Circuit Description

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 to accommodate the increased engine electrical or engine accessory load. The PCM monitors the following systems for idle-up signals:

• The electric rear window defogger.
• The exterior lighting system.
• The 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, the PCM determines whether engagement of the A/C compressor clutch is OK. To enable compressor clutch engagement the PCM provides a ground for the control circuit of the A/C compressor clutch relay. The PCM also increases engine idle speed to prevent a rough idle or a stalling condition when the A/C compressor is engaged.
• The power steering system. The Power Steering Pressure (PSP) switch is normally open to ground. Turning the steering wheel increases the power steering oil pressure. Increased power steering oil pressure puts an additional load on the engine that may be noticeable at idle. The PSP switch closes when the power steering system oil pressure is high enough to cause an undesirable idle condition. The voltage signal from the PSP switch to the PCM indicates less than one volt when the PSP switch is closed. The voltage signal is near battery voltage when the PSP switch is open.