Control system for internal combustion engines, having function of detecting abnormalities in engine speed signal detecting system

An air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine having an intake pipe, which comprises:

• an O.sub.2 sensor for detecting the concentration of oxygen present in exhaust gases emitted from said engine;
• a carburetor for producing said mixture being supplied to said engine;
• means operatively connecting said O.sub.2 sensor with said carburetor in a manner effecting feedback control operation to control the air/fuel ratio of said mixture to a predetermined value, said connecting means including an electrical circuit, a valve adapted to vary the air/fuel ratio of said mixture, and a pulse motor responsive to a signal outputted from said electrical circuit to drive said valve;
• a first sensor for detecing pressure in said intake pipe of said engine to produce an output indicative of said pressure;
• and a second sensor for detecting the rotational speed of said engine to produce an output indicative of said rotational speed;
• said electrical circuit including a first circuit responsive to said output of said first sensor for producing a first signal when the value of said output of said first sensor is smaller than a first predetermined value, a second circuit responsive to said output of said second sensor for producing a second signal when the value of said second sensor is smaller than a second predetermined value, a timer associated with said first circuit and said second circuit for producing a third signal when there is a concurrence of said first signal and said second signal lasting for a predetermined period of time, and means actuatable by said third signal, wherein said first predetermined value and said second predetermined value are set such that they are incompatible with each other when said engine is in a normal operating state.