This car did have a new mono valve insert installed by Mark at Stu Ritter Mercedes in Denver about 4 years ago but I didn't know for sure but suspected that he installed a genuine MB repair kit.

After receiving a new one and installing it, no joy. It would blow cold air until the coolant got hot then the blast furnace started.

After a lot of reading and testing I finally cracked the code. My mono valve and the coil which is the top portion of the removable part of the repair kit is good.
Putting a multimeter on the two pins gave me 4.80v on max cool. It should be 12v. Putting the positive probe on brown and negative proble on battery negative gave me 12v. That means the CCU is not switching ground.

To prove it I removed the two pin connector on the mono valve and used two jumper wires. One from battery positive to brown position on the valve and battery negative to the ground position on the valve and I get a thunk, valve closed no more hot coolant into the core and wonderful cold air.

The good news is the mono valve is good. The bad news is probably the CCU ground switching function is toast. All other functions work like auto where turning the temp wheel slightly cooler or warmer work with the exception that I won't get hot since the valve is now held closed.

Most owners with a mono valve problem is no heat. Usually it means it's stuck closed or you don't have 12v to the brown pin. Other like me have a ground problem in the CCU which leaves the valve always open. Or, more likely leaking past the rubber parts of the valve.

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