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Simrad AP20 autopilot compass gone wild

Old 08-21-2017, 02:46 AM
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Default Simrad AP20 autopilot compass gone wild

Been using an ap20 on our boat for about 15 years now and it has pretty much performed flawlessly. Recently, the compass reading started indicating a significant variance from the ships compass. Depending on direction, the difference is anywhere from 10 to 60 degrees. autopilot still works fine in AUTO mode as once course is determined, the actual compass heading is irrelevant, but can't handle NAV mode as the NAV steering relies on compass heading as well as the XTE data from the GPS. Haven't installed any new motors or electronics that might be affecting the rfc35 compass.

Tried installing a different rfc35 compass and it seems to display the same errors. Tried the compass calibration portion of setup and it failed. Would appreciate any ideas from users here.
Old 08-21-2017, 05:42 AM
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Sounds like you added something to the boat that is largish made of metal and too close to the compass.
Tool box, tackle, etc.
Old 08-21-2017, 10:20 AM
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Originally Posted by LI32 View Post
Sounds like you added something to the boat that is largish made of metal and too close to the compass.
Tool box, tackle, etc.
As indicated in my post, I haven't added anything, changed anything, or removed anything
Old 08-21-2017, 01:33 PM
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It doesn't take much of a misplaced ferrous or electromagnetic item to throw a heading sensor way off if it is close enough. Do you know where yours is mounted?

Cell phone in a jacket pocket stored there, fishing hook , pliers, `a steel bolt, small magnet or speaker or any active wiring not twisted can all confuse it if very close. Just curious, do you have a fluxgate or rate compass heading sensor?


Once you confirm no obvious influences are too near, go through the Sea Trial re-calibration procedure which should be done ~~every year or whenever new influences are installed.
Old 08-21-2017, 01:53 PM
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I’ve got the rcf35 fluxgate compass. The compass is mounted on the boat’s centerline, on the floor of a cabinet in main cabin. It’s been in that same location for 16 years, and the contents of the cabinet have not changed, nor has the installation of any nearby electronics. I guess I can try taking everything out of the cabinet and relocating to another portion of the boat, but it’s hard for me to believe that’s was causing the variation, as it all worked fine for 15+ years, and has only recently become an issue.
Old 08-21-2017, 11:09 PM
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Sorry for the length of the post. Maybe there will be something here to help a little.

I'm afraid this 1st won't help your present situation, but you might be interested to hear the new AP44 has display readouts for " Local Field % " ( which indicates the strength of influence an offending object has on the heading sensor) , and also "Field Angle" which even is an indication as to which relative direction from the heading sensor that influence should be found.

IMO, It's a fantastic feature, even if it confirmed there is presently no such substantial influence as you feel is the case.

You can see it on the AP44 halfway down in this thread:
http://www.thehulltruth.com/marine-e...ff-course.html

Now to a few suggestions and things to consider:

You might just check that RFC35 sensor to make sure it has not come loose to perhaps swing away from aiming along or parallel to the vessel centerline. Check connections at both ends .

If it is still correctly positioned, I believe there is a way to roughly determine which relative "field angle " to check , by slowly motoring in a large circle while watching the dash compass or even GPS heading as long as there is/are no current , wind or waves acting on the hull. You would have to carefully note on which headings the AP heading sensor readings agree or diverge the most with/from the heading source you still trust most.

Unfortunately , right now I can't quite wrap my head around which direction that would suggest you look for any "influence" , because it would be competing with magnetic N .
[ I "think" the direction from your fluxgate you should check would be where boat compass( not the boat centerline) points N when the magnetic compass and fluxgate compass agree best, because the influence and mag. N would be aligned.]

Anyone feel free to correct if you feel that would be incorrect.

It could be a failing fluxgate but I'd think your AP 20 might flash an error code if it was bad enough.
Sometimes it takes a couple /few tries turning that even, slow circle in 45-60 seconds (keep going for at least 1 1/4 turns) to successfully complete calibration, but if you keep getting "RFC Failed" either any influence is still just too strong or the sensor may be bad.

Has your dash compass been adjusted , and fairly closely matches indicated-heading with the GPS when straight underway & there is no current , wind or wave influence to offset?
FWIW- I've had a steel fishing hook at least 10" away from a dash compass on one of my boats that caused an error of up to 50* on some headings and down to 0* on others until I moved the hook another 6 or 8" away.

When you empty out the cabinet and anything near it , have someone watch the AP20 Standby heading readout to see if it starts to change noticeably and closer to the dash compass. And try to have the boat aimed such that error is greatest before you start. Don't compare to the GPS for this since that just shows which direction the boat is moving over ground.


You ...could.. also take a handheld compass from anywhere it seems to read close to what you think is "magnetic correct", and slowly move it toward that cabinet and nearby areas to see if the needle moves toward any possible culprits, but there will be loads of those on a motorboat.
The cabinet doesn't have magnetic catches by any chance?


Hope you are surprised and find something big or small which makes an obvious difference you can fix.
Best luck.
Old 08-22-2017, 03:14 AM
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TTaxi - Thanks for your interest in my issue and the suggestions you have offered.

Unfortunately, except for the removal of all items in the cabinet or nearby, I don't think there is much more I can do right now. The original rfc35 was not loose in its mounting and checking connections was one of the first things I did when this issue surfaced. I even obtained another rfc35, mounted it in the same place, and it too displayed the same erratic readouts. The ships compass reads relatively close to GPS headings while underway in all directions.

I'll attempt the cabinet cleanout this week and report back.

Thanks again
Old 08-22-2017, 01:32 PM
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On a bouncing boat , could something ferrous have slipped down unnoticed behind or next to the cabinet base? Flashlight batteries, etc.?
Old 08-28-2017, 04:01 AM
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Update - I removed everything from the cabinet and the counter above. Thought I noticed a slight change in the at the dock compass reading when some large diamond jigs were removed from the counter. But, on the water, compass calibration failed again. Additionally, while compass used to hold a steady but erroneous reading so that AUTO mode could be used, now the compass reading on the ap20 is all over the place.

Going to try a used j300x (autopilot computer) from ebay to see if I can get the system working again as I can't afford a whole new system at this time. Hate to troubleshoot by replacing parts one at a time, but I'm lost.

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