Sure enough, it was the vacum switch. Thank you everyone for your help, I appreciate it!

 

My 1999 blazer LT will not change to any 4x4 dunction the push buttons do not light up. When the temp outside gets below a certain temp the 4 x 4 will not do any thing sends a code of some sort and, service engine light comes on after it is in a warm shop the selector works fine but still have service engine light on.
I have disconnected the battery to reset the codes but it always comes back on.
Any thoughts?

 

I have a similar problem myself and I was wondering if anyone could help. My 1999 Blazer LT will not engage any 4 wheel drive operation. The buttons used to flash but they have now altoether stopped lighting. I took my Blazer to the delaership I got it from, and they said it was some sort of chip that was missing. Im guessing that there is some sort of computer that tells the truck to switch between 2wd 4lo and 4high. The dealership said if i could get the part they would pay for it, where do i find the exact name or part number of this chip? Any ideas? thanks for any help.

 

dmxmessenger,

If the Dealer is a GM Dealer then I would think twice about them working on my truck... If they can't figure out the part and location...do a search for TCCM and troubleshooting...that thread should be of some help...the TCCM is the (T)ransfer (C)ase (C)ontrol (M)odule and it is located in the dash on the passenger side...it controls the shifting of the Encoder motor on the transfer case... if it has any trouble codes in memory then it will not be able to shift the transfer case...