Looking at a wiring diagram for a USA version 126 pin 85 of the relay goes straight to ground. No control if it at all. The push buttons provide power to terminal 86, via the pressure switch on the drier, which turns the relay on passsing power from 30 to 87. The ground for this relay is location G102 which is behind the instrument cluster. I also see it shares a ground with a ACC control unit. Which part did you replace? I'm a little confussed on what part it is since there is 2 controllers and the push buttons for this system. If it is a bad ground then grounding pin 85 would make the system work. A simple test would be ground pin 85 then put the system in to EC mode and see if the compressor shuts off and on in any other mode.
This is the same for all years 1981 to 1985 126s' with Automatic Climate Control. Sounds like you have a OEM system so this should be valid for your car even though it is a Euro version but not promising you that. It could be differnet but for the most part this kind of stuff is universal for Mercedes.