I have a 300 SD, 1981
The AC is locked on Defrost mode so I can't get air to yhe front panel, only blows on the top, near the winshield, how I can bypass?
Please help!
Nicolas

 

Sounds like your vacuum motor that controls the air has failed. I think the default is the windshield air. There are two of them. They are located in the passenger footwell on the engine wall. Remove the well lining on the right side (passenger) and you will see two green cylinder things bolted to the bulhead between the engine and and the footwell. There are the two. The diaphrams get tiny holes in them and will fail. George Murphy can sell you the diaphrams by themselves and you can avoid the cost of replacing the whole thing. If you are handy at fixing such things I can find out whee I bought mine and let you know.