A vehicle brake slack adjuster includes a housing coupled to a brake actuator and supporting a driving worm wheel engaging the brake cam shaft and a driving worm screw meshed with the driving worm wheel and transferring a brake actuating force, imparted by the brake actuator to the housing, to the driving worm wheel and cam shaft. The adjuster further includes a wear adjustment mechanism including a reference arm fixed against movement and including a control ring gear, a pinion gear meshed with the control ring gear and configured for rotation about an axis, a clutch ring disposed about the axis and configured for rotation with the pinion gear, a control worm screw disposed about the axis, a wrap spring engaging surfaces of the clutch ring and control worm screw and a control worm wheel meshed with the control worm screw and configured for rotation with the driving worm screw.