Abstract: The present invention relates to an automatic gear box and a control therefor of a motor vehicle, including two pulleys connected by a transmission belt (driving belt), each of which is formed by a couple of disks, of which one is movable in regard to the other one, in order to vary the winding radius of the belt. The control for the automatic gear box includes an hydraulic circuit which incorporates a number of actuating cylinders, a couple of delaying valves and a valve for issuing a fluid gradually and under increasing pressure only if the inlet pressure is above a preestablished value.

Abstract: The valve is arranged to control the pressure of a hydraulic fluid by discharging a proportion of its flow, and comprises a mobile valve element arranged to control the opening of a discharge port of a first chamber and kept normally closed by the action of a spring, a piston element rigid with said valve element and arranged to define a second chamber which is in hydraulic communication with the first by way of a first bore of predetermined size, and a third chamber in hydraulic communication both with the preceding by way of a second bore also of predetermined size, and with the discharge outlet by way of a discharge port the opening of which is controlled by an interception member operated by an electromagnet; the pressure of said fluid is controlled by varying the discharge port controlled by said interception member and consequently the rate of a flow of control fluid passing through said first bore, second chamber, second bore and third chamber.