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The most common cause of erratic DFC operation is the Start Voltages of the Roll and Pitch Trim Motors.
A good Motor will begin to respond at around 2 Volts. Any voltage beyond 2.5 volts will cause erratic operation of the autopilot.
The autopilot is also designed with current monitors that will disengage the autopilot if excessive current is detected due to excessive drive voltages.
Have any inspections been performed on the operational voltage of both the Roll and Pitch Trim Motors?
Aging Relay Contacts may also play a part in raising the current draw.
This could also be caused by the tray. If the DFC90 replaced the STEC, the (gold or bronzed color) STEC tray may have been used. In this case, re-seating the unit CAN help. It is also possible that the guide pins were used and should have been removed before installation. In some cases it may be required to replace the mounting tray.