Okay folks, there a lot of threads on this issue but I am getting more confused by the minute. I have a 1999 GMC Jimmy with the push button 4X4 (4 button). When I press the buttons everything works correctly except the front diff wont engage. The transfer case is working properly and the drive shaft is turning. There is a cable running to the diff then under the batt tray. The cable moves freely but there is no vacuum going to the diaphram. Tracing the lines back there is a switch on the fire wall but no vacuum to it either. Where do I go from here? Don't know if this helps but my selector switch for the heater/AC controls won't work either. just blows out all the vents. I have been reading about a vacuum line on the back of the motor near the distributer that controls both the 4X4 and the heat/AC controls. Could this be the problem? I have a vacuum line that goes no where and wondering if this "T" near the dist is where it should go.