I found out the hard way in my 96 2500 that the 4wd quit working, the light switch is fine, I replaced the 4wd actuator in the diff , then I took the two lines with the check valves in them off and tried, no vacuum, could it be the switch on top of the transfer case or should I start replacing hoses and lines?
I can clean the switch if I have to , I think the original problem is it was a farm use vehicle right off the showroom floor till I bought it

 

Do you have vac to the t-case switch?

 

Check the steel vacuum line that runs along the frame for any breaks or holes in it. Also, I believe Chrysler still sells the complete vacuum line harness that connects the vacuum source, the vacuum pod on the front differential and the transfer case. I replaced all of those lines a few years ago.

 

They sure do and it's only like $35. So much better than trying to scab together broken plastic lines.

Also these systems can get tiny pebbles in the steel hardlines which will cause slow engagement or no engagement of the CAD. Blow shop air in the steel line at the crossmember and see if a commensurate amount comes out the other end on the framerail before the flex lines drop to the CAD.

Compare each hardline (there's two, of course) and if one seems choked compared to the other, something is wedged in there.

 

Mine the steel line got water in it and rusted shut, Couldn't get vac threw it. The rubber fitting on the cad leaked.

 

Mine collected moisture, and froze.... so one cold winters morning, when I needed 4x4, it didn't work.