Is it on both sides the same? Driver and passenger side have separate heater valves.
Heater vales are fully open if they do not get power. So first thing to check if they get power, power is pulsating. Remove the plug from the heater valves and check that. Here is the system explained from an E32, but it will help you.
http://bmwe32.masscom.net/johan/ihka/ihka.html