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733i heater valve

Gary,

Sounds like you are describing the heater control valve.  I have an 83 733.  When vacumn is not applied (a leak or from the control) the valve is open and hot water goes to the core and you will get heat.  Temperature control is maintained by mixing hot and cold air.  On defrost, the a/c is even utilized to remove humidity.  You need to fix all vacumn leaks to make sure all flaps are opening and closing to accomplish the proper mix (you can hear them when you push the different select buttons) and if there is still a problem, the board with the temp and airflow wheels might be the cause.  If you would like further info, please E-mail.

Gene M.