This started about a year ago and is getting more frequent. When I turn on the heat or AC from the off position, it only blows on the windshield even though I have the vents facing me selected. I have to cycle all the way to defrost and then back and eventually they change, albeit slowly. Sometimes I have to do this a few times before it changes. Is this something simple or do I need to replace the control module?

 

There are no electronics in the panel. The HVAC mode selector is actually a ganged vacuum valve. It diverts engine vacuum to the various actuators.
If it's acting sluggishly then you likely have weak vacuum, which is usually caused by a vacuum leak. Inspect all the tubing, valves, and reservoir and replace as necessary.

https://chevroletforum.com/forum/exp...module-110797/

 

Update:

The HVAC selector finally stopped working. It was stuck on defrost vents and would not change. There is a ball-shaped vacuum reservoir mounted to the bottom of the battery tray. Vacuum tubing runs from there, to a 3-way splitter under the blower motor, and then back to the intake on the passenger side of the engine, behind the doghouse. This tubing was split/leaking. I replaced all of it and now the HVAC controls are sharp and quick and work as they should. I've read that some models have a small vacuum pump next to this ball tank also, instead of getting vacuum from the engine.

 

Thank you for posting this update. I've got a small vacuum leak in the HVAC too and will have to start messing with it sometime.

 

Common problem.
The heat degrades those rubber hoses pretty rapidly. Mine are falling apart in my hand now too so I'll be doing some major replumbing soon as well. Luckily bulk hose is cheap at NAPA and similar. I believe each section is a different size so make sure you bring a sample with you.
Don't forget to change the check valve while you're at it. They're too cheap not to.


I believe that's diesel only. They're turbocharged which means positive pressure in the intake... no vacuum!