I know this has been discussed in other posts but nothing I've tried so far works. All functions of the ccu appear to work except for the heat functionality of the climate control. So far I have replaced components of the mono valve and it seems to be functioning as I understand it's supposed to work i.e. the plunger and diaphragm move in and out to control coolant flow to control temp. The Aux pump is operable and has flow. I have raised the vehicle on a jack while on a steep hill and filled coolant through the top rad hose to thermostat housing while at op temp so I know the coolant system is full, I have also replace the ccu with a junk year unit and it seems to not have made any difference to the original unit ( which I took apart and inspected for broken solder traces) Is there a step I'm missing. Any help would.be great thank you.

Did you use an OEM or generic "MTC" monovalve cartridge? The generics are well documented to cause problems with heat.

__________________
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!)

If you pour cold coolant into the thermostat it’ll close.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Sixto thanks for the input I'll revisit this coolant issue but I'm positive it's full. Could be wrong though is there an oe procedure to ensure all coolant lines and heating system is full and bled of air?

Diseasel300 pretty sure it was an Mtc purchased from pelican, the old diaphragm I took out looked in excellent shape. So I'm leaning towards that's not the problem also it worked like a champ before I pulled the motor in order to replace the front seals on the tranny. So I'm thinking either there is an air pocket or its pulling sir from outside the car through the vents