W126 monovalve stuck open
 

Friend of mine has a w126(1990) with the heat on full blast.Climate control seems to be fine,however I suspect a monovalve failed,stuck open.My question is,considering that we go in the summer season and heat will be useless for the next 5 months...can you shut the monovalve manually and repair it in the fall?,just to stop the antifreeze from getting in the heater core for the summer.

The monovalves are open by default, they are closed by electrical energy!

Therefore it is probably not closing.
In my C280 I shut the duovalve and the heat disappeared - but the thermostat no longer could control the compressor. The car became a freezer...
Jorge

Monovalve
 

check that you have power going to the valve and you can check it
by pulling just 4 screws it took me about 15 min to change and only a small amount of fluid loss

Sometimes just cleaning them up (the internal sleeve) with some 000 steel wool will do the fix. There is very thin tolerances between valve walls. If he has used some anti-freeze stop leak prior to this happening he is going to need to flush the system.