The biggest difference between machines and mechanisms for Deleuze and Guattari is that a mechanism is a closed system, whereas a machine is an open system. A machine is never closed, that’s the whole point of it always coupling with fluxes, opening connexions, and producing effects. A machine is not a structure, neither signifying nor cybernetic.
A second difference is that a machine is diachronic in all its parts and as a whole produced as one more part, whereas a mechanism is synchronic (even if it may contain diachronic parts, or becomings, they are territorialised).
A third difference is the perpetual and universal interactivity of machines far beyond any sort of contact within a closed structure – “withdrawal” is an improper concept in a machinic universe.
A fourth difference is that “machine” and “machinic universe”, according to Deleuze, are the most adequate concepts to underline the equation image=movement=matter. The universe is the machinic assemblage of movement-images.
NB: I have been summarising these points from Deleuze’s cinema course, lecture 5th January, 1982). This lecture provides an expanded treatment of the points that we can find on page 59 of THE MOVEMENT-IMAGE:
“mechanism involves closed systems, actions of contact, immobile instantaneous sections”
or as I have expressed it: machinism involves openness, interactivity, diachrony.