Troubleshooting electrical problem.

[wombat]

Got the plane back from annual, and the electric trim no longer works.

 

The switch for electric trim is getting 14V, and when switched 'on' it has 14V  on the downstream side too. (i.e. that switch is working properly)

The autopilot self-test fails.

The electric trim on the yoke fails to do anything. There is also no additional draw on the electrical system, so I don't suspect it's a problem with the servo.

The manual trim wheel works fine.

There was some minor work done behind the panel during this annual; adding two wires to the Garmin GPS and routing them to a new button on the yoke, as well as removing and re-installing the engine monitor and a slide-out/slide-in replacement of the GPS.   I expect this activity dislodged some wire or connector behind the panel.

The wire added to the yoke was on the right hand side, opposite the electric trim and AP disconnect and CWS buttons, so I expect those remained undisturbed.

 

Any ideas on what I can look for before I just take it to a shop and start paying for someone to figure it out?

[warren.huisman]

I’ve had this problem a couple of times and it has been broken wires where they exit the yoke tube and another time I had a bad micro switch.


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[Fly Boomer]

30 minutes ago, wombat said:

Got the plane back from annual, and the electric trim no longer works.

 

The switch for electric trim is getting 14V, and when switched 'on' it has 14V  on the downstream side too. (i.e. that switch is working properly)

The autopilot self-test fails.

The electric trim on the yoke fails to do anything. There is also no additional draw on the electrical system, so I don't suspect it's a problem with the servo.

The manual trim wheel works fine.

There was some minor work done behind the panel during this annual; adding two wires to the Garmin GPS and routing them to a new button on the yoke, as well as removing and re-installing the engine monitor and a slide-out/slide-in replacement of the GPS.   I expect this activity dislodged some wire or connector behind the panel.

The wire added to the yoke was on the right hand side, opposite the electric trim and AP disconnect and CWS buttons, so I expect those remained undisturbed.

 

Any ideas on what I can look for before I just take it to a shop and start paying for someone to figure it out?

Since it was on the opposite horn of the yoke, I suspect the wiring from the thumb switch.  I think those wires are pretty delicate.  If it's King, be careful with that switch -- they are hard to come by.

[wombat]

Thanks for the advice.  I had been assuming that if the electric trim actuator switch was disconnected the autopilot self-test would still work, but apparently not.   I'll investigate this part more.  

 

@Fly Boomer I'm confused about why you say 'since it was on the opposite horn of the yoke, I suspect the wiring from the thumb switch.'     Since it's on the opposite horn, I'd tend to NOT suspect the thumb switch, since there was never any need to mess with the wires in that horn.

Getting the wires through the yoke to get the thumb switch out is going to be a huge pain.  I can't get any of those wires to move at all.

[wombat]

I have opened up the electric trim switch in the yoke.  It's not wired anything like I was expecting.

 

It looks like I've got a whole bunch of stuff wired together there that shouldn't be that way, according to the diagram from this other thread:

 

[MikeOH]

While that picture makes my flesh crawl, I don't think the switch wiring is your issue since the trim worked prior; doubtful from the looks of that mess that it's been worked on recently.

From your description it sounds like the electric trim system has lost power.  I'd be crawling under the panel looking for disconnected wires associated with the electric trim system.  I'd start with the trim breaker.

[Fly Boomer]

2 hours ago, wombat said:

Since it's on the opposite horn, I'd tend to NOT suspect the thumb switch, since there was never any need to mess with the wires in that horn.

Not the switch (good news).  Rather the wires where they exit the yoke and head back into the panel.  If I understand you correctly, they were working on the horn opposite the one with the trim switch.  That sounds like they would not have touched the switch, but all those wires eventually have to go somewhere, and that's what I suspect.

[MikeOH]

I would suggest resoldering the red wire on the left; looks like it's hanging on by only a couple of wire strands

[wombat]

Thanks for all the input.   I did find this diagram of the yoke wiring from the KAP 150 install manual.

