On my 1999 chevrolet tracker when I try to engage the 4wd the light in the dash wont go on and the 4wd doesnt engage. I taught it was the vacuum but when I unplug the hose and engage 4wd it will start sucking. Does the vacuum is supose to suck all the time the 4wd is engage or it will stop after a couple of seconds? If the probleme is not the vacuum what could it be?

 

The vacuum actuates a diaphragm inside the front differential, I'm pretty sure, as I read that somewhere. If the diaphragm is bad, the 4wd won't engage and the light won't come on. As for the vacuum, it should be present whenever the 4wd is "called for" to keep the diaphragm sucked in and the front wheels engaged.