On my 91 300E i have the typical defrost stuck on all the time.

my question is this....the valve has 2 ports. when i apply vacuum to the port in the middle of the valve nothing happens. When I apply vacuum to the port on the very end the valve will retract. I read this is a two stage valve.

How does it work?


2nd question.

I have no vacuum going to the valve inside the car. Where do I need look next, vacuum switchover valve or control panel.