@Fly Boomer Despite the many other wiring problems the plane might have, the wiring coming out of the yoke look really good.  The two lone wires not in the wrap are the new ones they hooked up recently.   Everything else is tidy and looks great.

 

@MikeOH I don't have a separate trim breaker, just the autopilot breaker.  And it's providing power to both the trim switch and the autopilot itself successfully.   But maybe somewhere else there is a wire disconnected.    Yeah, that red wire is barely hanging on.   Everything is covered in silicone, the person that was in there last really gooped it up.  If I end up needing to do any soldering, I'm replacing all three switches.  They are Honeywell 1sx74-t SPDT switches, and be had for $38 to $48 (Mouser Vs. Digikey)    I actually just went ahead and bought three of those switches to replace these three.

[N201MKTurbo]

You should look at the disconnect relay and the wiring going to it. 

[Fly Boomer]

17 minutes ago, wombat said:

Despite the many other wiring problems the plane might have, the wiring coming out of the yoke look really good.  The two lone wires not in the wrap are the new ones they hooked up recently.   Everything else is tidy and looks great.

Whatever it turns out to be, please post back.  If it turns out to be the electric trim switch, I am interested to know how that got disturbed.  Thanks.

[wombat]

14 minutes ago, N201MKTurbo said:

You should look at the disconnect relay and the wiring going to it. 

I would love to, but do you know where might I find the disconnect relay?   Are you talking about the autopilot disconnect switch on the yoke?

I'm super frustrated with all of this, might just take it to a shop and pay my AMUs to get it working.   The plane's been down since November.

[0TreeLemur]

15 hours ago, wombat said:

I would love to, but do you know where might I find the disconnect relay?   Are you talking about the autopilot disconnect switch on the yoke?

I'm super frustrated with all of this, might just take it to a shop and pay my AMUs to get it working.   The plane's been down since November.

I feel your pain.  The KFC-150 in our J stopped passing its self tests intermittently in November.   The electric trim works, so I am pretty sure it's not the trim switch.  I've had a busy time at work and uncooperative weather preventing me from getting it worked on.   I did have  an A&P clean the contacts and reseat A/P computer with negative effect.  Flying back from his shop I mashed on the A/P Disconnect switch, and it passed the self test and worked the rest of the flight home.   Hasn't worked since though.  From reading about this, I'm convinced that it is the A/P Disconnect switch.  I just need to have a shop put it in and see if that solves the problem.

I've learned this:  The A/P disconnect switches are problematic.  All the current going to the servos passes through that switch.  The contacts are running near their rated limits.  They often fail.  When the resistance of the contacts in that switch goes up it causes the A/P computer to fail its self test.  A guy in the UK has a great web site describing the problems.  He has a KFC225 A/P, but it uses a lot of the same switches.

 

[0TreeLemur]

6 hours ago, wombat said:

I have opened up the electric trim switch in the yoke.  It's not wired anything like I was expecting.

 

It looks like I've got a whole bunch of stuff wired together there that shouldn't be that way, according to the diagram from this other thread:

 

 

Is this wiring diagram correct for the KAP/KFC150?

If so, I like it a lot better than the one in the install manual.

Thanks,

-Fred

[Will.iam]

10 hours ago, 0TreeLemur said:

I feel your pain.  The KFC-150 in our J stopped passing its self tests intermittently in November.   The electric trim works, so I am pretty sure it's not the trim switch.  I've had a busy time at work and uncooperative weather preventing me from getting it worked on.   I did have  an A&P clean the contacts and reseat A/P computer with negative effect.  Flying back from his shop I mashed on the A/P Disconnect switch, and it passed the self test and worked the rest of the flight home.   Hasn't worked since though.  From reading about this, I'm convinced that it is the A/P Disconnect switch.  I just need to have a shop put it in and see if that solves the problem.

