Yesterday,    hauled  some  cattle home,  and  our  sand is  pretty dry.. and  LOOSE.   So,  normally   I'd   kick it into  4x4  as  we get close to the   load out area.   I've learned to  kick it in    at least  100 yards  before  expecting the  light to come on.       Sometimes  it comes on immediately,   and  sometimes  I have to  stop...back up a little.   No matter,  yesterday,  Nuttin.

 

Crawled under to see if any vac lines were off.  All ok.    I need to  pop the hood to  check the  vac supply.. although my  hvac  doors  work ok.  

I did notice the  Vacuum motor on the  axle disconnect   was  'loose'.    Not quite   floppy,  but  easy to  turn on it's  nipple that attaches it to the  alum housing of the  axle. 

...   It's  been  almost  10 years  since  any of  my rams  needed this,  Kinda  forgot the   checklist for  finding  the problem.

 

If  I just disconnect the   vac tube at the actuator,    I should   feel  a vac    in one of the  lines..  correct?   and  by  shifting the  handle  on the  TC,   the other line should be  'vac'  correct?     I remember  the  little  vac   valve  on the TC  was   problematic  on  my   gassers  years ago.

At any time there should be vacuum on one side or the other.

 

http://articles.mopar1973man.com/general-cummins/39-transmission-systems/80-central-axle-disconnect-cad-system-troubleshooting

 

If no vacuum there go back up to the junction block at the top of the transfer case. From there the lines go along the top of the trans to the firewall under the hood where there is a small manifold where the vac line splits off from the pump and goes to the defrost controls and 4 wheel drive. Not sure how bad corrosion is in your parts, but I've had to replace the metal lines underneath where they run along the frame rail to the CAD.

One thing that I noticed on my 99 and on my friend's 02 Rams, is that the inside of the vacuum lines can get plugged up from the rubber hoses deteriorating over the years and the vacuum line gets plugged up. We took a blow nozzle on an air hose and blew each line out until we could feel air coming through them (with them disconnected from the vacuum switch on the transfer case, as well as, them being unhooked from the actuator on the front axle. Now my 4x4 kicks in so much faster than it ever did before, and the light kicks on just as fast now.

Edited December 17, 201410 yr by 99_Cummins_4x4

crawled under front end today and  pulled the  fitting off the  vac motor.    Vac  seemed a lot  more  suction on   the 2wd setting as opposed to 4wd.     I'll try  blowing out the  lines first.   I'll  try  swapping in a  known good   vac switch next.