Circuit Description

WARNING: This page is about a different car, the 2002 Oldsmobile Intrigue and 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.

Idle Air Control (IAC) valve is located on throttle body. IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a 2 phase bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time polarity of a winding is changed. Polarity can be seen when observing a test light connected between ground or B+ and an IAC valve circuit while the Powertrain Control Module (PCM) is attempting to change engine RPM (test light will flash on or off each time polarity is changed). The PCM does not use a physical sensor to determine IAC pintle position, but uses a predicted number of counts, one count represents one change in polarity which equals one step of the stepper motor. The PCM counts the steps it has commanded to determine IAC pintle position. The PCM uses IAC valve to control engine idle speed. It does this by changing pintle position in the idle air passage of the throttle body. This varies airflow around throttle plate when throttle is closed. To determine desired position of IAC pintle at idle or during deceleration, the PCM refers to the following inputs: engine RPM, battery voltage, air temperature, engine coolant temperature, throttle position sensor angle, engine load, and vehicle speed. When ignition is turned off after an ignition cycle, the PCM will first seat the IAC pintle in the air by-pass bore, and then retract it a predetermined amount of counts to allow proper amount of air to by-pass throttle plate for engine start-up. This procedure is known as an IAC reset.