I've learned this:  The A/P disconnect switches are problematic.  All the current going to the servos passes through that switch.  The contacts are running near their rated limits.  They often fail.  When the resistance of the contacts in that switch goes up it causes the A/P computer to fail its self test.  A guy in the UK has a great web site describing the problems.  He has a KFC225 A/P, but it uses a lot of the same switches.

 

for this reason i use the AP engage switch and press it again to disengage the autopilot as there is not all that current going through that button compared to the AP disengage button. Also i have found that if i push the alt button and it engages the ALT and FD then i know the AP test will pass. I do this so that i don’t try the test and have it fail with the continuous beeping that only a circuit breaker pull and reset stops. When i had Jake from bevins aviation look at it there is a voltage range that the computer checks on the trim for the computer to pass and usually if it fails it’s because it’s on the the low side of the range but for mine it was on the high side. Some time when i have mine fail i will try it without the Alternators on and a load on my battery to see if that helps bring my voltage back down in the range to pass. 
but like other here once my AP passes it will for for the rest of the flight. 

[0TreeLemur]

52 minutes ago, Will.iam said:

for this reason i use the AP engage switch and press it again to disengage the autopilot as there is not all that current going through that button compared to the AP disengage button. Also i have found that if i push the alt button and it engages the ALT and FD then i know the AP test will pass. I do this so that i don’t try the test and have it fail with the continuous beeping that only a circuit breaker pull and reset stops. When i had Jake from bevins aviation look at it there is a voltage range that the computer checks on the trim for the computer to pass and usually if it fails it’s because it’s on the the low side of the range but for mine it was on the high side. Some time when i have mine fail i will try it without the Alternators on and a load on my battery to see if that helps bring my voltage back down in the range to pass. 
but like other here once my AP passes it will for for the rest of the flight. 

Since buying this airplane in Dec., 22, I never used the big red ("Easy"?) disconnect button.  I always used the A/P button on the computer, or tap the trim switch.  In fact I only used it once just to see if it worked - and then these problems started!   Once I get it that switch replaced, I'll never intentionally use it again unless nothing else works.

[wombat]

Well, I'm giving up.   Time to call for help.    My inability to get more slack in the wires was a total killer.  I suspect if I took the yoke off the push-pull tube I'd be able to get slack, but.....   I've thought "If only I take this one more thing apart" for every step of the way.

 

If I can't get anyone to look at it in the next week or two, I'll probably go through each wire on the yoke harness, identify what it's connected to, so I can determine the expected state (open/closed) from every other wire and then I can test at the harness disconnect.   But this is also totally assuming the problem is in the yoke.  Well, there is at least one problem in the yoke NOW though. 

Everything is back together nearly as good as when I started looking, except one of the wires on the electric trim switch is now disconnected.   I still have three new micro switches on order.

[wombat]

So... The soonest anyone local can look at the plane is the end of March.  So I've been doing a bit more troubleshooting, and the switches I've checked in the yoke are all good, by matching wire colors and looking at the diagram, I was able to test the CWS, autopilot disconnect (closed on both pairs of wires), pressing the switch opens them, and the autopilot arm and the trim up and trim down switches.   

I think maybe the next thing is either check at the servo, or try to bench test the autopilot.

[MikeOH]

I wish I could offer specific advice for your particular symptom and autopilot.  However, as a career EE with plenty of troubleshooting experience I would be looking for loose connectors/broken wires ESPECIALLY in any areas that you think the shop would have been 'poking around'.  I've fixed more systems (that HAD been previously working) by simply LOOKING than I have by using meters and 'scopes and pouring over schematics.

I think your comment that removing the GPS 'moved wires around' and the engine monitor is exactly my point; and are the areas you should examine carefully.

Get a bright headlamp/magnifer and a comfy pillow and crawl under the panel

Also, did the shop mess around in the tail where the pitch and trim servos reside?  Check that wiring and connectors; unplug and check for dirt/corrosion and re-plug a few times.

