When you set it to heat does the air from the center duct stop? If it does the servo is being controlled. It is possible to adjust the temp wheel and get the servo to move farther to open the valve. It is likely that its shot if you get this movement and no water flows.
The pump only works below some temp. Try putting power to it and see if you get heat. If you do put a toggle switch on it.
Bosch Master, ASE Master, L1
33 years MB technician