Narrative:

[I picked up aircraft after] installation of integrated auto-pilot to the grt EFIS system. A stand alone auto-pilot had been removed and replaced with the new fully integrated unit. Everything had bench tested and checked out on the ground satisfactorily. I was returning to [home base]. Weather was VFR however I filed IFR to expedite leaving the [busy] area. Autopilot functioned ok upon leveling at 10;000 ft; however tracking was 'hunting for heading'. As I started my descent the autopilot developed a mind of its own; [and] was searching for altitude which I had pre-set in the EFIS which was driving the new autopilot. The servos were pulsating the control stick and I could not stop it. I slowed my descent and airspeed to try to diagnose the problem. I decided to divert to [a nearby airport] which was approximately 40 miles farther from my home base as I knew there were facilities there in the event I developed further problems. I contacted approach approximately 40 miles out and they cleared me to 7;000 ft and 220 heading to the airport. The autopilot would not level at 7;000 and deviated about 500 ft below and as I fought the stick to stop the oscillations it then zoomed up to about 7;300 ft. The stick was fighting me and I was attempting to hold 7;000 ft and during the button pushing of trying to control the autopilot I somehow lost contact with approach control. I was trying to shut off the auto-pilot but it would not release. After fighting with the controls I finally managed to get the autopilot off. I called approach again and they cleared me for the visual. I do not know why I lost radio contact for the few minutes but once the EFIS shut the autopilot off everything returned pretty much too normal. The remaining approach and landing was uneventful except that my body was shaking. The aircraft was taken to an avionics shop and remains there as we are trying to diagnose the problem.in retrospect; when the first issues developed I should have canceled IFR and continued VFR. I tried the master off switch as well as the off switch on the stick; to no avail; and as a result I wasted valuable time as I was caught off guard by the events. ATC was very professional to me after I busted the 7;000 clearance and finally got back on frequency with them after the autopilot was off. In as much as it was VFR I probably should have pulled the circuit breaker on the EFIS (which drives the autopilot) but I was hesitant to as I would have lost all navigation functions. I have always developed a habit of always flying the plane by myself for at least an hour after it comes out of maintenance before ever letting anyone else fly with me. This strengthened my reasoning for always doing that.

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Original NASA ASRS Text

Title: The pilot of an experimental aircraft reported having control difficulties after a new autopilot was installed.

Narrative: [I picked up aircraft after] installation of integrated auto-pilot to the GRT EFIS system. A stand alone auto-pilot had been removed and replaced with the new fully integrated unit. Everything had bench tested and checked out on the ground satisfactorily. I was returning to [home base]. Weather was VFR however I filed IFR to expedite leaving the [busy] area. Autopilot functioned OK upon leveling at 10;000 FT; however tracking was 'hunting for heading'. As I started my descent the autopilot developed a mind of its own; [and] was searching for altitude which I had pre-set in the EFIS which was driving the new autopilot. The servos were pulsating the control stick and I could not stop it. I slowed my descent and airspeed to try to diagnose the problem. I decided to divert to [a nearby airport] which was approximately 40 miles farther from my home base as I knew there were facilities there in the event I developed further problems. I contacted Approach approximately 40 miles out and they cleared me to 7;000 FT and 220 heading to the airport. The autopilot would not level at 7;000 and deviated about 500 FT below and as I fought the stick to stop the oscillations it then zoomed up to about 7;300 FT. The stick was fighting me and I was attempting to hold 7;000 FT and during the button pushing of trying to control the autopilot I somehow lost contact with Approach Control. I was trying to shut off the auto-pilot but it would not release. After fighting with the controls I finally managed to get the autopilot off. I called Approach again and they cleared me for the visual. I do not know why I lost radio contact for the few minutes but once the EFIS shut the autopilot off everything returned pretty much too normal. The remaining approach and landing was uneventful except that my body was shaking. The aircraft was taken to an avionics shop and remains there as we are trying to diagnose the problem.In retrospect; when the first issues developed I should have canceled IFR and continued VFR. I tried the master off switch as well as the off switch on the stick; to no avail; and as a result I wasted valuable time as I was caught off guard by the events. ATC was very professional to me after I busted the 7;000 clearance and finally got back on frequency with them after the autopilot was off. In as much as it was VFR I probably should have pulled the circuit breaker on the EFIS (which drives the autopilot) but I was hesitant to as I would have lost ALL navigation functions. I have always developed a habit of always flying the plane by myself for at least an hour after it comes out of maintenance before EVER letting anyone else fly with me. This strengthened my reasoning for always doing that.

Data retrieved from NASA's ASRS site as of July 2013 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.