I have a 99 ls four door ....had a mechanic check it out and tccm and push button and accutor is working....he thinks there is something in the front differantial is not working....I dont know much about 4x4s since this is my first one...but can do most minor repairs....anyone have any ideals what is wrong or have had the same promblem?.....it wont lock in ...light on push buttons dont light up but ...tccm clicks....the vacum module lol what ever u call it that is under the battery is working but it is not engaging in 4 wheel....

 

Can you hear the encoder motor try to shift the transfer case when you select 4WD?

 

Check the ATC fuse under the hood. If that is blown, it could be the encoder motor, front axle switch or even the cable to the actuator. Make sure you have vacuum to the actuator. In my case it was the transfer case itself. It was binding up on shift.

 

If the front differential is not locking in, that is controlled by vacuum. The vacuum switch located on the transfer case in notorious of going bad, check that first.

 

the vauum switch is on top of the transfer case, when you unscrew it from the TC the ball bearing at the end will be pushed in and hung. if it moved freely however it's probably not the switch. If it is the switch be sure to buy from a dealer because the auto parts stores carry a version of the switch that looks identical but when you connect the vauum lines they leak. had this problem on my 98 LT. the switch was about 15 dollars at the local gm dealer.

 

yeah if the encoder motor makes a click...i can hear that....thanks you guys...I appreciate the help very much...Ill check all that stuff tomorrow....and get back to yall....