Abstract: A driveshaft in the form of a crankshaft (12) or a camshaft (212) drives a camshaft (16, 216) by means of a chain (20, 220) which is trained around a sprocket (14, 214) on the driveshaft and another sprocket (18, 218) on the driven camshaft. The driven camshaft is selectively advanced or retarded in its position by lengthening or shortening one portion (20a, 220a) of the chain and simultaneously lengthening or shortening another portion (20b, 220b) thereof by extending one hydraulic tensioning device (22, 122, 222, 322) and simultaneously retracting another hydraulic tensioning device (24, 124, 224, 324). A spool valve member (50) is controllably positioned within a valve body (48), in response to instructions from an engine control unit (58) which acts through a pulse width modulated solenoid (56) to increase or decrease a load acting on one end of the spool, to permit or prevent hydraulic fluid from flowing from one tensioner to another as a result of a pressure differential between the tensioners.