Simrad AP 26 autopilot interfaced with new Garmin GPS

Take the boat out and put the autopilot in setup mode, swing/calibrate the compass on the autopilot, then do the seatrial for the autopilot. See if that helps.

Sort of reads like a battle between the fluxgate of the pilot and the GPS. Would assume here that the Garmin is using the GPS for heading information and that isn't in harmony with the data coming from the pilot compass. You could perhaps point the Garmin to use the pilot fluxgate compass as its heading source and see if that corrects the issue. Or, as stated, recalibrate the pilot from scratch and get the systems all in harmony.

In auto mode, the AP26 uses the Simrad autopilot compass for heading and that is it. Should not be getting any heading information from the Garmin for steering the boat.

Never stated that the AP is using the Garmin compass. Said that the AP and Garmin are using different compass sources, and that those two sources are not calibrated. Thought I was clear. Meant to be clear...

Had a mess of an issue with my AP, MFD, and Radar all misaligned. Thank you, Highseas. AP wanted to go 300 degrees, Waypoint was due north, and radar overplayed land on top of my position. Boat Icon was going sideways everywhere. Once all were dialed back in, and I told the MFD to use the AP fluxgate as its heading source, no more issues.

This sounds like a calibration problem as Capt J. already pointed out. Go thru the sea trial set up again then see what happens. I am baffled by the talk of a "Garmin Compass". The plotter has no compass. It will read COG (course over ground) based on the boats movement thru the water as determined by the GPS. This will often vary slightly from "heading" which is magnetic and will come from the AP if the AP and Plotter are integrated. If the "heading" and "COG" vary by a large margin then their is a calibration problem or magnetic interference with the AP compass. Once the sea trial set-up is redone I would just put the AP in heading hold and see if it can drive a straight course. It should be able to do that totally independently of the plotter. If it can do that, then you can monkey with asking it to drive to a point or follow a route which depend on the AP and plotter exchanging data via N2K.

Don't be baffled by the talk of "Garmin Compass". The reference is very simple. Every MFD uses some source of data for heading. So does the auto pilot. Making those one in the same simplifies the task of calibration, if it is possible to do so.

I don't see any reference to something the OP has that could be considered a "Garmin Compass". I consider a compass to be a magnetic or gyroscopic device that determines heading. He makes no mention of having anything that would be a "Garmin Compass". My interpretation of his set up is Garmin Plotter, traditional analog Helm Compass (like all boats), and Simrad AP (also with compass). So i'm not clear on what the "Garmin Compass" would be. The only thing OP seems to have that's Garmin is a plotter. The plotter it's self has no ability to determine "heading" on its own because it has NO compass. If you do not have an external heading sensor connected (ie AP compass, Garmin Steadycast or other external magnetic or gyro compass), the plotter will display NO heading data. What it will display is course over ground COG based on the boats movement thru the water. Boat stops moving and COG becomes meaningless.

OP stated "I noticed that the heading on the GPS (146 degrees) my compass (150 degrees) and the AP26 (105 degrees) " His GPS is displaying COG, which matches pretty close to his compass heading which I presume is a helm mounted magnetic compass. This makes sense. One of his problems appears to be his AP is displaying a heading that is WAY off from the Garmin COG and the helm compass. This is likely either a calibration problem of the AP or there is magnetic interference with the AP compass. Easiest thing to try first is simply recalibrate the AP via the sea trial set up.

My point for OP is simply confirm that the AP can steer a straight course and displays a heading that matches relatively closely with COG and helm compass. This can be done with zero interface between the AP and plotter. If all that works then he can get bogged down in if the AP and plotter can talk to each other.

Every MFD requires access to some device that provides it with electronic heading information. Most basic systems will get that data from a GPS. More elaborate will get the data from the AP or some other additional fluxgate-type appendage. Same MFD. Different setups. The OP didn't mention every feature incorporated in the system installation. So much of the banter here is speculation.

When a modification is made to a system installation, variables for potential conflict are introduced. Here the owner has added a new MFD to pair with an old AP. The headings are conflicting along with various other conflicts. That isn't the fault of the new MFD or the AP. It's an installation, setup, calibration issue. If the MFD is looking at your watch for time of day, and the AP is looking at mine for the same, and out watches are set to different time zones, well, they aren't going to collaborate very well...

In my Ray MFD settings (using LH3) I can go right to a setting from which I can choose amongst the available sources of heading for the MFD to rely upon. When I switched from the same Simrad AP to the new Ray AP, I had similar conflicts appear. Going into this setting component I was able to point the devices to rely upon the AP compass that had been added with that installation, and then everything lined up. During the AP calibration, everything dialed in as would be hoped.