I have a 2006 Savana 3500. The heat control for the front only puts out full heat or full cold, but trying to adjust the **** anything in between does nothing.
Heat and AC works fine, just not able to adjust temperature.

Any ideas?

 

that will be a malfunction in either the control panel, or the temperature blend door actuator.

 

Do you know how to diagnose which one it might be?

 

are you good at electrical troubleshooting with a meter and reading diagrams?

 

Sure. I tested the variable resistor on the heater control ****.
25 ohms to 9200 ohms

 

schematics and drawings are on charm.li
Next I would probably look at the actuator and door to see what they're doing when you twiddle the ****. It could be binding up or jumping teeth or something.