What was found
DURING CRUISE FLIGHT WITH AUTOPILOT ENGAGED THE A/P DISCONNECT SWITCH IN PILOTS YOKE GOT WARM, STARTED SMOKING AND A/P DISCONNECTED. PLANE WAS LANDED AT 26N AND SUBSEQUENTLY THE A/P CIRCUIT BREAKER WAS PULLED AND COLLARED WITH "A/P DEACTIVATED" PLACARD INSTALLED IN PANEL. PLANE WAS THEN FLOWN BACK TO TTN, THE A/P DISCONNECT SWITCH REPLACED WITH NEW SWITCH, THE A/P REACTIVATED AND FUNCTIONAL TESTED GOOD. THE A/P DISCONNECT SWITCH IS A NORMALLY CLOSED SWITCH THAT GETS 28 VDC FROM THE A/P CIRCUIT BREAKER AND PROVIDES 28VDC TO ALL 3 AUTOPILOT SERVOS AND THE A/P COMPUTER "AP DISCONNECT" INPUT. THE SWITCH IN QUESTION DEVELOPED AN INTERNAL RESISTANCE, HEATED UP WHICH INCREASED THE RESISTANCE RESULTING IN THE SMOKE AND A/P DISCONNECT.