Electrical/Accessory Load Idle-Up Signal Diagnosis: 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 in order 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 A/C compressor control module sends an A/C On signal to the PCM. Once the PCM is aware of the request for A/C, the PCM will determine whether engaugement of the A/C compressor clutch is OK. The A/C compressor control module requires a signal from the PCM in order to allow operation of the A/C compressor. The A/C compressor control module will not turn on the A/C compressor clutch relay unless the PCM pulls the A/C Cut-out Control circuit low. The increase in the idle speed prevents 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.