The idle air control (IAC) valve is attached to the underside of the throttle body with four bolts. The IAC valve enables the powertrain control module (PCM) to easily control engine idle speed by precisely metering the engine's air intake at closed throttle. The IAC valve opens and closes the idle air (bypass) passage according to signals from the PCM. The IAC valve contains an engine coolant passage that enables the IAC valve to operate more efficiently at cold temperatures. The PCM determines the correct engine idle speed by using input from various sensors and switches in order to assess engine status and requirements.

The powertrain control module (PCM) uses the IAC valve in order to control engine idle speed. The PCM communicates with the IAC valve by varying the ON time of a repeating ON/OFF (duty cycle). A magnet inside the IAC valve operates a rotary valve that controls the opening of the idle air (bypass) passage in the throttle body. The idle air passage allows air to enter the engine without passing over the throttle valve. The strength of the magnet in the IAC valve is related to the current flow in the IAC circuit.

The PCM increases the ON time of the IAC valve command in order to increase the idle air passage opening. A larger idle air passage opening allows more air to enter the intake resulting in an increase in engine speed.

The engine idle speed is controlled by the powertrain control module (PCM) through the idle air control (IAC) valve. There are several reasons for idle speed control:

1. Maintain the engine idle speed at the specified RPM at all times. The engine idle speed can vary due to any of the following reasons:

•	A change in the load applied to engine (when the rear defogger is operating, the automatic transaxle is shifted to R, D, 2 or L ranges, the A/C is turned ON, the headlights or stop lights are turned ON, etc.).

•	A change in the atmospheric pressure.

•	A change in the engine's condition over time.

2. Improve the starting performance of the engine.
3. Improve the driveability of the engine during warm up.
4. Compensate for the change in the air/fuel mixture ratio when decelerating (dash-pot effect).

The circuit opening relay (1) provides power to the electric fuel pump. The circuit opening relay is controlled by the powertrain control module (PCM). The PCM provides an electrical ground for the coil of the circuit opening relay which causes the contacts in the relay to close. When the ignition switch is turned to the START position, the circuit opening relay will be energized to allow the fuel pump to operate and pressurize the system. The fuel pump will continue to operate if the PCM receives an ignition signal when the engine is started. The fuel pump operates as long as the circuit opening relay is energized by the PCM. When the circuit opening relay is de-energized, the fuel pump will stop operating.

The circuit opening relay is located in the relay box on the LH side of the engine compartment near the battery.

The malfunction indicator lamp (MIL) is located in the instrument panel gauge cluster. The MIL is illuminated by the powertrain control module (PCM) in order to notify the vehicle operator of an engine control system malfunction.

The MIL receives ignition positive voltage anytime the ignition is ON. The PCM illuminates the MIL by providing a ground for the MIL circuit. The MIL should be illuminated whenever the ignition is ON and the engine is not running. Illuminating the MIL whenever the ignition is ON and the engine is not running provides a check of the bulb and circuit. The PCM will turn OFF the MIL once the engine is started or running if there are no engine control system malfunctions.

Many of the conditions for illuminating and extinguishing of the MIL are performed by the PCM in accordance with OBD II regulations. For more information on the MIL function, refer to Powertrain Control Module (Common OBD II Systems).

The Powertrain On-Board Diagnostic (OBD) System Check guides the user to the correct diagnosis and repair of the MIL circuit.

The powertrain control module (PCM) can prevent the engagement of the A/C compressor clutch through the A/C compressor control module. The PCM provides a ground signal to the A/C compressor control module that enables the A/C compressor control module to operate the A/C compressor clutch relay. The PCM allows the A/C compressor clutch to operate at customer request except during wide open throttle engine operation.