autopilot woes

[blue-max]

 I am having a difficult time trying to  sort out the fault in my autopilot. Its given great service but this year it has developed a fault - won't keep a course , gradually turns to port . I have checked the obvious components - bought another fluxgate compass and had the course computer professionally checked. Display ST7000+ does not show any heading change  when the compass is rotated in the cockpit (to simulate a sea trial) Maybe the fault is with the display or a sea talk cable. I wonder about the drive unit  but there are no noises that would indicate slipping clutch - wheel turns to port and starboard when buttons on the display are pushed. I don't have  any manuals on board about the drive unit. Its a lewmar chain driven to pedestal shaft unit. Looking on line  this type was fitted by Bavaria in the factory - named Constellation. But being described as suitable upto 39 feet. My boat is 47 feet so  I wonder whether another model has been fitted. Any help much  appreciated. 

[blue-max]

Having found an old lewmar catalogue I think my drive unit could be a Mamber which iscoupled to a rotary sprocket drive but it would be good to have that confirmed

[Yngmar]

If your fluxgate compass is being rotated and this doesn't result in a change in compass heading on the display, then either the fluxgate compass or the course computer have a problem. It has nothing to do with the drive unit and it makes no sense to investigate in that direction.

Did you check with a handbearing compass for magnetic interference near the fluxgate?

[PEA-JAY]

I had a similar problem once, in my case it’s a linear drive and it wouldn’t hold course. I couldn’t figure out what had happened as it worked perfectly prior to haul out. I called Raymarine UK and an engineer asked if I had removed the drive, which I had done. There were two black and red wires going to the drive that I logically reconnected black to black and red to red. In fact in my case they should have been inverted!

[JEN-et-ROSS]

A few years ago our Raymarine autopilot/linear drive began to misbehave and wouldn't hold a steady course and could suddenly veer off course...
Turned out we had stored a folding bike in the port aft cabin and it was up against the hanging locker in which the fluxgate was located..
Due to this experience we keep all metallic objects at a discrete distance...
Incidentally we use this to test the autopilot periodically by waving a screwdriver over the fluxgate and listening for the drive unit frantically responding ....

[Klausen]

Mine worked fine until this year. Then i changed the freshwaterpump and put the old one in the locker under the starborad settee...

[blue-max]

I had a Raymarine agent check the course computer - it was good , and I have another fluxgate to check on the original. No metal interference. From internet research  it seems others who  had this problem (and replaced components unnecessarily) found that there was a poor connection in the system. So I will focus on that next  time in the boat. Drive unit - originally Whitlock Constellation has a design weakness - plastic cogs  - which  are prone to shearing. So i will check on that  and hope to  find a spare. probably upgrade the computer and display.