An electronic butterfly valve adjusting mechanism wherein the current supply to the adjusting motor (6), which is equivalent to the sum of all of the mechanical moments acting on the motor shaft, is monitored and a "window" is preselected within which the motor current/moment must lie if the butterfly valve and its adjusting components are operating normally. If, in the static case, the sum of all of the mechanical moments acting on the motor shaft is detected to be outside the "window" then it is concluded that a mechanical fault condition exists and the maximum possible angle of the butterfly valve is arranged to be reduced to a safe level. The driver can also be warned by a suitable visual signal.