Idle speed control device for an engine

An idle speed control device for an engine having an inlet air passage and a throttle valve disposed thereon, comprising:

• an inlet air bypass passage connecting the portions of the inlet air passage upstream and downstream of the throttle valve for supplying inlet air to the engine without passing through the throttle valve;
• a two-solenoid rotary type idle speed control valve disposed on said inlet air bypass passage having an opening solenoid for urging said valve to open and a closing solenoid for urging said valve to close;
• a bypass air control means for generating a control signal which controls the degree of opening of said idle speed control valve in such a manner that the idle speed of the engine coincides with a predetermined target speed;
• a drive means for driving said idle speed control valve by supplying electric current to said opening solenoid and said closing solenoid in accordance with said control signal;
• a failure detecting means for detecting the failure of said solenoids;
• a bypass air flow detecting means for detecting the amount of air flowing through the inlet air bypass passage when the failure of the either of the solenoids is detected;
• and,
• an emergency control means for controlling said bypass air control means when one of the solenoids fails in such a manner that said bypass air control means generates said control signal in accordance with the amount of air flow detected by said bypass air flow detecting means.