Steve, thanks for the comments and part numbers. My wife just got back so I was able to take another look. It seems that in defrost, the center vent is open and the fan goes to high speed. In normal operation, it appears that the defrost vents are still open and I think the whole system is stuck in recirc.

What is the best way for a DIY to check for vacuum leaks on the valves?

tks again for all your help.

__________________
Litton
'90 420 SEL (sold)
'72 280 SEL 4.5
'98 ML320 (for sale)
'86 560SL
'05 Jeep Grand Cherokee Ltd (offroad in style)
'87 Chevy Blazer (AZ Pin Strips)