300D Idle controller?
 

Is there a manual idle adjustment near the fuse box under the hood? I was going thru old threads and found a post that mentioned a numbered knob, or plastic thingy to raise or lower the idle a little. After warmed up while driving(or parked), the idle goes down to about 550 rpm. The book says it should be 670 +\- 20 rpm. Also when revved and let off, it drops to 500, then up to 550. It's kinda rough that low. I have a 1990 300D 2.5.
Thanks aplenty.

Yes you have electronic idle control. It LOOKS like a knob, on the driver's side fender if I remember correctly, but you unplug it and plug it back in a new orientation to change idle speed.

If its running at 500rpm then the idle control isnt working at all.

Interesting. Mine runs at 500rpm about half the time when idling. If my idle control is indeed toast, what does it take to get it back?

Could just be a fuse or loose connection. Check that the plug on the injection pump is seated and contacting.