Electrical/Accessory Load Idle-Up Signal

WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

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

Electric rear window defogger.
Exterior lighting system.
A/C system. When A/C is selected from the HVAC control unit the PCM receives a 12 volt signal. Once the PCM is aware of the request for A/C, PCM determines whether engagement of the A/C compressor clutch is okay. In order to enable compressor clutch engagement, PCM provides a ground for the control circuit of the A/C compressor clutch relay. The PCM also increases engine idle speed in order to prevent a rough idle or a stalling condition when the A/C compressor is engaged.
Power steering system. See POWER STEERING PRESSURE SWITCH under INPUT DEVICES.