I've been trying to figure out what's wrong with my truck, I followed this:
http://www.dodgeram.org/tech/repair/..._engagment.htm

1. Shift into 4WD and move truck a couple of feet
2. If light is still off, park truck, block the wheels, set the park brake, jack right front wheel, and try to hand spin the wheel
   • If wheel doesn't turn, 4x4 is functioning properly but there is a wiring problem: use an ohmmeter to check the 4x4 switch, wiring, fuse, and bulb.
   • If wheel turns freely, the front axle or transfer case is not engaged: continue...
3. Attempt to hand spin the front driveshaft If the driveshaft WILL spin, the transfer case is not engaged Check the transfer case shift linkage, problem is in linkage (or in the tran$fer ca$e internal$ <-- yes this means $$, ouch!)
   • If the driveshaft will NOT spin, the transfer case is engaged properly: continue...
4. USE CAUTION! With transmission in neutral or park, start engine, measure vacuum on the two vac lines at front axle shift motor. One line should have approx 20" vacuum, other line should have none.
   • If NO vacuum measured, look for vacuum leak or bad vacuum switch.
   • If both lines have vacuum, look for a bad vacuum switch or ruptured CAD actuator diaphragm.
   • If one line has 20" +/- vacuum:
     • Move transfer case lever to 2WD, measure vacuum again. Lines should have swapped vacuum signals.
       • If No: vacuum switch bad or 4WD shift linkage needs adjusting
       • If Yes: Vacuum motor bad or stuck, or shift fork stuck: continue
5. Remove vacuum shift motor from the axle, attach vac lines to shift motor, engage/disengage 4WD. Does shift motor plunger move at least 1/2"?
   • No: shift motor bad
   • Yes: Check axle shift fork and vacuum motor for binding

Did it all. Driveshaft was locked, but wheel turned. Checked the lines, got vacuum from one. Put it into 4, got vacuum from the other. Pulled the assembly off the axle, had someone switch between 2 and 4, and the fork moved back and forth. Put it back together, still no 4wd.

 

do you have a broken axleshaft?

 

does the four wheel drive indicator light on your dash come on when you put it in four wheel drive. When the vacuum diaphram slides the collar over to lock the shafts together it pushes in the switch that activates the 4X4 light. If it's not coming on, the collar and plunger aren't moving correctly.

 

Not that I know of... how would I tell?

 

Good point. I was getting vacuum, but I don't have a vacuum gauge so I don't know how much I was actually getting. If the vacuum control isn't pulling enough pressure to move it, that could be my problem.

FYI, my 4x4 light never has come on, but 4low always has. Bulb burnt out?

I was planning on replacing the vacuum system with the cable actuator, but I just haven't got around to it yet.

 

Check you shifter linkage it's a small rod coming from the transfer case there is just a small bolt locking it in you can adjust that

I used to put my truck in 4x4 and it would go a few feet then go to neutral when I adjusted the linkage all was well again

 

good work so far, but can you put the truck up on 4 jack stands and start at the t-case and work down stream. check things in this order.
does front drive shaft turn.
do front wheels turn (probably not since you wrote this post)
does vacuum switch from one line at the cad to the other.
remove cad.
does the fork slide over.
can you move the coupler byhand on/off the 2 axle pieces.
(there's a bushing in there that might be worn out causing out of alignment. also coupler or shaft could be buggered up).
can you get the light on the dash to work (its activated by switch on cad, only when everything slides all the way over).

 

possibly a Posi-Lok is in order?

 

front drive shaft locks to t-case when in 4
wheels don't turn
vacuum switches over, but not sure how much vacuum, no gauge
fork slides over
have not tried to move coupler by hand. thanks, i'll try that
light has never worked, except 4lo. didn't think to check it when moving fork by hand.

 

Ya,but I wanted to make sure that the problem was isolated to the CAD system first.

It's $202 from the company, anyone know of a cheaper place?