I have searched and can't seem to find this exact problem.

2000 blazer lt auto climate

On the first start of the day and if it's cold out I can control the selector to any place I want, floor defrost face. After the vehicle warms up I hear a air hiss and all the heat comes through the front vents. Even if the selector is at defrost or feet. What could this be? Thanks.

 

The hiss is from the selector switch leaking. Will likely need to replace the switch, but may be just a loose connection for when it gets warmed-up.

Pull the trim panel off and take a look at.

Good luck.

 

The selector switch? I have pulled out the hvac panel and looked it over. Nothing out of the ordinary.

 

Did you pull the HVAC panel and then run the engine until warm and check defrost operation?

 

No I haven't had a chance to do anything to it yet but want to this weekend. So unhook the hvac panel and run the vehicle? What am I looking for?

 

I made a video of it doing it this morning. Towards the end you can hear it switching from defrost to vent.

http://youtu.be/hTYYWr_gowA

 

Anyone help me with this?

 

Okay watched the video and have an understanding of the problem now.....air flow is switching from defrost to dash.

I always thought that defrost was the default safety position...so if vaccum was lost, air flow would be direct to defrost. So doubt this is a vacuum leak.

Wondering if the controller circuit is failing once it gets warmed-up and just switches air flow. Before replacing the HVAC head unit, I would crefully take it apart and check the solder connections for corrosion and reflow every solder joint on the circuit board. I have solved many problems doing this as all those temp cycles crack solder joints. Be sure to use a soldering iron, not a gun.

Let me know what you do.

 

The head unit? As in the switches (*****)? Or is there something behind that in the dash.

 

Well I got a different unit to see if that fixed it. It did not. So now what.