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.
Defective operation of the Trim and Autopilot Disconnect relays may also be an area of concern.