Start with the basics. The default operating mode of the system is "heat" so that if the system takes a dump it will still run in heat just from the air being forced through from outside.

The monovalve must be energized to close. If it doesn't receive power, you get full heat. They also tend to bind up over time staying open even if commanded to close.

Check Fuse #5. It powers just about everything ACC related. If it is blown the monovalve will be open and the pushbutton unit will be dead. You also won't hear the aspirator motor behind the glovebox whirring.

Depending on when your car was built, it'll have a 25A fuse in #16 or it'll have an external fuseholder next to the fuse box with a strip fuse in it for the fan. If the older style with a fuse in #16, they're prone to melting and failing. The strip fuse in the external holder is prone to cracks, it is the same type fuse used in the fusebox for the rear defrost.

Check the fuses mentioned and report back.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]