2002 taurus 3.0 v6 when turning on defrost R.P.M'S drop

Hello !! i'm new to the site and i have been reading some of the post . just looking to see if anyone might have some insite with the vehicle i bought for my wife . I'll start by saying that i've only had it for a month ... 2002 taurus i have replaced the computer on it so i went on by giving it a full tune up . 3.0 v6 with 91,000 on it . so i noticed it yesterday when i turn on the heat or defrost for the winshield that the R.P.M drops from 1000 down to 500 or 600 R.P.M but only in park . i can put it in drive or reverse and it's fine. a/c compressor does ingage and it works fine . does anyone have any clue lol

One purpose of the Idle Air Control (IAC) servo is to increase the air to the engine to accommodate increased loads on the engine. These loads include electrical loads (blower motor, brake lights, headlights, etc.), power steering loads (steering turned to one of the locks, steering at low speed), and an engaged A/C compressor clutch.

My first suspicion, if the engine otherwise idles smoothly, is that the IAC is either not working properly or the passages to/from it are restricted.

-Rod

Hey Rod thanks for the reply. I'm aware of the load once I turn the heat on this is more like it wants to stall out . The RPMjust jumps around. Thanks