Search the Community

Hello, fellow Mooney drivers. I'm writing to ask for advise from those of you with experience in older autopilots and autopilot servos. My 1990 M20J has an old but well functioning Century 2000 autopilot that is coupled to a Century NSD-360 HSI and GNS-530W. The autopilot meets my mission needs well and I have no desire or plans to replace it. A couple of months the autopilot navigation computer failed while engaged and the only indication was a totally dark computer display, no popped circuit breakers, no adverse indication on the controls. Recycling the autopilot circuit breaker had no effect. Testing by an avionics shop confirmed a blown internal fuse in the computer that was easily replaced. After 3 hours of diagnosis no cause for the blown fuse was found, and the computer was found to be operating normally. Now it's happened again, and the CFII/A&P flying with me has suggested the roll or pitch servo (no yaw damper) may be demanding too much voltage or amperage or both from the computer while engaged. I'm requesting your advise on how to troubleshoot the servos before again replacing the internal computer fuse. After the first fuse failure and repair, the airplane flew multiple sorties with the autopilot always passing the preflight check, and remaining powered (cannot be turned off) during the flights but not engaged for navigation or altitude hold. On the flight with the latest computer failure (expecting another blown fuse but not certain yet), we had the autopilot engaged for approximately 1.5 hours during which we completed coupled ILS and RNAV approaches, with everything functioning well. Towards the end of the flight, with the autopilot engaged in the heading mode and altitude hold, we were experimenting with maneuvering the aircraft by turning the heading bug, and that's when the computer again failed in the same mode, by disconnecting, with a totally dark computer indicator, and no adverse indication on the controls. Recycling the autopilot circuit breaker had no effect. This is why the current thinking is one or both of the autopilot servos may be malfunctioning by demanding too much voltage or amperage from the computer to operate and causing the fuse to blow. After downloading the EDM-830 data post flight and reviewing the 1-second recorded voltage history I found no spikes that could perhaps cause the failure, and the voltage varied between 13.7 and 13.9, with the large majority of the time at 13.9 volts. Any thoughts or suggestions on how to test the servos for proper operation and/or what else may be causing the blown fuse would be most welcome.