124 have a "mono-valve" located close to the battery. It is a different design than the ones used in 123 and 126 cars, on these 124 cars the entire valve would be replaced rather than just the plunger/diaphragm unit.

The ACC dash unit controller is the weak link in the climate control chain, rebuilt is a reasonable choice and sometimes a solder reflow will get things working if you're solder iron capable.

Yes,I saw it there in front of the battery. I had a W123 several years ago and I remember having to fiddle with the mono valve - seems I replaced the diaphragm. So I checked at Pelican Parts and I can get a new one for about 130 bones

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Dale