Here is the problem that I am having....Truck works in 2HI fine and the light is on for 2HI. When I go to engage 4HI nothing happens and the light doesnt come on for 4HI it stays on 2HI...When I put in netural and press the button for 4LO it will engage into 4LO and both 4HI and 4LO lights will be on. Please advise...Thanks

 

90% of the time 4wd problems are vacuum related. Start at the vacuum actuator on the top middle of the fire wall, look at the vac lines to make sure they have no cracks or brakes in them.

 

I had the vacuum problem before and had it fixed. When I had that problem the light would still change and you could hear something happening but it wouldnt engage but after the leak was fixed it worked fine.. Im wondering if there is another place to look. Thanks for your input!!!

 

Have you replaced the TCCM or are you still running the original?

-They fry themselves and the pin connector corrodes up because Chevy thinks die-electric grease is too expensive to use on their vehicles.

Does the service 4x4 light come on?

-If it has it has stored a code and will make your search so much easier.

 

There is another place to look: the TCCM. You didn't say if you have 3 or 4 buttons on the dash, but if you have 3, then there's a way to get the TCCM to tell you what's going on. Check out the "4x4 no worky" thread in the DIY section. It may save you from throwing money at the problem.