Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Speed Control Solenoid Valve Cross-Section:

PURPOSE
This system controls the bypass air flow by means of ECM and ISC solenoid valve for the following three purposes.
^ To keep the engine idle speed as specified at all times
The engine idle speed can vary due to following reasons.
- Load applied to engine (when electric load is applied, air-conditioner is turned ON, A/T is placed in gear etc.)
- Variation in atmospheric pressure
- Change in engine itself with passage of time
- Other factors causing idle speed to change

^ To improve starting performance of engine

^ To compensate air/fuel mixture ratio when decelerating (Dash-pot effect)


LOCATION
On the throttle chamber.

Idle Speed Control Solenoid Valve Control System:

OPERATION
ISC solenoid valve opens the bypass air passage when it is turned ON by ECM and closes it when turned OFF.

ECM detects the engine condition by using signals from various sensors and switches and while repeating ON and OFF cycle of ISC solenoid valve at a certain rate (12 times a second), it controls bypass air flow by increasing and decreasing its ON time within a cycle.

While the engine is cranking, ECM keeps ISC solenoid valve ON (maximum ON time rate within one cycle) so as to obtain better start of the engine.

When the accelerator pedal is depressed while the engine is running (the idle switch is OFF and the throttle valve is at other than idle position), ECM sets the ON time of the ISC solenoid valve (rate of ON time within one cycle) so that it is determined by the engine conditions and keeps it. When decelerating, on the other hand, ECM reduces the valve ON time gradually (i.e., the bypass air flow is reduced gradually) to obtain dash-pot effect.

When the vehicle is at a stop, the throttle valve is at the idle position and the engine is running, ECM controls the bypass air flow by increasing or decreasing ON time of SC solenoid valve so that the engine speed is kept at a specified idle speed.

With an A/C equipped vehicle, when the A/C is ON, a certain amount of the bypass air is supplied by the A/C VSV independently of this system. The bypass air supplied by this system is used for fine control to keep the idle speed as specified.