I have all the symptoms of a failing monovalve on my 94 E420 i.e. occassional and random full heat from the side vents when on full a/c. At the moment it is very intermittent but I know it will get much worse. My car is a late model with the combined monovalve/bypass pump which does not have a rebuild kit and which costs $300+ from Mercedes. Do I have any option but to replace this?. I have considered a manual on/off valve in the hot water inlet line to the monovalve for use in the summer.....is this advisable??

I had problems with my duovalve and cleaned it myself.

My line of thinking was that if the only other option was to replace it, I might as well have a go at repairing it. I did so, and its been fine ever since.

Food for thought.

I replaced the monovalve in the evening, a relatively simple procedure and all is now fine with my a/c.

If you're up to it, you can pull it out and disassemble it to see what you might be able to fix. Could be as simple as a broken wire. The link below is a write-up for the rear duovalve in a S600, but some of it may apply or help.

http://v12uberalles.com/Rear_AC_System.htm

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1995 S600, 1 of 618 (sold)

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