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]

Hi guys, we've redesigned URO Parts Heater Valve Repair Kit 000 835 0644R for easier, smoother operation. We're thinning and softening the boot material a bit so the valve cycles more freely. The updated version should be available in a month or two.

__________________
URO Parts online catalog: https://apaindustries.com/catalog

Is the OE cartridge still available? Google only comes up with the MTC repair kit. But apparently, a BMW inset fits into the W126 as well:

https://www.benzworld.org/threads/monovalve-mtc-bosch-or-bmw.2507129/

I think I will take the risk and purchase this one.

I've heard that the BMW inserts fit. I haven't tried it so I can't give first hand data on that. The MB OE cartridge is still available at the parts counter or through various vendors online (this forum's parent site sells it, not sure if they post to The Netherlands tho...).

As far as the coil resistance goes, the '83 500SL is 15.2Ω, '91 350SD is 14.6Ω, and the spare from my '86 300SDL is 14.9Ω. I'd say your coil is fine.

__________________
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]

Thanks for checking the coil resistance.

I ordered the BMW valve, but right after ordering I received a notification that the delivery time had been changed from two working days to unknown (backorder).

I checked the Pelican website, but I could not find the OE cartridge. Only the MTC Kit appears to be available.

Hmm. That's disconcerting. I did some searching this morning for the MB one, and like you said, was listed NLA everywhere, including the dealer parts sites. Fantastic!

Plan B involves the MTC cartridge, but disassembly of both your original and the MTC one. The primary issue with the MTC cartridge is a weak return spring (causing the valve to suck closed when it should remain open and give a no-heat condition). Several people have reported success transferring the MTC diaphragm assembly over to their OE spring cartridge. If you try this, make sure to be very gentle with your OE cartridge and pay close attention to how it goes together. If you get it apart and it's rotted out with rust, you may or may not be able to salvage it. Give the internal parts of it a good blast with brake cleaner to flush out any sludge or rust that may be present and make sure the rod moves freely without binding.

__________________
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]

Problem solved. I got hold of an (original) rebuild valve, and since then the problem has not yet occurred. I cancelled the BMW valve, as they had no idea how long it would take to deliver.

Many thanks for all your suggestions!