The valve core should move with a good "thump" like a solenoid (that's essentially what it is). Not sure what the coil resistance should be, I can measure the ones in my cars tomorrow. 14Ω give or take doesn't sound wrong.

I'd suspect the monovalve cartridge is bad. Either a corroded shaft internally, or a hardened diaphragm. My 300SDL had wonky behavior that was the monovalve cartridge as the issue. No rips or tears, but the internal guts of it were corroded and making it bind.

Shell out for the OE cartridge, the MTC aftermarket ones are known to have weak springs and block coolant flow when there should be heat.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]