Abstract: The adjustment device (1) comprises a selector (4) which is capable of being actuated by an operator, a coder (5) which is driven in rotation by the selector (4), when the latter is actuated, an arithmetic unit (UC) which receives the signal emitted by the coder (5), formats it, makes a computation to deduce the selected value therefrom and is capable of transmitting the selected value thus computed, a first detection means (7) which is capable of detecting an actuation of the selector (4), a second detection means (8) which is capable of detecting the emission of a signal indicating a change of the selected value, and a comparison means (9) which deduces a malfunction when only one of the first and second detection means (7, 8) has made a detection.