Since you are loosing the compressor as a symtem you must decide whether the problem is an inside control problem (pushbutton controller et al) or a compressor relay problem under the hood. The way to tell is at the low/high pressure switch on the drier. The pushbutton controller tells the compressor controller to work by grounding the signal coming fronm the compressor controller. If both terminals are battery voltage (at the low pressure switch) the inside system is at fault. If the two signals are low near 0v probably just less than 1v then the compressor controller system is being switched but is inactive for an under the hood problem.
The common problem on your year 190 for a pushbutton control problem is: the lack of full movement or improper voltage from the mode door potentiometer. As the door closes the voltage drops (I think...I have to look it up every time). If it doesn't move all the way or if the potentiometer is out of adjustment the compressor on signal is never given. 190s also had a fair share of evaporator sensor problems.
The under hood problems with the compressor controller are usually related to the belt protection system which matches engine speed to compressor speed. The usual problem is bouncing belts or slightly slipping clutches (watch for oiled down clutch).
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician