The DFC-90 requires Aircraft Power to be supplied to "A/P Disconnect" P1-17 and "Trim Interrupt " P2-14. If either input is missing the Autopilot will Disengage. The most common cause of DFC-90 problems is the connector mating into the old S-Tec 55x Tray. Insure that the unit is securely seated into the rack. …
The following is a theory of operation of the pitch/pitch-trim system. The attached drawing will help, and it is advised to opening the drawings while reading through the paragraphs below. The lines highlighted in blue are grounds, and the lines highlighted in yellow are bus voltage (28V). Pitch Down Trim Relay …
NOTE: The Diodes and Resistors called out on the Flap Interface Drawing Are NOT included in the installation kit from Avidyne.