My 94 S-500 has been doing something strange. Occasionally when you start the car, the AC system will direct the air to the defroster ducts near the windshield. No air will come out of the center two ducts, no matter if they are set on heat or AC. Turning the system off and on does nothing. Directing the flow to the floor does not work. Going to manual from Auto does not work. The only way to get the system back to flowing out of the dash vents (all 4) is to turn the car off and on a number of times till something resets or rests and the vents open correctly. Also the floor setting for heat has never produced any measureable flow, even when the system appears to be running correctly. The car only has 16 K miles on it and the evaporator was replaced around 4 K ago. Problems were there before the Evap. was changed. Directing the system through the Air Filter only reduces the flow slightly.

The Outside Temp. is normally around 70-80 F. and the car int. could be 80-90 F. I might add that the end vents do blow a little all the time but the major flow is to the defrosters.

Could this be a pump that is going bad and does not have the Vac/Pressure required to operate the vents properly?

I would welcome any suggestions.

Thanks,
Mark

Mark, hi.
Did you ever figure this out? Did a search and found your original post.
My '96 S500 has started doing a similar thing... when first starting out in the morning, the heat comes out the defroster vents and some out of the end vents in the dash. Nothing out of the floor vents no matter what position you select.
Shut the car off for 10 minutes.... restart it, and everything works normally...
If you figured this out, would appreciate knowing what the problem was.
KenP