I'm trying to track down the cause of a "defrost only" situation with my 300SD. I'm tired of cold feet!

I am curious to know what - if anything - causes a CCU to go in "safe mode" besides vacuum leaks.

Here's what I've tried so far:

-Replaced foot-space flap vacuum actuator
-tested for vacuum at the line that feeds all the switchover valves = OK
-plugged each line from the switchover valve to pod(at the switchover valve) to see if I could get the CCU to go out of safe mode (thinking maybe if there was a vacuum leak between the switchover valve and pod this would be a way to diagonse).
-Actuated the foot space flap with a Mity-Vac so that circuit works

What am I missing?

Could it be electrical? I.e., bad CCU?

A tangential question: what is the function of the "oriface" between the leg-space flap switchover valve and pod? It seems to slow the "action" of the pod.

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1985 300SD 99K