Circuit Description

Powertrain Control Module (PCM) receives an idle-up signal when certain engine electrical loads or engine accessory loads are present. PCM increases flow of air through idle (bypass) air passage of Idle Air Control (IAC) valve when receiving an idle-up signal. Additional air flowing through idle air passage causes an increase in idle speed. Idle speed is increased in order to accommodate increased engine electrical or engine accessory load. PCM monitors the following systems for idle-up signals:

• The exterior lighting system.
• The HVAC system blower motor.
• The electric rear window defogger.
• The A/C system. When A/C is selected from HVAC control unit, A/C compressor control module sends an A/C ON signal to PCM. Once PCM is aware of request for A/C, PCM will determine whether engagement of A/C compressor clutch is okay. A/C compressor control module requires a signal from PCM in order to allow operation of A/C compressor. A/C compressor control module will not turn on A/C compressor clutch relay unless PCM pulls A/C Cutout Control circuit low. Increase in idle speed prevents a rough idle or a stalling condition when A/C compressor is engaged.
• The power steering system. Power Steering Pressure (PSP) switch is normally open to ground. Turning steering wheel increases power steering oil pressure. Increased power steering oil pressure puts an additional load on engine that may be noticeable at idle. PSP switch closes when power steering system oil pressure is high enough to cause an undesirable idle condition. Voltage signal from PSP switch to PCM indicates less than one volt when PSP switch is closed. Voltage signal is near battery voltage when PSP switch is open.