hi Brad, What is the Price for your switch?

[xcrmckenna]

hello, where did u get the micro switches that u rewired? 

Sorry just saw your post. I ordered them from skygeek.


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[Healthpilot]

My trim switch Bendix King is acting up. The metal rod that connects the two switches came out about half an inch. I pushed it back in but now the trim switch goes on and off. I think a spring inside unsprang. Is there a way to set it back where it belongs and fix it? These things seem to be incredibly expensive and there seems to be no alternative in the market. Thanks for your help!

[carusoam]

Expect that it can be disassembled and reassembled…

People have had to replace the tiny micro switches at the middle of it all…

PP thoughts. Only, not a mechanic…

Best regards,

-a-

[Healthpilot]

Apparently there is a 'tool' that makes it easier to reset the springs inside but I have no idea who sells the tool.

[Warren]

There is a small piece of spring steel - think wire about .020" diameter that provides the spring/center in effect on the back side.  It may have fallen out or been damaged.  Hopefully you will have enough length in the wires to undo the screws and pull the switch far enough to inspect it.