Heater vent modes not working: solved

[BlueDove91]

I recently replaced my vacuum lines. When complete, my heater vent modes wouldn't work (stuck on defrost). 

I was familiar with the potential issue of the line leak and the vacuum ball, but the ball was nearly new and the lines were brand new (with flexible silicone lines). 

There was no vacuum leak, but the modes were still not working. Which was a head scratcher.

 

The problem ended up being the check valve in the line where it connects to the engine manifold. 

Apparently when I put the new lines on, I put the check valve in backwards. Flipped it around and now the vent modes work as they should. 

Wanted to post this fix in case it helps someone out there. 

 

Truck is a 91 4.0 HO

 

[JZLAJeep]

Nice work. Thanks!

[robfg67]

Huh, you would think that the design would only allow it to be connected in only one direction. Good catch.

[Eagle_SX4]

I can never tell what direction the flow is on those check valves. I have had this happen before and always blow through them to find out which way goes towards the engine.