A four wheel drive torque transfer mechanism with a hydraulic control system is switchable between two wheel drive and four wheel drive during running of a vehicle having an automatic transmission. A reducer permits direct coupling or reduced speed ratio drives. A parking gear associating with the transfer device requires only small modification to the conventional parking mechanism. The hydraulic control system automatically actuates the reducer to provide a reduced speed-high torque output when a governor output responsive to vehicle speed, falls below a predetermined level.
