1. Regulating mechanism for the refrigerant flow to the evaporator of refrigerating systems or heat pumps with a compressor, which is arranged in the refrigerant circulation system and is connected or connectable, respectively, with the input of a condenser on the pressure side and with the output of an evaporator on the suction side and which is connectable with the output of said condenser or of a collector on the input side thereof via and expansion valve, an actuation motor for controlling the opening coefficient of said expansion valve, said output of the condenser being directly connected to said expansion valve or arranged as the output of said condenser, comprising a computer (10) for controlling said actuating motor (4) of the expansion valve (8) as a function of set values and as a function of the signals from sensors (5,6), which are sensitive to temperature and are arranged on a refrigerant line at the input and output sides of said evaporator (3) to indicate an input temperature (T.sub.E) and an output temperature (T.sub.A), said computer controlling the actuating motor such that in the event the input temperature (T.sub.E) at said evaporator lies below a threshold value, said expansion valve increasingly opens when the difference between the input and output temperature (T.sub.E,T.sub.A) at said evaporator exceeds a set value or such that said expansion valve increasingly closes when the difference falls below said set value, and closes completely when said input temperature signal (T.sub.E) lies above a threshold value.