Abstract: A planetary gear system employs a retarder to provide fluid coupling of power during startup and fluid braking during deceleration. At idle with the output shaft stationary, an input shaft rotates in one direction and the retarder rotates in the opposite traction. When the retarder is activated by adding working fluid, the resultant torque coupled to the output shaft rotates the output shaft in the same direction as the input shaft. The retarder rotor can be mechanically braked to produce a first mechanical gear. Further coupling rotates the input shaft, output shaft and retarder rotor at the same speed in the same direction to form a second mechanical gear. Hydrodynamic braking can be performed in the second mechanical gear by adding working fluid to the retarder. A control system is disclosed which varies the level of working fluid during acceleration to a value which permits the input shaft to rotate at a required value regardless of the acceleration input signal.