The monovalve is bad or the CCU isn't sending voltage to the monovalve coil. It's as easy as that.

Start with the monovalve cartridge. Buy OEM only, Aftermarket will leave you disappointed. The OEM part is less than $40 and commonly fails. If the cooling system isn't maintained, it'll rust and seize in place. If $40, 4 screws, and 30 minutes of your time is too much to ask, drive with the windows down.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)