Hey guys, does anyone know how to correct the vent door, some call it a blend door. I have an 88 and 90 Gen 1, both only blow out of the vent, wont blow defrost or heat vents on floor. Please help.

 

That is default mode. There is a small vacuum line that goes to the manifold, probably busted or just disconnected. I'd rather work on those than the newer ones with the electric actuators for the various doors in the heater plenum

 

The "blend door" regulates how much heat you get(yours is cable operated). Your problem is the lack of vacuum to control the actuators.

 

Look under hood on firewall. Should see 2 small hard vacuum lines. One goes to heater control actuator and the other goes to a vacuum junction point if you will, normally behind throttle body on intake manifold.

 

Maybe I’m confused. The temperature control (hot and cold air) works
what is the solution to move the air flow to the defrost, vent and floor when I choose?
thanks guys.

 

find this black hose under your hood. Trust me, I just fixed this problem on my 89. If it doesn’t have a good vacuum, you’ll never get the blend door to move between zones.

 

Thanks Langston! So I have found these two small vacuum lines, neither seems to be broken or have damage, what is my next step?

 

See if you are getting vacuum TO the control head. (in the dash) Temp is controlled by a cable, pure mechanical connection, where the air comes out is vacuum controlled. If you don't have vacuum, most of the air comes out the defrost vents, and a little bit will come out on the floor.

 

Yours is an issue on my list of repairs for my truck as well, mine being 86 though. I have the 86 FSM, you want me to post you the climate control pages and see if they’re any help?

 

Polye86,
yes sir please do.
thank you