As the autopilot has worked for as much as 2 1/2 straight, I don't think it is the wrong switch. However, it could be a ground issue.
Also, when I flew the other day, I could press the AP74 button to reengage, and it would engage for just a second or two. I could change the armed modes and they all light up when pressed, as they should for pre-arming.
My partner in the plane flew yesterday. See his comments below.
On the trip down, once established in a cruise climb and relatively on course, I engaged the AP. It held the GPS course and, per its programming, climbed to the desired 5500 foot altitude where it dutifully leveled off. Without any input from me and without any reason that I can gather, a little over an hour into the flight it shut off and refused to come back on for more than a second or two.
On the return flight, it refused to work at all. In fact, it quit working altogether. By that, I mean that when I was relatively stable and on course and at a constant altitude, I attempted to engage the AP in HDG, TRK and NAV mode and it would stay “on” for no more than a second or two and then disengage.
Several times, after it disengaged and I tried to power it “on” again, I got a funny screen. See Image2 attached. Look at the bottom left of the screen. As you can see, the AP is “OFF/OFF” As that is the case, there should NOT be anything appearing at the bottom right of the screen, but you can clearly see “305/4480FT” This portion of the screen should only display this information when the autopilot is ENGAGED.
During several tried of turning the thing back ON after it simply switched itself OFF, I got an error message on the EFIS screen which said something like “Autopilot Error”. And then, regardless of what buttons I pushed or however many times I attempted to reset the AP74 unit, all I got was what you see in image1, “ERR/ERR.” Interestingly enough, when I pulled the breaker for the AP74, the D180 gave me a bus error telling me that the AP74 was not online. So, the EFIS sees the AP 74 and apparently sees the servos but still shows ERR/ERR.
Houston, we have a problem. I don’t know if it is in the AP74, a Servo, the Efis itself, or what…….