DTC P0505: Idle Speed Control System: Description

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of air which by-passes the throttle valve via Idle Air Control Valve-Auxilary Air Control valve IACV-AAC valve. The IACV-AAC valve changes opening of air by-pass passage to control amount of auxiliary air. This valve is actuated by a stepper motor built into the valve, which moves valve in axial direction in steps corresponding to ECM output signals. One step of IACV-AAC valve movement causes respective opening of air by-pass passage (i.e., when step advances, opening is enlarged). Opening of the valve is varied to allow for optimum control of engine idling speed. The Crankshaft Position Sensor (CKP-POS) detects actual engine speed and sends a signal to ECM. ECM then controls step position of the IACV-AAC valve so that engine speed coincides with target value memorized in ECM. Target engine speed is lowest speed at which the engine can operate steadily. The optimum value stored in ECM is determined by taking into consideration various engine conditions, such as during warm up, deceleration, and engine load (air conditioner, power steering and cooling fan operation).

The IACV-AAC valve is operated by a stepper motor for centralized control of auxiliary air supply. This motor has four winding phases and is actuated by output signals of ECM which turns ON and OFF two windings in sequence. Each time the IACV-AAC valve opens or closes to change auxiliary air quantity, ECM sends a pulse signal to stepper motor. When no change in auxiliary air quantity is needed, ECM does not issue the pulse signal. A certain voltage signal is issued so that valve remains at that particular opening.