Control system for internal combustion engines, provided with an exhaust gas recirculation control having a fail safe function

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 system and an exhaust system, said air/fuel ratio control system comprising:

• sensor means for detecting the concentration of an exhaust gas ingredient emitted from said engine;
• fuel quantity adjusting means for producing said mixture being supplied to said engine;
• an electrical circuit operatively connecting said sensor means with said fuel quantity adjusting means in a manner effecting feedback control operation to control the air/fuel ratio of said mixture to a predetermined value in response to an output signal produced by said sensor means;
• an exhaust gas recirculation passageway communicating said intake system of said engine with said exhaust system of same;
• an exhaust gas recirculation control valve having negative pressure-actuatable means and arranged across said exhaust gas recirculation passageway;
• a negative pressure passageway communicating said negative pressure-actuatable means of said exhaust gas recirculation control valve with said intake system of said engine;
• and a solenoid valve having a solenoid and arranged to open and close said negative pressure passageway;
• said electrical circuit including a power transistor connected to said solenoid of said solenoid valve for energization of same, an abnormality detecting circuit arranged to compare the level of an input applied to said power transistor with the level of an output of same and generate a first signal when said two levels are out of a predetermined relationship, a timer arranged to generate a second signal when said first signal is continuously generated by said abnormality detecting circuit for a predetermined period of time, and emergency means actuatable by said second signal generated by said timer.