So, I'm not sure if I have bad lines or what, but, I can hear my vacuum has lock when I put the truck in 4wd. The vents will default to defrost. Truck is a 2006 V10 250. Should I still suspect the hub's, or go after changing lines?

 

Does the airflow default to the defroster ONLY during the short interval immediately after switch is set to 4WD?

 

No, it will stay at the defaulted defrost.

 

Then it isn't the vacuum lines to the hubs

Start with checking the vacuum hoses under the hood that lead through the firewall. Check the other lines that are tee'd into that one. Those vacuum lines near the battery seem to degrade due to the battery. Check the solenoid that supplies the vacuum to the hubs when energized, those solenoids have a bad habit of cracking and leaking vacuum all the time. Check the reservoir.

 

Hey, thanks!!! I'll start going through the other lines and the reservoir and report back.