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.

My first advise is to check the coolant level.

Yes, I also suggest to check the coolent level. Then check the heater valve, if it not open then hot coolent cannot enter to the heater. You can chect it but unplug the wiring and give it 12 volt power, and see wheather it work or not.

If not it's problem, check the potential meter of your temperature control, as mind have problem few days ago, you can try to clean it, and see wheather it better or not.

If not as well, try to test the temperature sensor.

If your test everything and can't find anything go wrong, then, might be the ECU causing problem.

You are getting some good replies here, but don't forget to see that the heater core is clear. To do that, you can simply disconnect the hoses at the firewall that go to and from the heater core and blow through them to see that it will flow.

Good luck,