The (electric) mono valve cuts off coolant flow through the heater.
The push button unit controls voltage: 12V = CLOSED and 0V = WIDE OPEN.

Here are the links you need to read for education on function/diagnosis/repair.

I suggest you Bookmark this master link for future need.
Fast navigation Do It Yourself Links
Fast navigation Do It Yourself Links



Climate Control:
Climate Control:



Monovalve Problems
http://www.peachparts.com/shopforum/tech-help/138522-monovalve-problems.html#post1026761

Mono Valve Voltages
Mono Valve Voltages

Mercedes-Benz A/C Control Diagnostics
http://www.peachparts.com/Wikka/AcDiags

Climate Control Repair
on the 1983 300D Turbo (W123 Chassis)
by BoostnBenz
2/16/04
http://www.peachparts.com/Wikka/W123ClimateControl





Have a great day

__________________
ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/