Diesel engines are extensively used in agricultural machineries such as water pumps and power generators. The protection and maintenance of these engines have been a major concern especially in remote areas. In this paper the relation of diesel engine temperature with fan belt and radiator water was investigated. In the normal condition, the fan and radiator control the engine temperature, so temperature increase would be limited. If, by any reason, the fan belt snaps or radiator leaks, the engine temperature will overflow the defined limit leading to weak operation. An electro-mechanical system was designed to protect diesel engine based on the critical temperature. The system constituted an electro-valve and electronic devices which were supplied by two voltages from an electronic circuit. This circuit switched the diesel engine off if the engine temperature exceeded the critical temperature.