The monovalve is controlled by the ACC controller if you have ACC.

Default is +12V and no ground to the monovalve so default is full coolant flow through the heater core. I'm not sure if this is the case in max heat mode. It is the case in defrost mode.

The ACC controller sends a ground signal to the monovalve to stop the flow of coolant. This is the case in max cool mode and off mode, if off is a mode, with the engine running. In other modes the controller pulses the ground signal to control heater core temperature.

I'm pretty sure the philosophy holds in MBs with dual zone climate control.

I'm don't know if MBs with manual AC have electric or mechanical valves.

Sixto
93 300SD 3.0