Here’s a way to figure it out–
On a cold morning, when you start your car up on a stock Engine Control Unit, it idles higher than normal right? That’s because this valve is partially open letting more air in to idle the car up so that it warms up faster. Now… if you sit there until it warms up, does the idle drop back down to normal all of a sudden? If so it’s a Fast Idle valve (generally two wires, ground and 12v+).
Alternately, does it slowly idle back down to normal a little at a time, gradually reducing the idle speed as the engine warms up? If so it’s either a PWM valve or stepper motor valve.
To determine if it’s a stepper or PWM valve, find the valve on your engine, it will be attached to the intake manifold, either directly or via a hose. How many wires does it have? Generally a PWM valve will have 2 wires, sometimes 3, and a stepper motor IAC will have 4 wires, sometimes 6. The MS2/MS3 stepper circuit was designed with GM/Jeep valves in mind, however it works great on many different stepper IAC systems, including most all USDM, Euro, and Japanese vehicles that use steppers, among others.
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