Narrative:

Aircraft had a recent avionics upgrade garmin 650; G750 and auto pilot.filed ZZZ direct zzzzz [then to the destination] 11;000 feet. Departed ZZZ weather was 500 overcast and 1 mile visibility. Takeoff and departure was uneventful other than icing conditions enroute (all anti ice was on) at approximately 25 miles from zzzzz intersection center inquired about my altitude stating I was 300 to 400 feet high with the correct altimeter setting; I reset the altimeter which corrected the problem; shortly after that the autopilot disengaged causing the aircraft to yaw hard. Hand flying the aircraft I contacted center about the autopilot and request to return to ZZZ. After completing the turn towards ZZZ it became apparent that I was having a problem trimming the aircraft for level flight; which wanted to pitch downward even after adjusting the trim wheel which was stiff to turn. During this time period ATC notified me of low altitude and to take immediate action which I did; holding the aircraft in level flight. I tried engaging the autopilot which would nose over abruptly. With not being able to trim the aircraft properly I was having difficulty maintaining heading and altitude; while trying to program the GPS for the approach into ZZZ. I had to go missed due to low visibility I then diverted to ZZZ1. I continued to have difficulty holding altitude and programing the GPS while enroute to ZZZ1. Tried the autopilot again which still nosed the aircraft downward. Flying into IMC and icing conditions at night with not being proficient with the new avionics contributed to not being able to maintain altitude. Not sure about mechanical issue other then I think the trim tab deice was not working properly causing the trim issues.

Related reports:

Google
 

Original NASA ASRS Text

Title: AC690 pilot reported autoflight and flight control trim system malfunctions during hazardous weather resulting in weather diversion.

Narrative: Aircraft had a recent avionics upgrade Garmin 650; G750 and Auto pilot.Filed ZZZ direct ZZZZZ [then to the destination] 11;000 feet. Departed ZZZ weather was 500 overcast and 1 mile visibility. Takeoff and departure was uneventful other than Icing conditions enroute (all anti ice was on) at approximately 25 miles from ZZZZZ intersection Center inquired about my altitude stating I was 300 to 400 feet high with the correct altimeter setting; I reset the altimeter which corrected the problem; shortly after that the Autopilot disengaged causing the aircraft to yaw hard. Hand flying the aircraft I contacted Center about the Autopilot and request to return to ZZZ. After completing the turn towards ZZZ it became apparent that I was having a problem trimming the aircraft for level flight; which wanted to pitch downward even after adjusting the trim wheel which was stiff to turn. During this time period ATC notified me of low altitude and to take immediate action which I did; holding the aircraft in level flight. I tried engaging the autopilot which would nose over abruptly. With not being able to trim the aircraft properly I was having difficulty maintaining heading and altitude; while trying to program the GPS for the approach into ZZZ. I had to go missed due to low visibility I then diverted to ZZZ1. I continued to have difficulty holding altitude and programing the GPS while enroute to ZZZ1. Tried the autopilot again which still nosed the aircraft downward. Flying into IMC and icing conditions at night with not being proficient with the new avionics contributed to not being able to maintain altitude. Not sure about mechanical issue other then I think the trim tab deice was not working properly causing the trim issues.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.