I don't think the 126 has a blend door. It cycles the monovalve to control the heater core temperature. The airflow always goes over the heater core; during cooling it just isn't supposed to add any heat to the airstream.

You know, your car may have a temperature sensor for the heater core. If so, you could tap into it with an inexpensive digital multi-meter to measure the resistance. That would give a good idea as to whether the monovalve was functioning correctly.

- JimY