Fluxgate compass deviation

[Clay Greene]

Any ideas on why the fluxgate compass for my Raymarine S1 autopilot would suddenly start to experience 30 degrees of deviation?  The compass has been in the same location for the last three years.  We calibrated it by doing the two 360 degree turns when we installed it and it indicated it had two degrees of deviation.  It has always tracked the GPS heading and the pedestal compass very closely.  A couple of weeks ago, we noticed that it was 45-60 degrees off.  We recalibrated it last night and it reported 30 degrees of deviation.  We have not put anything metal in the compartment in which it is installed.  In fact, I took everything out of that compartment and it had no effect.  We did make some changes to the networked instrument wiring this summer that ran through the autopilot "brain" but it does not seem likely that signal interference from one of these other wires would be causing this result.  Thoughts? Ideas?  Thanks!

[Ken Juul]

If I remember correctly the mounting instructions said not to place anything metal with in 3 ft of the fluxgate.  Have you changed/added anything in the compartment next to it?  Have you re routed any electrical lines that may be effecting it?

[Terry Forshier]

When I installed my autopilot (similar raymarine) and fluxgate compass I had to not only do the 360's but I also had to calibrate the compass system. If I recall I had to reset the display to corespond to the GPS and the other systems on board. (I also have a tac tic fluxgate compass for that system and I have the binnacle compass on the helm and a RayM GPS) so I should know where I am going. All four now have  varied somewhat over time even though I set them all the same to start. Maybe location in the boat maybe just "ghosts"?
Have you reset the system to the now correct compass settings? If you have then does it revert back to the 30 degree variation?

[Stu Jackson]

1.  You did something, and then it changed.  Not sure I understand how you can then ignore the implications.

2.  Cell phone in your pocket?  Been reported to have happened...

[Clay Greene]

As far as we can tell, we have not added anything metal within 3 feet of the compartment.  I took everything out of the compartment and it had no impact on the deviation. 

After we did the two 360 degree circles, we set the heading according to the GPS (you just need to press "auto" in the calibrate heading page and it takes the heading from the chartplotter through a NMEA 0183 connection into the Sea Talk system).  The fluxgate compass reading at the autopilot control heading was right as long as we stayed in a straight line but it deviated about 45-60 degrees again as soon as we tacked. 

And I understand about following the implications of the wiring changes, but I don't get how signal interference from another wire could cause a consistent deviation problem. 

[Terry Forshier]

I don't know if this will help at all but when i initially set mine up it was off too. Reading the manual I checked and the wires to the control box were the wrong color and in the wrong holes. I kept turning and could not calibrate. Might check those.

[Clay Greene]

Thanks - I will check that out!