Idle Speed Control System

Fig. 109 Idle Speed Control System:

CIRCUIT OPERATION
The idle speed is controlled by the amount of air that is allowed to bypass the throttle plate through the auxiliary air control (AC) valve. The AC valve is turned ON and OFF at a rate between 100 and 200 Hz. The ECU calculates the duty cycle (frequency of ON/OFF signals) necessary to maintain the proper idle speed, compensating for transmission gear position, electrical and accessory loads. In addition, the throttle body is equipped with an idle up solenoid and fast idle control device (FICD). They are activated by the ECU when the power steering load switch closes or an excessive electrical load is applied to the charging system, to allow an increased amount of air to bypass the throttle plate.

Fig. 110 Auxillary Air Control Valve:

AUXILIARY AIR CONTROL VALVE
The auxiliary air control (AC) valve controls the idle speed. It receives a pulse signal from the ECU which opens and closes an air passage allowing a precise amount of air to bypass the throttle plate.

IDLE-UP SOLENOID VALVE AND FAST IDLE CONTROL DEVICE
The idle up solenoid valve and fast idle control device (FICD) open additional air passages in the throttle chamber. The ECU provides the ground circuit to the solenoids. The windings in the solenoids pull plungers which allow additional amounts of air to bypass the throttle plate, raising the idle speed.

Fig. 111 Auxillary Air Control Valve Operation:

Refer to the chart for AAC valve operation.

Thermo Element:

COLD FAST IDLE DEVICE
Cold engine fast idle is controlled by the throttle body mounted thermo element. The thermo wax element is a wax pellet operated plunger. Coolant is circulated through the element. When engine temperature is low the wax pellet contracts, extending the plunger which in turn opens the throttle. As coolant temperature increases, the wax pellet expands retracting the plunger, dropping the idle to curb speed.