a lot of people have problems with their 4x4 not working, for various reasons. one of the common reasons is the vacuum actuator going bad, or the vacuum lines going to it. i have gotten a PM from somebody, and i figured i could share some of the info with everyone.
in s10's, when you put it in 4wd (4hi or 4low) with either the 233 (push button) or 231 (manual shift(also 207)), the transfer case has a switch of sorts that tells the front axle to engage. this switch routes vacuum to one of two places. if you are in 4wd (hi or low), the vacuum goes to the actuator. if you are in 2hi then the vacuum gets routed to an orb where it is just stored. this line also supplies the vacuum.
now, the switch it self is on the drivers side, almost at the front of the t-case. it has three vacuum lines going to it. the switch works similar to a ball point pen, meaning there is a round ball that is either pushed or not. the ball gets pushed by the shift fork inside the t-case, when the case is shifted into 4wd. now, there are three vacuum lines going to the switch. one line supplies the vacuum, and stores it in the orb when not in 4wd. one line vents to the open air, and the t-case vent piggy backs onto it. the other line goes to the actuator. all three lines run together up to the passenger side of the bell housing. then two go one way, and one goes the other. the vacuum supply/orb is the line that breaks from the pack and follows over to the drivers side. the other two lines, the vent and the actuator line, continue to run along the passenger side frame rail where they end near the vacuum actuator. one line plugs into the actuator and the other is just a vent that clips onto the inside of the outer fender well.
the actuator takes the vacuum, when supplied, and has a diaphragm that pulls on a cable. this cable runs down to the passenger side of the front differential, and moves a collar back and forth. this collar either connects the passenger side cv shaft to the carrier, or it disconnects it. this is how the 4wd system works on an s10, and all IFS 4x4 gm products (with a few variants).
now the pictures:
this is the switch installed on a 231. note the vent line coming off the t-case
here is where the t-case vent line meets up with the open air circuit.
you can see the lines running along the trans and bell housing
this line goes off to the orb, and supplies the vacuum from the engine
this line will follow the wiring harness and go the orb, located behind the drivers side headlight
this is the orb, aka the vacuum canister
this is where the switch gets vacuum, off the intake
this is the other two lines that go off to the passenger side
they end behind the passenger side headlight
this is the line going into the actuator
the actuator is under the battery tray
now onto the switch. the switch looks like this
if you look at the top, where the lines go in you will notice a little "key"
this lines up with a similar spot on the female end
the three vacuum lines must be in the right spot in order to function properly. take the female end of the vacuum lines, looking at the open end towards you like in the above pic. if you orientate it to where the key is on top, the lines are as followed.
if anyone has any questions, or wants any more pics, just ask.