Idle Speed Control (ISC) is required to provide an acceptable idle speed when the engine is cold or when loads (AC etc.) are applied. Without idle speed control, an engine will idle too slowly when cold. Often the cold idle speed will be so low that the engine stalls. Likewise, extra load loads will decrease the idle speed and may cause the idle to stall or become unstable. The ISC system regulates the engine idle speed by adjusting the amount of air which is bypassed around the throttle. This air may be bypassed using either a solenoid or stepper motor.
G4X ECUs employ a sophisticated closed loop idle speed control system that provides factory like idle speed control. This system can be used to provide idle speed control using either a 2 or 3 terminal ISC Solenoid or a 4 or 6 terminal ISC Stepper Motor. While a solenoid and stepper motor operate very differently they both achieve the effect of bypassing air around the throttle plate to increase the idle speed.
Some engines use a non-electronic system to control idle speed as the engine warms up. This system contains a bimetallic strip, which is heated by the engine coolant and/or electrically. As the engine warms up, the amount of bypassed air is reduced and the idle is returned to its normal level. The ECU does not control this type of device. It is intended ONLY as an idle up when the engine is cold.
See also:
·Two Terminal ISC Solenoid Wiring
·Three Terminal ISC Solenoid Wiring