This is probably a strange question but one I really would like to know the answer too. I would really like the answer for a 500SEL but for any CIS with electronic idle control equipped car would be great.
When the accelerator is OFF idle, what does the IAC do? Does it stay where it was when last idling? Does it open up? Does it close up?
I have built a new digital idle control unit that works great, idles at 600RPM in or out if gear, A/C on or off, when warmed up, 800RPM when cold, 700RPM for a few seconds after a warm start up to give the fuel system time to stabilize. I just don't know what to tell it to do when driving down the road. It probably doesn't make any difference but inquiring minds want to know.

PaulM

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84 500 SEL (307,xxx miles)