Have a 98 Monte. Am having problems with the heater not switching between the different zones. I'm getting good flow to the defrost. When I switch to a different zone, such as floor, there is no change. There is a little bit of heat flow to the floor but most is still coming through the defrost. If I go to panel/floor I hear a small difference and get a little heat out of the panel vents, but still, most comes out of the defrost vent. Fan works on all speed settings.

Any ideas?

Thanks

 

Vacuum leak probably. I believe all of that is controlled by vacuum.

 

If its a 3800 check the back of the UIM for broken vacuum line. It's 100% controlled by vacuum. If that checks out did you recently have the dash apart at all to do stereo work etc? The plug on the back of the HVAC isn't always held on very well and can partially slip off. The mode doors are moved by vacuum diaphragms down by the passenger foot area make sure they are getting vacuum when you change the positions.

 

Check the hard, thin vacuum line running along the cowl behind the engine. It starts at a rubber plug or vacuum tree at the top of the engine. It then runs along the top of the the left strut tower, between the battery and the fuse box, then through the firewall to the vacuum ball canister. This controls the face, feet, defrost selector.