I've searched 30 pages of the archives, no luck. 91 300SE. The system seems to have a mind of it's own. It will work fine at start up, at some point, 30 min to an hour later, warm air will come out the center vents and the small upper side vent, the lower side vent will continue to have A/C. Rotate the temp control to minimum, 15-30 minutes later things return to normal. When the center vents are blowing warm, if the temp knob is rolled to heat, the floor vents open as expected, turning back to cold, floor vents close and center vents blow hot again. Blower acts normally, adjusting speed to try to attain the setting of the temp selector. Monovalve has been replaced, twice in the last 2 years. The overhead temp sensor test ok, the sensor fan works, the tube is intact. The temp sensor in the heater case tests good. All the fuses are ok, they were replaced about a year ago. Had a mityvac on the tubes at the vacuum contol manifold. All seemed to hold vacuum. The situation is further complicated by the airbag on the passenger side. Makes it hard to check operation of the servos. Does the dash need to come off to get to the servos? Any suggestions? Help! Wife is loosing faith if my DIY abilities!

The dash does not need to be removed to check or R&R the vacuum elements (servos). Removing the passenger airbag is easy enough, just be sure to disconnect the battery first. It is held in by one bolt that goes up thru the kneee bolster. I just got done replacing three bad vacuum elements out of the six on my 420 SEL, two of the bad ones were on the recirculate door just in front of the passenger airbag and the other bad one was the defrost element, the hardest one to get at. A real pain but much better than pulling the dash, did that once on a 380SEL with no airbags, nevermore!!

If you've tested all the vacuum elements at the switching block like you say and they all held, I would guess they are all OK. I would look elsewhere for your problem, like the pushbutton controller for example.

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Chas

Had a 380SL, 450SL, 380SEL, 420SEL, 500SEL

Got a Euro 500SEC right now.

Thanks for the reply. I was reviewing the CD last night. I realize now there is more to testing the servo's than pulling a vacuum and listening for the actuation. I'm pretty sure it is either a servo or switch over valve, think I'll retest by the book. Thanks for the airbag info. I give things that go boom great respect. Very hard to find information on it.

On the advice of Kirk Gilchrist at www.euromajic.com I opened up the monovalve. The diaphram was good but the screen was covered in white oily foam. Flushed the system after changing the head gasket, guess I didn't get it all out. Cleaned and back together, hopefully this will solve the mystery.