The primary cause is probably lack of freon. The climate control system has to mix refrigerated air with heated air to arrive at the setting on the rotating temperature dial. Yes, the tube or pipe going from the in car temp sensor has to be there and connected at either end, but without refrigerated air to measure, I would think the system would be erratic at best. You might leave it in econ mode to see if theres an improvement in comfort. Describe "nothing at all in between?" do you mean the air stays extremely hot or it goes to cool. This year of car is famous for the MONO valve diaphram tearing which will make the air temperature erratic. This valve is located on the firewall just to the right of the battery facing the engine. Remove the four screws holding its lid on (remove radiator cap first!) engine must be cold! and lift the internals up and out. Tug on the black rubber diaphram around its diameter, you will most likely find it torn.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.