first of all, it is south dakota cold. is it natural for cold air to come out of the defrost vent when warm air is coming out the bottom vent and under the seats? it seems that they are offsetting each other. the defrost vent is warm when it is only on, but cold when either in the economy mode and/or air compressor mode. i've read before on the forum it might be the monovalue, but not sure. thanks for any help and happy holidays

Replace the monovalve then recheck.

__________________
Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

where is the monovalue located on the motor and are there any special tools needed to repair it? i am buying the repair kit from fastlane, i assume that's the way to go

The monovalve is inboard of the battery on the firewall under the diagnostic port.

- remove 2 philips screws behind the transmission dipstick
- move the diagnostic port out of the way
- remove 4 flathead screws on top of the monovalve
-- it might help to disconnect the 1/2" OD hose above the monovalve
--- go easy because the fitting is plastic
- lift the monovalve cartridge and top plate out of the housing
- note the order of washers as you remove the cartridge
- installation is reverse from this point

The amount of coolant I lost didn't even register in the reservoir. Maybe there's a pocket of air in my block now.

The monovalve usually fails through tearing of the rubber seal on the valve or the rubber diaphragm.

[added on 03 Jan] I've added a monovalve album to http://photos.yahoo.com/tabijan for some visuals to go with these descriptions for future reference on monovalve cartridge repair and/or replacement. Corrections are always appreciated.

Sixto
91 300SE
81 300SD

unfortunately after i changed with monovalve with sixto's directions (thanks man), the cold air out of the defroster still exists. any other ideas, or maybe that's the way it is.