climate control 1987 300D

I have a 1987 300D that will not deliver heat unless I turn the temp wheel as far as it will go. It takes a few minutes to start but after I do that it will finally blow heat. If I turn the temp. wheel at all to a lower temperature after the heater starts it will quit blowing warm air and start the air conditioning. I have replaced all of the air conditioning and some of the heating system including the push button control unit. Any suggestions would be very helpful.