4wd doesn't work
 

02 Zr2 Has 4 hi, lo 2 hi and auto 4wd

Press any button from 2wd hear noise under truck and light will flash and eventually return to 2wd steady. Can press 2wd while 4wd light flashing and return to 2wd immediately.

Edit: I inspected under the hood and found the vaccum line that goes to the thing on the firewall disconnected and there's no vacuum line at all going to the actuator! How can i fix this? The actuator is under battery tray? Do I just go buy vacuum line?

There is a cable link between the actuator located under the battery tray and a front axle diff. That actuator is vacuum operated. If You got any vacuum leak, then it won't shift the cable. Check vacuum lines and actuator. Check cable link.
A front differential is equipped with a proximity sensor that gives the feedback signal to the control logic. LED will keep on flashing until a valid signal from the sensors is recognized.

I replaced the vacuum hoses. When transmission in neutral switching to 4LO there is a clunking sound that seems like it's engaging but 4LO continues to flash. If I try to go from 4LO to 2HI with transmission in park - it stays in 4LO with 4LO lit up steady and 2HI flashing. If I go from 2HI to 4HI I hear the clunk that sounds like engagement and if I try to go from 4HI to 4LO 4HI stays steady and 4LO flashes. Before I replaced the vacuum hoses I could hear a noise when pushing the button but never a clunk from the front end.

"LED will keep on flashing until a valid signal from the sensors is recognized. " - think it could be the sensor?

From what I've read sounds like encoder?

If I am not mistaken here, I remember some saying that You may need to be rolling at a very little speed to change over. Could be that a transfer case needs to synchronize to complete the mechanical gear change over. Worth checking.

The sensor I meant is in the front differential assembly and it is only for 4WD front axle engagement check (front axle in/front axle out), and isn't a factor for Lo/Hi. And from Your latest post looks it is not the culprit.

The Lo/Hi is changed over into a transfer case, so a different place. The transfer case is driven by the encoder motor, like You say.

Leaks in any of the under-hood vacuum lines can result in the front axle not engaging. (These leaks can also mess with the HVAC system control.) So just to be sure, take a look at a link I have in my signature (below) that describes all of the hoses I replaced on my 1999 that also has the 4-button controlled transfer case.

As for how the system is supposed to work, I'm attaching an excerpt from my 1999 manual that describes how the 4-button transfer case is supposed to work. (Did you know the transfer case can be put into neutral??) This will answer Mike's questions.

With the clunk sounds you are reporting it is likely the transfer case is working fine. How is the transfer case control module (TCCM) doing? It is under the passenger side kick panel and can go bad too. When starting to go bad, its relays will be clicking back and forth before setting. Eventually it will get worse and then never set.

Good Luck

Even though I hear a clunk in 4lo the 4lo light never goes steady until I say try to shift from 4lo to 2hi not in neutral, because it has to be in neutral to shift from 4lo. But if it wasn't in 4lo why would it go steady and 2hi flash like it can't go into 2hi. I have replaced the red and the blue vacuum lines. The diameter of the nozzle on the actuator is different that the diameter on the valve at the firewall. The last person used the same vacuum hose diameter as coming from the T to the valve then it goes to a vacuum adapter to another piece of hose that is the size on the actuator. I assumed this would be fine since the source vacuum line is the same, it's just staying that diameter for longer. I guess my next step would be replacing the other vacuum lines though I don't see how to do the one that disappears into the fender well

It does not disappear, there’s a little fitting (if your talking about driverside between cruisecontrol module and brake system) reach with fingers and you will feel a indent and a nipple, plugs onto that. That hose is typical for being unplugged and not knowing where it goes. Check that.

Regarding the line that goes into the firewall, that one should be of a different material, a tough smooth plastic type. On mine there is a almost transparent female fitting on the end. This line is part of the HVAC system in the passenger compartment and controls many of the air-duct actuators.

The best way to test if there is a leak is to use a hand vacuum pump. This will help diagnose if there still is a leak after replacing the hoses. However, even if there is no leak, I still suggest to people that they replace the under-hood vacuum hoses because after at least 15 years, they will rot and start leaking if they have not started leaking already.

Climbed under yesterday to test the front drive shaft - can't turn it by hand - is that normal in 2WD?

It should spin free and easy in 2wd, at least on the three button cases like what I have. You'll need to determine if it's either the tcase engaging 4wd when it's not supposed to, or if it's the axle disconnect being engaged when it's not supposed to.

Lift the right front wheel off the ground. If the axle disconnect is open, like it should be, that tire will spin, without turning the driveshaft. If the front driveshaft turns as well, the axle disconnect is the issue.

If that wheel spins independent front the front driveshaft, but the driveshaft doesn't spin by hand, then the tcase is actually in 4wd