Autopilot steers to port when engaged in AUTO mode

[feickenberg]

I own a 1999 Jeanneau Sun Odyssey 34.2. She’s equipped with a ST4000 PLUS that was installed in 2012. The autopilot is connected to a wheel drive with lever to steer the boat when the autopilot is engaged. She is not equipped with a rudder reference transducer.

Until 3 weeks ago the autopilot worked perfectly fine. Without that I made any changes to the system or the boat the autopilot now always steers to port in several small steps once it gets engaged (AUTO mode).

Wheel Drive:
When I engage the wheel drive with the lever and leave the autopilot in STANDBY I can turn the wheel to port by pushing the -1 and to starboard by pushing +1 buttons which indicates that the wheel drive is working properly.

Fluxgate Compass:
When I press the disp button the heading that is shown is always very similar to the angle that is shown by the ‚regular‘ compass that’s mounted on the pedestal. The displayed angle doesn’t jump and is quite stable (not changing more that what you’d expect because of the movement of the boat). The fluxgate compass is mounted in the main cabin. I checked the resistance between the different wires as described in the „Fluxgate Compass Test“ instruction here in the forum. Resistances are in the range as they are described in the article. Thereby I conclude that the fluxgate compass and wiring are also ok.

The problem:
When I engage the wheel drive with the lever and push AUTO the wheel gets always turned in small steps to port until the end. When I change the course by pressing -1; -10; +1 or +10 the bars below the course indicate correctly the difference between course and heading, but independent if the course needs to be changed to port or starboard the wheel drive turns the wheel always to port until the end.

https://drive.google.com/file/d/1V2N4CRt...sp=sharing

Any idea what’s wrong with the ST4000 PLUS?