This has been on going for the past year. the blend works fine. fan speed works correctly. the selection **** for ac, max ac, etc will only allow air through defrost vents.
vacuum leak?


1998 4.6 auto super cab

 

See HVAC FAQs, link is in signature below.

 

all the vacuum cylinders that direct air thru the ducts are controlled by vacuum from the motor... vacuum line runs to your selector switch, then to each cylinder thru the selector valve... when you loose vacuum all the cylinders close and the air goes to defrost..... You have a leak on the hose at the firewall, behind the selector valve, or inside the selector valve itself ...... check hoses first.... have seen cracks in the TEE or hose at the firewall, right side of motor at firewall in front of glove box.

 

Thanks for the help.

 

Very common problem. I have had the same issue.