Vacuum question for '89 300E
 

When I engage the recirculating air button on the dash, the red light comes on and I hear some sort of flap or door moving (I presume shutting or seating over an opening). I presume that it's vacuum actuated.
I find that I can engage the air recirculation the red light comes on and when I look at it again later and the light is off.
Is there a specific time frame that this is supposed to stay on? If there's a vacuum leak, will that cause this to disengage? If so, how to fix (I know, start with the easy stuff and work from there)??
Thanks for any help you can bring to me on this.

Check the owners manual. I'm pretty sure the 100% re-circ mode shuts off after 30 minutes, but check it out. Let me know how it goes.

on the 126 that flap is located behind the glove compartment.It is operarated by two vaccum actuators and i agree with ILUVMILS.
It does deactivate itself after 30 minutes.
If it cuts out real soon, it might be either the switch(most likely, as happened to me ) or loss of vaccum.-
Once I had lost vaccum because one of these vaccum lines had become undone at the switch. Real easy fix
good luck

Just because the light is on doesn't mean the door has functioned properly. Or, in other words, if there is a vacuum leak the light won't reflect it.

Improperly functioning recirc doors are a common cause of poor duct temperatures. Most if not all models go to the first stage of recirc door closing upon choosing AC. If the door stays in the total vent position it will cause 10-15deg rise in duct temps on 126,124, and 201 bodies, maybe more.