John,
The climate control in the '96 W140 is made up of several component "groups".

1. Climate Control Push Button P/B unit (in dash)
2. Vacuum valve block controller (behind the glove compartment)
3. Vacuum tank and vacuum source
4. Sensors (temperature/sun load/pressure,etc)
5. Electrical cabling/harnesses between sensors and P/B unit

In short, its a complicated system that will kill you in dollars if you just "hunt and peck" for the problem. The procedure to troubleshoot this system occupies several "pages" in the Mercedes manuals.

The good news is that the P/B unit (the brains of the system) is able to diagnose a number of basic problems and will store a DTC when it finds an error/problem. Also, the unit will allow you to command individual flap controllers throughout the system.

If you're not familiar with this system, I strongly recommend you get someone to perform the diagnosis for you via P/B unit testing or DTC downloading via the Mercedes Hand Held Tester (HHT) or other good scanner.

On my car, the problem I was having with flap control was in the vacuum valve block controller but it only happened in cold weather. The troubleshooting I had to go thru got me pretty familiar with the system and took a few weeks of my spare time. Your problem could be in any of the above components or the vacuum/electrical lines between them.
If I had to do it again, I'd pay Mercedes to tell you what is wrong and then make the decision whether they fix it or you do. Paying them an hour's labor will save you time in the long run.

Hope this helps.

__________________
KenP

1996 S500 W140 Coupe