Nice explanation on the Rear A/C. Better than Alldata, which does not specifically call out a "rear duovalve", until you hunt around a bit and interpolate. I have a W140 1993 600SEL. The right rear a/c outlet runs hot all the time. I checked both rear heater core temp sensors and rear a/c control module. Left rear a/c outlets seems to work ok hot and cold. Front a/c and heater controls seem ok. DTC indicates open or shorted right duovalve. Are there two duovalve assemblies on these cars? I can locate my duovalve at the left front engine compartment, next to the hydraulic ride system reservoir. Does this car have duo valve control both the front and rear A/C heater coolant return flows? Have relooked your diagrams and they helped a lot in showing the "rear duovalve".
Thanks

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Bill & Monika Mercer