Anyone know how autohelm works. It s has an st5000+. Original spec I think. But keeps saying it can't hold course and goes back to standby.
Hi Chris,
You didn't mention if it was working when you trialled the boat?
Nearly all autopilots work very much the same, unless they are high tech and computerised. The Raymarine ST5000 plus was a good unit and slightly above entry level units. They work by using their own compass, usually a Fluxgate type, which very often disagrees with the plotter and/or ships compass. The reason is mainly that they haven't been swung i.e. swung to the boats deviational magnetic fields or the ships compass hasn't been swung. They can also be connected via cabling to other GPS or plotter units.
The control head units of this type usually have a Compass course display with buttons which are +1 - 1 as well as +10 - 10 then Auto and Stand By. As you no doubt know the buttons are course changers of either 1° or 10° plus or minus. It'll probably have a rudder reference transducer and a sterndrive actuator.
This for me sounds like a problem with the compass wiring or calibration or in fact it maybe more than one fault so you may have to take each a step at a time. You could try calibrating it. I think you have to press and hold the stand by button in for a second or two, the screen might read "cal" or something like that. Then turn the boat in a very slow speed circle about 3mins to do a complete circle. Keep turning until you get a reading, I can't remember exactly but I think you get a deviation reading as well as a heading. If this works, press and hold the stand by button for about a second again to save.
See if that works.

Edit.. forgot to say to check wiring first, if you're able to, sometimes it's tucked away out of reach. Possible corrosion somewhere, connectors etc.