Change to 4x4, AC switches to defrost vent, why?

[Coadman]

On my 2000 F350, my son noticed when he changes to 4x4, the AC quits blowing out of the front vents, and switches to coming out the defrost. Is this supposed to happen, or is a vacuum problem in a valve somewhere?
thanks,
Jim
By the way, love my truck!

[Dave]

Probably leaky seals in the auto hubs

[Coadman]

Are they easily replaced?

[davespark]

My standard caveat of "I'm not an expert" but...There's a couple of possibilities, there's a yellow O-ring on the hub that's part of the vac-u-suck hub system, and there's an O-ring on the hub itself too. seems like the Yellow one is the likely culprit. I think you can find some guides on it, like Welcome to guzzle's 4x4 Needle Bearing Maintenance Web Page Even though that's aimed at the needle bearings, it'll still show taking off the hub and where the O-rings are I believe.

I just replaced my hubs with Warn Hubs and blocked off the vacuum lines myself...

[Mykol]

A little off topic, but will Guzzle's 4x4 Needle Bearing Servicing description work on dually front axle?

[tweener]

leaky auto hub seals...they are not that easy to replace...they can be installed without the special tool, but it can be quite a PITA..

If it were my truck, I would plug off the hoses and use the hubs manually if yours have that option, or install manual hubs...the auto hubs cause a lot of grief.......i know the work great when they work, but when they fail, they suck in contaminants, water etc into your hub area...I prefer manual...and a lever in the cab too..

[firefighter56]

It could be something as simple as a cracked vacume hose that goes to the hub, or a rusted nipple that the hose connects to. I'd start by checking the simple outside stuff first!

Doug