The idle air control (IAC) valve is attached to the underside of the throttle body. The powertrain control module (PCM) uses the IAC valve in order to control engine idle speed. The PCM controls the IAC valve by varying the ON time of a repeating ON/OFF signal (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 throttle body is concurrent with the current flow in the IAC circuit.
The IAC valve contains an engine coolant passage that enables the IAC valve to operate more efficiently at cold temperatures. The IAC valve enables the PCM to easily control engine idle speed by precisely metering the engine's air intake at closed throttle.
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:
The main relay is controlled by the powertrain control module (PCM). The PCM provides an electrical ground for the coil of the main relay when the ignition is ON or when the engine is running. The main relay provides power to many of the engine control components including the PCM. The following is a list of some of the PCM controlled outputs that depend on the main relay for proper operation:
The main 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 2 regulations. For more information on the MIL function, refer to Powertrain Control Module (Common OBD 2 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.
The upshift indicator lamp is located in the instrument panel cluster assembly and is controlled by the powertrain control module. The purpose of the upshift indicator is to provide a display that indicates to the driver when to upshift the manual transaxle for optimum fuel economy. The powertrain control module determines the optimal upshift point based on engine speed and load. The upshift indicator lamp will not be operational under any of the following conditions:
