For DSallee:

I have the Century IV in my 601P. The lateral and vertical guidance comes from an Aspen PFD Pro (Legacy) through the Aspen ACU. Attitude reference is still the pressure driven attitude gyro that is also a backup attitude display to the Aspen.

Laterally, autopilot NAV mode to GPS and VOR/LOC courses is good for basic leg types. Can't fly a hold or some of the curved approach segments in NAV because the later Garmin 430 software loads don't pass the legs well to the Aspen. On the other hand, Aspen GPSS steering to HDG mode works quite well and will fly the holds, but the magenta path will only be on the G430, not the PFD. Straight non GPSS HDG select tends to wander back and forth...likely an issue with slow turn rates and the response of the Aspen heading output.

Vertically the altitude hold is reasonable and attitude climb and decent are good but there are no advanced functions like automatic altitude capture. Glideslope capture is a bit abrupt, you sure know the mode changed to GS! I do have the glideslope enable wire from the Aspen to the ACU, so the GS mode will also fly a GPS style GP on approach.

Unfortunately the Century IV is getting to the end of its useful life, and the experts that still know how to service and repair these analog electronic components are retiring.

Thinking about a solution to this. Dan I may have some serviceable C-IV components you can try, just lmk.