Control theory changes through the years with the 190. I presume since the temp sensor was involved that your car is at least an 88 or newer. (the later system changest the cycling rate due to engine temp and at a point will discontinue the compressor totally).
All 190s (after 1984) use a compressor control relay that uses a number of signals to allow the compressor to work. Since the system works with defrost the engine signals to the compressor controller must be OK. You have a problem with the decission to cool. In defrost this is not a decission. The defrost position puts the compressor on no matter the temp selected.
The most common reason for the inside decission to be wrong is the feedback signal that the pushbutton controller gets from the mode door reostat. Untill the door gets fully closed (the system is still in heat until closed) the voltage from the potentiometer is too low. This voltage is adjustable on newer cars and has been the source of a couple bulletins. The vacuum to the door actuator or the integrety of the actuator can cause the door not to fully close and thusly not achieve the necessary output voltage. Does good strong airflow come from the center vents when in A/C mode? Is air coming from the defrost when in A/C mode? If so the doors aren't working fully due to leaks in the vacuum system.
Bosch Master, ASE Master, L1
33 years MB technician