I sold my 380sel, so I'm just lookng at my repair manual. It seems like a posible cause maybe no coolant flow through the heat exchanger.

The repair manual says that the monovalve shoulld have > 11 volts with the ACC set to off.
It also says that the auxiliary coolant pump should have > 11 volts when the ACC is set to defrost and if it does, touch aux coolant pump to verify operation.

I believe that no (or limited) coolant flow through the heat exchanger would give the problems you describe.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126