Bob, I really couldn't tell you how to test the sensor, I don't have the circuit diagram to figure it out iether. I changed mine since I had an extra sensor sitting in the garage. I do remember I had to take the valve cover off to change it.
I think the sensor is there to shut the compresor off if the engine temp is to high, and so it would turn the auxiliary fan on. This is I think the only link between the aux fan and the compresor so it probably means the sensor is stock on one position. I am sorry I can't help you here but I am sure the anwer is in this forum somewhere, just gotta look for it.
'92 190E 2.6