The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

self-acting
    adj 1: designed to activate or move or regulate itself; "a self-
           activating sprinkler system" [syn: self-acting, self-
           activating, self-moving, self-regulating]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "self-acting":
   arbitrary, automated, automatic, automatous, autonomous,
   cybernated, discretional, discretionary, elective, free, free will,
   gratuitous, independent, nonmandatory, offered, optional,
   proffered, self-active, self-adjusting, self-closing,
   self-controlled, self-cooking, self-determined, self-determining,
   self-directed, self-directing, self-dumping, self-governed,
   self-governing, self-lighting, self-operative, self-regulative,
   self-starting, self-winding, self-working, semiautomatic,
   spontaneous, unasked, unbesought, unbidden, uncalled-for,
   uncoerced, uncompelled, unforced, uninfluenced, uninvited,
   unpressured, unprompted, unrequested, unrequired, unsolicited,
   unsought, voluntary, volunteer, willful