KFC 200 problem

[rainman]

I have the KFC 200 autopilot in my 1979 231. It has been great for the three years I've owned the plane. Recently it stopped working and my avionics shop sent it in for repairs. The unit actually smelled like "burning electronics". Some components were replaced and it was returned. When reinstalled, it only worked for a few minutes. They sent it back to the repair shop again and the power supply and motherboard were replaced. It seemed to work for several hours, then failed again when turned on after a long cross country. I diverted to my avionics shop and the same problem was noted. The unit was sent off again. When reinstalled it did not work at all. The unit appears to be "on" but does not engage. All of the servos have been tested and the wiring tested for current. I have a super avionics shop and they are working closely with the repair shop. Any similar experiences? Ray

[Piloto]

If the unit worked at the shop bench for a long time I have to assume the problem may be related to your plane wiring. Maybe a momentary short on the outputs to the servos, that happens under vibration. Check the bus voltage while in flight. Anything over 15VDC can cause damage to some avionics. The bus voltage should be 14VDC at over 2,000 RPM. These problems are a nightmare to find the cause, you are not alone.

 

Good luck

 

José 

[rainman]

Thanks José. The problem occurred on the ground without the engine running after the last install. The wires from each of the servos have had a fuse installed and the servos have been removed and tested. I intend to be patient and let the shop sort it out, but it is frustrating. My neighbor is an A/P, and he states that doing the diagnostics can be a real money losing, time sucking, pain in the rear. Of course, my annual is due in Sept so I may need to interrupt the investigation and fly the plane off to Dugosh for that before the end of the month, then back to the avionics shop. The logistics of all this can be a real challenge. Ray

[flyboy0681]

Was the head unit sent to Merle's fixit shop, Autopilot Central or Mid-Continent?

[Piloto]

If the unit is not engaging check the CWS switch and wiring. I had a similar problem and found out that the CWS wires on the yoke were chafing with the yoke and causing intermittent operation while in flight. Check also the A/P disconnect switch and wires on the yoke.

 

José

[rainman]

José, the CWS switch has been replaced and engages and disengages correctly. We'll check for chafing. Thanks. I'm in San Franciso for a meeting and will be back next week to check progress. Ray

[rainman]

Autopilots Central is the repair shop . They've been very helpful...but still not solved.

[rainman]

Problem was solved, a short in one of the wires to one of the servos kept overloading the unit and causing the burning. The unit has been repaired again, and the wire replaced. Thanks for all the input. Ray

[Piloto]

Glad you found it, I know the feeling. To prevent chaffing I do not ty-wrap directly to the metal but to a ty-wrap already on the metal. Specially in engine areas like EGT/CHT probes and others. These chaffings are not visible and cause some nasty intermittent problems.

 

José 

[aaronk25]

Glad u found it. I had a altitude hold issue and it was a loose connector pin to the pressure can. Central autopilots do great bench work but even better if you can leave the plane with them as a person can spend a lot of money rebuilding servos and computers when the real problem lies in the airframe....

[rainman]

Thanks guys, good advise. José, I'll ask how the wire was secured, directly to the metal, or to another Ty-wrap. Ray