Troubleshooting electrical problem.

[0TreeLemur]

28 minutes ago, wombat said:

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.

This is how my trim switch is wired looking at the back, of the gang:

I gather that power comes in the yellow wire..  The mechanism on the arm switch depresses the arm microswitch whenever it is deflected up or down.  When that happens, it energizes the orange wire, and the normally open center taps on the other two trim switches.  Pushing the switch in the down direction energizes the up trim servo, while pushing it in the up direction energizes the down switch servo.  

Just like yours, the center switch is reverse orientation from the two on the ends.   My wire colors are different from yours, but the function should be exactly the same, because the jumpers between all three switches are the same.

 

[wombat]

Well, after going back and double-checking things, I think everything in the yoke is good.

My actual wiring is almost exactly like @0TreeLemur's is.   

I think the next thing is to see if I can find somewhere to bench test the KC192, since that's the easiest to remove. (One static air source and one small hex from the front panel then it slides right out.)  Or maybe there is a way to get more diagnostic info than "Beeping forever because something isn't right"

My notes from in the cockpit as I was looking and testing today, and the barrel connector the wires go to, with the numbers for the pin on them.   Kind of a hassle, but if I had just looked at the wire colors and this *first* I would have saved myself 8 to 16 hours of frustration.

 

[Will.iam]

For a data point my trim wheel does not move when my KFC-150 does it’s test and passes. 

[wombat]

I got my pitch trim motor back from Autopilots Central.  They said that what they did (replace a couple of resistors and remove some capacitors) shouldn't have made a difference.  But I plugged it in, powered on, and the electric trim worked and the autopilot passed the self-test.

Super exciting.   So I flipped the power off and back on, and... It failed again.    Can't get it to work again.    

I've heard that the autopilot disconnect switch will sometimes test good with a multimeter, but fail to actually carry load, so I used short jumper wires (14 gauge) to connect both of the the connections that go through the autopilot disconnect.    There was no change, unfortunately.

 

I'm taking the plane to an avionics shop on Tuesday and they said that if I get it there in the morning they can test out all of the autopilot wiring during the day while I wait.

Since there was the single success, this makes me suspect something in the autopilot itself.   So I expect I'll be sending that off in the near future.

[wombat]

Well, the avionics shop seems to have not recorded that I was scheduled to go in tomorrow, so now I'm back to doing more self-testing of the wiring.

 

The folks at Autopilots Central have been extremely helpful.

[MikeOH]

@wombat

You might want to call Bob Weber and get his input: https://webairconsulting.com/

 

[wombat]

Thanks, I was not aware of Bob and his consulting business.   If I don't figure out something tonight I'll give him a call tomorrow.

 

[wombat]

Well, I fixed it.  The problem was me.  And a few unexpected limitations of the electric trim and the autopilot.

 

The power to the electric pitch trim servo is routed through the Garmin G500.  (Not a GFC 500, just a G500)  The G500 must be powered on for about 22 to 28 seconds before it closes the relay for the electric trim.    Additionally, the autopilot itself must be powered on for about 3 to 8 seconds before the electric trim will work or you can successfully initiate an autopilot test.    I didn't expect this.

 

The problem started when I picked the plane up from annual.   The plane is still fairly new to me, even though I've got 90+ hours in it, so when picking it up from annual, I thoroughly reviewed my checklists before doing anything.   And after getting the engine started, before doing anything else, I reviewed the checklist.  Then I immediately turned the avionics power on, the electric trim, and hit the autopilot test button.  The autopilot test failed.  So leaving the engine running, I turned electric trim off, avionics off, and master power off, then turned them all back on.    And the electric trim didn't work, and the autopilot self-test failed.  

So I flew home, and spent a dozen hours testing what was going wrong.    But since I was always just testing the electric trim and autopilot, I would turn the master power on, turn the avionics on, turn the electric trim on, and then hit the AP test button.  A lot of times, because I didn't need the G500, I'd pull the breaker for that before testing.  Which, unknown to me, would ensure that the pitch trim servo would never get power.

After talking to Steve at Autopilots Central, he told me what voltages I should expect to see on each of the wires going in and out of the yoke, and where that power was coming from.

It all 'clicked' for me when I was testing the voltage on the 4 wires to the AP disconnect switch with the power on.   All four should have about 14V.  One going into to the AP disconnect has power from the autopilot circuit breaker.  The other power in comes from the electric trim switch.  (And unknown to me, through the G500!!!).    The power out from the first input goes to the sonalert and the flight computer, the power out from the second goes to the electric pitch trim servo.    I didn't know which wire was which switch or if the wire was input or output.  Just that pins 3 and 6 went across one switch, and 4 and 8 went across the other.

Since the sonalert was working when the AP would fail its self test, that meant I should expect to see +14V on the first one, both input and output.    When I measured it, I did find +14V. That was pins 3 and 6.

On the second one, I found no voltage on either of them.   So while I'm sitting there staring at the multimeter, I hear a tiny 'click' and the voltage goes to 14V.   I reach up and try the electric trim and I hear the servo spin.  I press the autopilot test button and it completes successfully.    

So I turn the avionics and master power off, and immediately turn them back on.  No voltage.   So as I'm staring at the multimeter again, I hear a tiny 'click' and voltage goes to 14V again.      So after about another 30 minutes of turning stuff off and then back on again, and pulling breakers, I finally come to the conclusion that after the electric trim switch, power goes to a latching relay on the G500 and from there to the AP disconnect.   Until the G500 has been powered on for about 30 seconds, that switch will never get power.   Once it has gotten power, I can pull the G500's circuit breaker and it will maintain power though.       

 

So ends the saga of the electric trim that wouldn't.  Just need to get the servo re-installed and I'm back to 100% functionality!

[Z W]

Congrats on persevering through what I'm sure was a frustrating struggle.  And now you know your plane that much better.

These birds are all unique creations at this point after years of modifications and improvements with unique installations.  Probably why it costs so much to get them worked on...

[wombat]

I talked with the avionics guy and figured out why the G500 needs to be booted up for the electric trim to work.  

The autopilot needs to have an attitude reference.   Historically it would be the KI-256 attitude indicator, but that was replaced with the G500.   The G500 has an output wire that lets other avionics know if it has a valid ADAHRS.     That signal determines (via a relay) if the autopilot servos can get power, since if you have no attitude reference, the autopilot cannot operate correctly.     So the electric trim (which is controlled by the autopilot) and the autopilot cannot function without the G500, therefore they cut power to the servo if the ADAHRS reference isn't good.

[ZuluZulu]

@wombat Thanks for documenting all this, I've gotta imagine this is going to help somebody out there save a lot of stress and grief someday down the road.  It must feel great having finally figured it out!

[Will.iam]

Congrats on another mystery solved.