Autopilot malfunction

[Geoffreykwright]

I've got a Raymarine C 70 chart plotter.  Every so often the GPS malfunctions - it still knows the location of the boat (lat, long) but the heading can be as much as 180 deg. off my actual heading (as shown with my magentic compass on the pedestal.  Sometimes I re-calibrate the electronic GPS (tell it the heading I am on based on the ship's compass) but sometimes it reverts back to these phantom headings.  Has anyone experienced this?

My boat is year 2000, an not sure if this chart plotter and GPS was installed by the PO or the factory - so I am not sure how old the gear actually.  Its not that big a pain, except when I am trying to sail with my auto-pilot...at which times I suddenly find myself way off my intended course.

I know it can be an issue if you've got a metal part too close to the 'pickup' (not sure the name of the part) for the compass - i've checked and nothing there.

Any thoughts?  Thanks in advance.

[Roc]

Is there an external fluxgate compass for the unit?  Do you know where it might be mounted?  There might be a yellow sticker somewhere on the boat telling you where it is (if the installer put something to show where it's hidden).  If you have a tool box, or something that causes compass interference, sitting next to the compass, that might be the problem.

[Geoffreykwright]

Its in the hanging locker in the aft cabin.  Issue is, its been working fine for a long time - just starting to lose its way now.

[Roc]

I'm guessing there is no metal in or near that hanging locker?

[Jon W]

I've learned the first thing to do is check all the electrical connections for corrosion and tightness before doing anything more expensive.

[Ken Juul]

The flux gate (compass) is normally a part of the autopilot.  The chart plotter should figure out the course on it own using gps data.  May need a software update or reload.

[Dave Spencer]

Geoffrey,
Here are a few ideas and musings to help diagnose your issue.
1.  A GPS / Chartplotter cannot know the "Heading" of the boat.  It only knows its position and can determine its Course Over Ground by comparing change in its position over time.  COG is not the same as Heading although they are often very close. 
2.  The Heading of the boat can only be determined by a compass.  In your case, you have your binnacle compass that doesn't "talk" to the chartplotter or autopilot, however, your autopilot certainly has a fluxgate compass that "talks" to the autopilot brains to keep the vessel on track. 
3.  It isn't a certainty that your autopilot is connected to your chartplotter although many are.  It sounds like yours is likely connected to your chartplotter if you say the Heading shown on your chartplotter can be off 180 deg.  Does this happen suddenly or does the heading accuracy on your chartplotter gradually degrade? 
4.  Is the autopilot functioning when the heading on the chartplotter falls off by up to 180 deg?  If so, does the autopilot command the boat to alter course to follow the degraded heading?  It sounds like it does based on your comment.  And it sounds like it happens suddenly rather than gradually based on your comment.  "Its not that big a pain, except when I am trying to sail with my auto-pilot...at which times I suddenly find myself way off my intended course."
5.  Do you know what kind of Autopilot you have?  Is it an Autohelm ST4000 or a newer Raymarine SPX or EVO or another brand?
6.  I had an Autohelm ST4000 when we bought our boat in 2010.  It had worked for many years before 2010 however in 2012, it started to misbehave and do what I called "Crazy Ivans" (after the Russian submarine manoeuvre in "The Hunt for Red October")  i.e.  After holding course for an hour or so (sometimes more, sometimes less), it would suddenly veer off course by 30 degrees or more.  Stopping and then starting the Autopilot would allow it to resume normal operations until the next "Crazy Ivan".  I lived with it for a year and then replaced the Autopilot.
7.  If your equipment is modern, there may be software downloads available as Ken Juul suggested.  Check Here.https://www.raymarine.com/service-and-support/software-updates/  Jon is also right on to double and triple check connections before pulling out your credit card.
8.  If the equipment is older, there won't be s/w updates available and if the connections all look good it may be time for a new Autopilot.  If you are patient, you might be able to find someone who can troubleshoot and repair legacy equipment but there's no guarantee that it will be fixable. 
9.  In your comments you said "Every so often the GPS malfunctions"   I suspect the GPS/ chartplotter is working fine but is simply displaying the heading that is incorrectly reported by the Autopilot.

Good luck with your detective work.  Let us know how it turns out.