pressure sensor 15 of the engine which generates at its output a signal a t this signal a t is high when the engine temperature measured for example by means of a sensor in cooling water of the engine is smaller than a predetermined value in this example when it is smaller than 20 u00b0 c the temperature sensor 15 is designed as a switching member in the form of an operational amplifier 16 with a feedback the inverting input is supplied via a voltage divider 17 with a constant reference signal whereas the non inverting input is supplied with the measured temperature signal u03b4 n the switchover of the output signal a t when the temperature approaches the value determined by the comparison reference signal thus occurs suddenly the function of the protective circuit 9 of fig1 will be explained below in connection with time plot diagrams in fig2 the monostable multivibrator 10a serves for detecting the aforementioned operational condition 2 occurring after the stalling of the engine to this end the interval of the transitory switching stage of the multivibrator 10a is adjusted so that at a given limit value of the rotary speed that means which is smaller than 300 rotations per minute condition 2 be shorter.