One detail to look for -- where is power originating for the AP unit?
I have always been told not to rely on the Sea Talk instrument bus with its small-diameter 12 volt pos. wire for the amp requirements of the AP drive motor.
Feed the AP with a its own #14 or #12 wire.
I realize that this is like powering the bus "from both ends."
I installed our A4000S unit in '95 and over the many years it has only had one board replacement (and is now, internally, a "plus" model).
It has been a reliable crew member, overall!
Happy wire researching.
ps: thanks for the great pics of the drive unit. I was wondering what was inside that casing!