Source : Webster's Revised Unabridged Dictionary (1913)

Self-acting \Self`-act"ing\, a.
   Acting of or by one's self or by itself; -- said especially
   of a machine or mechanism which is made to perform of or for
   itself what is usually done by human agency; automatic; as, a
   self-acting feed apparatus; a self-acting mule; a self-acting
   press.

Source : WordNet®

self-acting
     adj : designed to activate or move or regulate itself; "a
           self-activating sprinkler system" [syn: {self-activating},
            {self-moving}, {self-regulating}]