Device of controlling the idling speed of an engine

A device of controlling the idling speed of an engine comprising a main intake passage, a throttle valve arranged in the main intake passage, a bypass passage branched off from the main intake passage upstream of the throttle valve and connected to the main intake passage downstream of the throttle valve, and a control valve arranged in the bypass passage, said device comprising:

• a step motor actuating the control valve for controlling the amount of air flowing within the bypass passage;
• first means for detecting the engine speed to produce an output signal indicating the engine speed,
• second means for detecting the operating condition of the engine to produce an output signal indicating that the engine is operating in an idling state,
• electronic control means operated in response to the output signal of said first means and the output signal of said second means and producing continuous control pulse signals at predetermined first time intervals for rotating the step motor in a stepping manner at a first speed in a direction wherein the engine speed approaches a desired engine speed when the engine is operating in an idling state, and;
• power supply control means including an ignition switch and inserted between said electronic control means and a power source, said electronic control means producing continuous control pulse signals at predetermined second time intervals which are longer than said first time intervals for rotating the step motor in a predetermined direction at a second speed which is lower than said first speed when the ignition switch is turned to the OFF position.