Idle Speed/Throttle Actuator - Electronic: Description and Operation

Fig. 119 Idle speed control system.:

The Bypass Air Control (BAC) Valve is an ECU controlled component of the idle speed control system. The system regulates the amount of intake air allowed to bypass the throttle valve under the full range of operating conditions.

The BAC valve consists of an air valve that functions only when the engine is cold and an idle speed control (ISC) solenoid valve that operates throughout the engine speed range.

The air valve is the thermowax type that is open when cold to allow bypass air into the dynamic chamber, increasing idle speed to shorten warm-up time and ensure good driveability. As the thermowax element is heated by engine coolant routed through the BAC valve assembly, the air valve gradually closes.

After engine warm-up, the ECU controls the ISC valve to maintain good idle quality under a wide range of operating conditions. To make proper idle speed adjustments, the ECU utilizes inputs from these devices:
^ Atmospheric pressure sensor
^ A/C switch
^ Electrical load signal
^ Inhibitor switch (A/T)
^ Throttle sensor
^ Intake air thermosensor
^ Neutral and clutch switches (M/T)
^ Water thermosensor
^ Idle switch
^ Ignition coil
^ Power steering pressure switch