Perhaps others can provide a more technically accurate response, but I solved a similar problem I was experiencing with my now sold '85 190E by unplugging a line that went to a switch that rested above and to the right of the water pump. (Aux temp switch??) Are you sure you have freon?
'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold