Idle Speed/Throttle Actuator - Electronic: Description and Operation

Purpose
The throttle body (1) assembly is attached to the intake manifold. The throttle body is used to control air flow into the engine, thereby controlling engine output. The throttle valve within the throttle body is opened by the driver through the accelerator controls. During engine idle, the throttle valve is almost closed, and air flow control is handled by the Idle Air Control (IAC) (3) .The throttle body also provides the location for mounting the Throttle Position (TP) (2) sensor and provides a manifold vacuum source for various components. Vacuum ports are located at the throttle valve to generate vacuum signals needed by various components.

Operation
Engine idle speed is controlled by the PCM through the IAC valve (1) mounted on the throttle body. The PCM sends voltage pulses to the IAC valve motor windings causing the IAC valve pintle (3) to move IN (toward the seat) or OUT (away from the seat) a given distance (a step or count) for each pulse. The commanded location (steps away from the seated position) can be observed as a number of counts displayed on a scan tool. The pintle movement controls the airflow around the throttle valve (2), which in turn, controls engine idle speed: Pintle Extended=Decrease RPM=Lower Counts. Pintle Retracted=Increase RPM=Higher Counts.

The controlled or desired idle speed for all engine operating conditions is programmed into the EEPROM of the PCM. The programmed engine speeds are based on coolant temperature, park/neutral switch status, vehicle speed, battery voltage, and A/C refrigerant pressure (if equipped).

The PCM learns the proper IAC valve positions to achieve warm, stabilized idle speeds (RPM), desired for the various conditions (P/N or Drive, A/C ON or OFF, if equipped). This information is stored in PCM Keep Alive memories (information is retained after ignition is OFF). All other IAC valve positioning is calculated based on these memory values. As a result, engine variations due to wear, and variations in minimum throttle valve position (within limits) do not affect engine idle speeds. This system provides correct idle control under all conditions. This also means that disconnecting power to the PCM can result in incorrect idle control or the necessity to partially depress the accelerator when starting, until the PCM relearns idle control.

^ Engine idle speed is a function of total airflow into the engine based on IAC valve pintle position plus throttle valve opening plus calibrated vacuum loss through accessories.

^ The minimum throttle valve position is set at the factory with a stop screw. This setting allows enough air flow by the closed throttle valve to cause the IAC valve pintle to be positioned a calibrated number of steps (counts) from the seat during controlled idle operation. The minimum throttle valve position for this engine is not the same as the minimum idle speed associated with other fuel injected engines. The throttle stop screw is filled at the factory following an adjustment.

Important: Do Not try to remove the filler and readjust the stop screw. Misadjustment may set a DTC P0506 or a DTC P0507.