After searching the website I have not been able to find where any one else has had the same symptoms that I am having with my 300. But similar.

I have a 79 300SD that is blowing very hot air through the A/C vents on the right and left but air does not blow through the center vents. Also hot air blows through the heater vents on the floor. When the A/C is not on hot air still comes through the vents. The faster you drive the hotter it is and when the A/C is on it blows fairly cool at an idle. When you stop the A/C is able to cool the car down again. I live in the Dallas Texas area it has been 100 degrees lately and it is unbearable to drive my car at this time.

The compressor has been replaced with a new one (Not a reman.), receiver/drier, low pressure hoses and recharged with a low pressue of 30 and the high pressure is 300. The monovalve was leaking coolant and I replaced it with one from the salvage yard from 1980 model that fortunately is made of metal instead of plastic. After reading through portions of the forum I took a pair of needle nose vise-grips and effectively shut off the heater hose on the drivers side of the cylinder head and there did not seam to be any change.

The car has been sitting in my grandfathers hay barn for the last 6 years with 135,000 miles. The only thing he was aware of is that it had a leak that I fixed with new hoses.

Your help is appreciated.

Tony