I have a 1998 olds 88 with auto climate control. The upper ducts will not blow. When hot outside the A/c blows out the defrost only. It acts like it is in a purge mode or default to defrost only. The actuators I can see work at various times when I change settings. These include a vacum diaphram close to the radio, a white arm run by what looks to be an electic gear reduction below the glove box and just the passenger side end of a little white shaft that rotates 1/4 turn between the other 2. I'm sure there are other acutuators but I can't see them without major disassembly. The A/c blows cold at 45 degrees.

The diaphram located high above the accelerator was not working. I traced the blue vacum line to a module behind the glove box. The ploblem was the multi-line vacum connector at the module had deteriorated inside plugging the blue line, others were in various degrees of plugging also. Looked like some type of chemical reaction. I cut out the connector and spliced the lines, works better than ever.

 

Thanks for that heads up! My wife's Buick is experiencing the same trouble. I was always fearful of tearing into all the controls under the dash but suspected something like this. I am going to check her car out this weekend. Thanks!