Ok got it now. There seems to be an error in the diagram in that the depth transducer should not be connected to the autopilot control head.

I SUSPECT, I don't know, that there is a clash similar to what the ray technician was on about. Lifting the red wire is only removing the positive 12v from the st bus. Here is what I would do:
(i suspect the multi as being a bad boy)
Disconnect equipment so that you have the S3, the GPS125, and the ST6002 ONLY connected together, everything else off....
You should be able to power up and press "auto" and the ap should be able to follow that course forever or until you press "stby" or "track".

Test this please. Then one by one add other instruments until you find the one that is creating the problem.

On Gilana, I created a seatalk BUS that is powered directly from the regulated 12v power supply, the seatalk only uses the black and yellow, and goes to every ST instrument. I had a multi display in the aft cabin, and when I stopped the plotter to save power, the multi caused new headings to be locked into the st6000+ control head. Good luck and let me know

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