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.

Check the connectors, dirty connectors can cause the DFC to auto disconnect. (photo is from an R9, DFC100 install)