900 S Coupe L4-1985cc 2.0L DOHC Turbo EFI (1998)
When the accelerator pedal is not activated and the throttle is closed, the engine only receives air via the idle air control valve.
The ECM controls the degree of opening in the valve to keep idling speed constant. This means for example that the valve opens slightly more when the
A/C compressor is switched on or if DRIVE has been selected, in order to compensate the idling speed, which would otherwise fall.
The idle air control valve is of single coil type and supplied with power from the main relay. It is controlled from pin 49 of the control module by PWM
at a frequency of 500 Hz. The valve opens more the longer the control module keeps pin 49 grounded.
The engine control module is programmed to maintain engine idling speed at 900 ± 50 rpm. Rapid changes in idling speed cannot be compensated for by
changing the flow of air. Instead, ignition timing at idling speed is so arranged that engine speed is maintained constant.
If there is an open circuit to the idle air control valve, the opening in the valve is set by a spring so that the idling speed is at least 1000 rpm when the
engine is warmed up.
