My 1982 300d turbo looses heat when the blower is turned on high. When the blower is on low I (my wife) gets hot air. This occurs at all speeds. The engine thermostat is operating, the engine is at proper temperature, the blower is working, the pushbutton unit is also working correctly, (recently replaced) and the auxillary coolant pump is working(also new). Could my problem lie in that valve on the firewall? Thanks Kevin