Hi, new member here... I recently bought a '99 S10 Blazer, with a 4 button 4wd and I tried to read already existing related threads but started this anyway.

The Problem: I'm pretty sure that my 4wd comes around in "auto" position when rear end looses traction, but if I press 4wd HI, the light stays on but it still runs 2wd...
Is that even possible? could this be one of those easier "check vacuums etc." - repair?
4wd LO light won't stay on...

Would like to have one of those good old stick 4wd trucks, but S10 is so handy and small in the city....

 

If you can verify for sure that the auto 4x4 works then there should be no vacuum issues as the front axle remains locked in auto 4x4 and it is vacuum controlled. chances are if 4hi doesn't work the auto 4x4 doesn't either. can you hear the TCCM clicking under the passenger kick panel and the encoder changing on the transfer case ? these can usually be heard and if you can I would check if the front diff is locking.

 

when changing from 2 hi to 4 hi, couldn't hear tccm click, but when changing to 4 lo, heard the click for sure and then heard the sound of front drive getting on. But getting on and off the 4 lo needed the car to be on neutral gear and to let it roll a bit... a pro tip from a friend.
I don't know, is that 4 hi activation less easier to notice? and does the 4wd in these cars work so that it feels like it has more drive at rear axle?
last 4wd I had was a '95 Suburban, and it felt as 4wd as one can, when the lever was pulled...

haven't inspected the vacuum control yet, as the 4 lo at least worked for sure...

 

the 4Hi activation is definitely smoother, If 4 lo works then the vacuum part of the system should be functioning and it is possible that another part of the system is not . I tried to copy a link but it didnt work . If you type 4 button 4x4 in the search you will see a thread called "4WD problem" started by Nixoncc.....in this thread there is a link for your system , lots of great info in there.

 

The TCCM can fail but I don't know if this is what is happening with you. Be sure the check the underhood vacuum lines for leaks.