I have a 2000 Blazer 4x4 with 4.3, auto trans and computer controlled hvac. Has good heat but when driving it will randomly change direction of where heat comes out. I have it set to floor and it will change to dash vents, it use to go back to floor but now seems to stay on vents. Sometimes I can move **** and it will change back but not always. Not sure where to start, any help will be greatly appreciated.

 

Sounds like a classic case of leaking vacuum hoses that supply the vacuum that control the air deflection doors. If there is a leak in the system so that the vacuum reservoir leaks down during those times when the engine is under load and cannot supply as much vacuum, the actuators will default to their default positions.

I was going to point you to my write up on this but I saw you already found it. But I'll put the link here anyway in case someone else finds your post. https://blazerforum.com/forum/2nd-ge...replace-95425/

 

Thanks Christine_208, what I was not sure about was my 4x4 works fine except when it is very cold like we had a week or 2 ago. Does the hvac run off the same vacuum connection or from somewhere else and if I had a vacuum leak would that effect the operation of my 4x4 also?

 

Tankerdave,

Yes, there is a single vacuum supply hose attached to the upper intake manifold/plenum that supplies vacuum for both the HVAC and 4x4 engagement. I've read of instances where the 4x4 won't stay engaged because of the lack of vacuum. Also from what I've read it appears that HVAC issues tend to arise first or, alternately, are simply noticed first.

Below are just two of the threads that show the vacuum hose routings. You might find ones that are clearer than I was able to find.

https://blazerforum.com/forum/2nd-ge...4-issue-83365/

https://blazerforum.com/forum/engine...ng-help-93443/

With the diagrams, replacing the vacuum is not very hard.

Good Luck

 

Thanks again Christine_208, I will start with the vacuum lines and go from there, being a 17 year old truck sure it can't hurt replacing them. Hopeful that will fix my issues and all will be good.