hello all im new here and im haveing a issue with my 99 gmc jimmy sls 4x4 4.3v6. the problem is when i put it in 4hi or 4lo i can hear the transfur case engaging but the front dif wont, i checked the cable, and the actuator and there both good, i just checked all vacumn lines and there all good, i just changed the vacumn switch on the transfur case and still not working. can any one give me any ideals

 

Welcome to the forum!

How many buttons on the dash switch, 3 or 4? Engine at idle, transmission in park or neutral, select 4WD: Pull the vacuum line off of the vacuum actuator under the battery, the line should have manifold vacuum, (around 17"Hg).

 

there are only 3 2wd, 4hi and 2lo and i did check for vac and theres none, and i did check for vac at switch and there is vac going in but not out it could be a defective vac switch that i put in

 

The connector at the vacuum switch has 3 lines: The source line comes from the vacuum reservoir up near the brake booster. One line goes directly to the vacuum actuator under the battery tray. The third line goes up a few inches to a vacuum tee. One side of the tee goes to the transfer case vent tube, the other side goes up alongside the transmission dipstick tube to a filter. The actuator must have vacuum in 4WD, and must not have vacuum in 2WD. The vacuum switch has a spring loaded ball that is pushed in when 4WD is selected. It connects source vacuum to the actuator line. Some aftermarket replacement vacuum switches have smaller diameter nipples so the lines leak at the switch. Vacuum will leak out and not supply enough vacuum to the actuator.