Autopilot won't hold course

[Captain Spike]

I have a 2000 28' with Volvo outdrives and a Simrad AP25 autopilot. It used to work great but now it won't hold a course for more than a few minutes before the off course alarm comes on. It doesn't make a difference whether it's set to "go to" a waypoint on the plotter or just to hold a bearing, it strays and over corrects until the alarm sounds. Any ideas where I should start looking for the fault? Also last time I used the boat the steering seemed very vague and wandery, are the two faults connected?

[Split Decision]

I would first check you steering fluid. If that is low, the autopilot won't be able to correct. This happened to me when I had a leak and was low on fluid.

[Captain Spike]

Thanks, I'll ask the mechanic to check that out.

[Bluefin 32]

Another possible culprit is your heading sensor. They do go bad and will produce this result without necessarily triggering an error message. Have you tried redoing the compass calibration procedure? Does the heading shown on the autopilot agree with the course shown on your GPS?

[chevz71]

I had my toolbox within inches of my heading sensor and my ap was constintly deviating, moved it and all is good

[Nutz]

Try also going through the installation setup on the unit that was done when it was first installed.

[Over Billin]

I had the same issues, 2002 Volvo / outdrives:
Fluid was low / air.
Steering knuckles / joints / pins were dry and corroded from lack of service, I removed cleaned and applied water resistent anti-sieze. Still working on getting fully greased thru zirks.

To gain access, I had to remove live well, which opened everything up.

[Captain Spike]

Thanks for the suggestions everyone. I'll get it checked out.