All you have done is verify voltage. You must inspect the diaphram. When mine tore, it would blow warm only in the max hot temp wheel setting, turned anywhere else the heat would quit and not return unless car was turned off and restarted. My experience is its best to leave the wheel somewhere in the middle and select auto center button for fan and center push button for mode. And leave it alone all year long! except for occasional defrost or econ. operation. Some folks operate it as if it were a manual system? which I personally think causes early failure of vacuum motors and switches. I owned my 81 300CD 10 years and never had to repair it except for burned out bulbs in the control head once and one MONO valve repair kit. I would also verify that the aux. electric water pump works as well. At idle or low car speed it turns on to provide extra push to the coolant flow to the heater core. Some have suggested adding an in line fuse to that pump and I think thats a good idea. They say a siezed pump can damage the control head.

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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.