Toyota's A.D.D. (Automatic Differential Disconnect) system includes this actuator assy on the left front axle. When in 4WD a sleeve inside is moved into a position connecting the left front axle. The unfortunate part of this system is that it includes electrical, vacum, and electro-magnetic systems leaving way too many possibilities for failure. The system failed on me during the first snow of Dec. 2010 making it so I could not engage 4WD.