Troubleshooting HVAC controls

[bryancald]

Does anybody have vacuum diagrams for a diesel 240 HVAC system?
I can get heat/no heat with the temp slider.
What I can't do, is change where the air flows (it will only flow through the dash).
Now that it's winter, I need to defrost the windshield.

Is there a vacuum solenoid or something that fails routinely on these systems?

Thanks!