I have a 1990 190E 2.6 and I'm having a problem with the air conditioner not blowing cold air. It works fine at the lowest setting, but when I push the low setting to the third and fourth (Hi) positions it blows hot air. If the sun is directly on the car and the outside temperature is more than 84 degrees the air conditioner will not cool the car off. I end up putting my windows down and opening the sun roof to get some relief. It seems to cool fine if it is cloudy, night or less than 80 degrees. Even when the air is on low and I feel cold air coming from the vent I can hear a hissing sound. Any ideas? Thanks.