KFC-150 issues...

[tschat]

Ok, the background story here will take a while, but please work with me.  System was fully functional until a 2 years ago.  At that point, it would hold heading just fine.  If the plane is trimmed out prior to enabling altitude hold it would work just fine (unless bad turbulence).  The AP appeared that it would not adjust trim, if pitch wasn't sufficient.  Spent a lot of time researching the issue and found there are torque limit switches in the pitch servo that would likely be the issue.  Before I had a chance to test them, the AP quit passing self test.  Contacted a shop and they said to pull the trim servo, pitch servo and the AP box and they would test them out.  Pitch and trim servos passed.  The AP box had a cold solder joint.  All 3 units were returned an reinstalled.  The AP passed self tests but still no trim on altitude hold (yolk electric trim has always worked through this entire story).  Local mechanic recommended checking/tensioning the pins on the AP cage, as those can be weakened over time as well as the limit switches.  He performed these tasks and now the unit non longer passes self test.  (Yes, I told you the background story is long).  When in self test, the trim servo only received 2 pulses (should be 2 up and 2 down from my research).   I put a meter on pins K(CW) and R(CCW) of the trim servo.  Sure enough K is seeing 2 pulses of 24v and R was seeing nothing.  So, next we pull the AP box to check continuity from the AP cage to the trim servo.  Ummmm..... There was no pin in molex pin in slot 14 bottom connector.  Quadruple checked we were reading the wiring diagram correctly and slept on it.  I got folded back like a pretzel under the AP and I saw a wire with a pin on it in that general location.  I toned it out and sure enough it was pin R on the trim servo.  It was bent up a bit, so it had to be straightened out and then it was slid back into the connector.  Now is when things get strange.  The unit continued to fail self test (still 2 pulses on K only).  Now, when the AP is off, the voltage on pin R is 0v.  When the AP is powered up, the voltage is 2.48v.  We pulled the AP again to double check the pin didn't recess back and also ensure that we didn't have any load on the wire.  Pin in the connector was in alignment with the rest and it has high impedance to ground (unplugged from the servo).  Any idea if the AP got damaged when the pin got pushed out?  Any idea about 2.48v source?  Actually, any ideas how I can further diagnose the failure?  Thanks.

Thomas

KC192/KS270/KS272

Edited February 11 by tschat

[KSMooniac]

Start here:

Your Aircraft Autopilot Technician - WebAir Consulting

 

[Will.iam]

 https://bevanaviation.com
especially if you can fly to their shop. 
@Jake@BevanAviation is extremely knowledgeable on KFC-150 autopilots. 

[PT20J]

If everything bench checks OK, it must be wiring. Bendix-King used PCB edge connectors for the Silver Crown line which are not the best connectors in the world. When I had a King autopilot, I fixed a couple of issues by taking a very long blade screw driver with a small tip and filing it down so that it would fit in between the dividers between contacts on the connector in the tray, and then using it to carefully flex the contacts toward the center of the connector to make better contact. I also cleaned the gold plated contacts on the edge connector of the unit with some alcohol.