I don't understand. My 4WD works fine.

 

the round silver looking thing is a vacuum diaphram with a pivoting shifter fork and it has 2 vacuum lines running to it from the transfer case. its the cad (central axle disconnect). if those lines get badly damaged, to where it leaks vacuum, then the cad won't work and your front axle wont couple up, which is needed for 4wd operation. (my vacuum lines are kinked under the trans cross member but they still work). as a related item, when the cad engages, it trips a switch on the end of unit which has a 2 wire connector. when this switch closes, it turns on the 4wd light on your dash. if these wires are badly damaged, then you may have a working 4wd but no light. ( i had no light until i cleaned the mud out of the connector).

 

i'm starting to guess... i think the check valve is in the return cooler line, and its often removed when a trans is rebuilt because it provides a place for trash to collect and block the line. somebody correct me if i'm wrong.
mine has only done this a couple of times with no explanation, so i haven't worried too much about it.
if yours does it all the time, then there may be something wrong, such as a weak pump ??
somebody else help me out, thats all i know...

 

What are the indications that make you say that your 4WD is engaging ("works fine")? If the electrical connector is disconnected ... no 4WD light. If one of the lines is toast ... well ... it may still be sealed even though it's crushed so the CAD may still be operational ... but I doubt it.

Seriously ... how do you know it's ACTUALLY working?