4x4 still works but diverts to defrost ??????

[rdodger]

okay , so i'm posting another question about my beloved PSD , today we had a bunch of snow dumped on us and i figured i would get the front differential fluid changed then try out my 4x4 for the first time . before i tried it i had read a few post on the vacuum problem where the heater diverts to defrost and 4x4 doesnt work . i go to the truck and flip the switch on the dash with the hubs in auto and the light came on and 4x4 worked ( i checked in the snow) then after a few second the heater went to defrost so i was wondering why this is happening . sorry so long and thanx in advance .

[YoungMT Hick]

only thing i know is that it is a vacuum leak. I unpluged my vacuum pump and it goes straight to defrost and nothing else. Check the vacuum pump to see if that is working. then check the vacuum valve behind the glove box to make sure that is plugged in.

Were you able to get out of 4x4?

[rdodger]

ok so everything looks plugged in but i'm not sure where to look for leaks or how to check to see the vacuum pump to see if its working , thanx for your help

[Davey~do]

The majority of the time it will be one of either two things:

1. The vacuum line to the hub has either been knocked off, dry-rotted, etc. right where it attaches to the hub itself. It's just a rubber hose slipped over a little nipple on the hub so if that's it you can probably just cut the bad section off and slip it back over.

2. One of your seals has an internal leak. If that's your problem you'll just have to replace the seals in your hubs. They're not very much money. If number one doesn't work out borrow a vacuum tester from someone and try to pull a vacuum on each hub and see if it will hold.

As far as testing the pump you should be able to just pull the vacuum lines off of it, plug the ports and see if it will quit running. If it does you can just put the lines back on one at a time and see which one causes it to keep running.

[Gibby206]

Okay I have the same issue and realize it is a common issue. My vacuum pump seems to run a long time after switching the dash switch over to 4Hi. It also makes allot of noise, would this indicate a bad pump?? The A/C eventually switches over to defrost than the pump seems to stop..

Any thoughts would be greatly appreciated.