 

Edited February 18 by MikeOH

[Shadrach]

On 2/10/2024 at 7:15 PM, MikeOH said:

I would suggest resoldering the red wire on the left; looks like it's hanging on by only a couple of wire strands

Does that look like it’s from wear and tear or installation by someone who should be banned from touching a soldering iron?  
I recently repaired the connections in my factory oil temp probe and I was trying figure out if the connections deteriorated or were poorly constructed. It’s likely a 57 year old part, but the wires are definitely not.

[MB65E]

I’ve replaced a few AP disconnect switches on M20s. Most were mid 90’s vintage. Stupid expensive switch. New switches always solved my issues. 
-Matt
 

[MikeOH]

36 minutes ago, Shadrach said:

Does that look like it’s from wear and tear or installation by someone who should be banned from touching a soldering iron?

The later; that whole switch 'assembly' is a soldering disaster.  And, no idea why they 'gooped' it with RTV; makes it nearly impossible to rework without a ton of cleaning.  I think OP is smart to start with new switches.

[0TreeLemur]

Yesterday my A&P (who is not a B-K A/P expert) replace the "Easy Button" disconnect switch.  While the KFC-150 still doesn't pass its self test, a new symptom arose after replacing the disconnect switch.   Now while doing the self test, you can hear the trim motor actuate 4 times, but the clutch fails to engage.  I can tell that because the trim wheel doesn't move, and I don't hear the loud "clunk" of the clutch engaging. Before replacing the disconnect switch, I don't recall hearing that trim motor at all during the self test.  

I'm planning to make an appointment tomorrow with a shop that specializes in autopilots...   I've done all I can do.  The manual electric trim works.

[0TreeLemur]

On 2/10/2024 at 5:52 PM, wombat said:

I have opened up the electric trim switch in the yoke.  It's not wired anything like I was expecting.

 

It looks like I've got a whole bunch of stuff wired together there that shouldn't be that way, according to the diagram from this other thread:

 

The diagram above doesn't go with the trim switch shown in the photo.   The diagram shows the three center pins wired together.  The photo shows the jumper going from the pin where the green wire is, down to the center pin on the other two switches.

I'm not sure why they use red and white twice each in the photo.  I don't see any color stripes.

[wombat]

I took a much closer look at the wiring diagrams and the wiring on the switches.  It does seem to be wired correctly, which is not shocking because it worked this way when I bought it.

All of the switches operate normally(*) when measured with an ohm meter back at the disconnect point behind the panel.  I've checked the CWS (Normally open, closes when I press it)   Autopilot disconnect (two connections that are normally closed, and open when pressed)  and the autopilot arm is normally open between 8&9, closes when I press it.   Up and Down are normally open to 8 but connect it when pressed.

 

(*) Upon further thought, I neglected to check both poles of all switches, I just checked one of each when compared to the common (Green, line 8).   So there is still a chance this is something I can fix on my own!   The CWS and Autopilot Disconnect are fully correct though.  But the three micro-switches for up, down, and arm might still be experiencing a failure.   

Thinking about it, the way I described it, (up and down normally open to 8) would indicate a faulty 'arm' switch state....   They should be normally closed to 8 unless 'arm' is pressed, at which point the should be open to 8 unless Arm is pressed, in which case they should be closed to 8 AND 9.    Hmmmm...  This might be the problem.

 

@0TreeLemurThat's what I thought at first, but if you look at the codes NC (Normally Closed) and C (Common) on the diagram, they have two switches one way and one of the ends the other way around.  Mine is physically set up so that both ends are the same and the middle one is reversed.  It ends up doing the same thing and you can install them in any setup as long as the two on the "up" and "down" switch are not the same.  While I'm not actually happy with the wiring that's been done here, it was obviously working fine for a while.  If there is a problem, it lies within one of the honewell switches.

I've never done the autopilot test without the engine running, so I am not aware of any noises it makes.  Bad on me.