I have done a fair amount of research on this, but I am stuck. I have a 1999 jimmy with the 4 switch 4WD buttons including auto. I cannot put it into 4WD. Here is what I know:

1. I have the 4WD with the two wire actuator on the firewall that controls the vacuum.

2. When I turn on the ignition, the service 4WD light comes on and then goes out. The lights for auto, 2wd and 4hi also light and auto and 4hi go out. 2hi remains on.

3. The light for 4Lo never comes on.

4. Fuses are fine. I have pulled both the ATC in the engine and the 4WD fuse in the interior for 30 seconds - no change.

5. The vacuum actuator under the battery is working when I bypass the vacuum switch on the firewall (connect the two hoses that go to the switch) and the cable seems to be being pulled properly. However, even with vacuum applied and the cable pulled, I am still not in 4wd.

6. Pushing any of the buttons currently does nothing, either stationary or moving. Hi2 stays lit. A couple of days ago I pushed auto4wd and it seemed to do something, but then the service 4d light came on in the dash. It has not come on since then, nor does the auto 4wd button appear to do anything.

I thought that you could engage 4wd by pulling on the cable, and in fact there were manual locking cables to replace this system, however it appears that even with the cable pulled I do not have 4wd.

There appears to be a lot of information relating to the non auto 4wd transfer cases that has confused me. I tried to get my vehicle on ramps today to look at the transfer case, and could not because the grass was wet and I had no 4wd. I am not sure if my vehicle has the three vacuum line switch in the transfer case and whether that would be relevant to this situation.

Any thoughts, suggestions, or advice would be most welcome.

 

I solved number 3 by removing and connecting the big connector to the tccm
Under the passenger kick panel. I now have all lights lighting when I turn on the key, but still cannot get the vehicle in 4wd

 

It sounds transfer case or computer related. Check to see if your front drive shaft is spinning when you put it in 4wd. Im pretty sure the computer doesnt care if you have 4wd engaged and no vacuum to the dif. Otherwise people wouldnt have all the stuck in 4wd issues or the no 4wd with lights showing its in. However i dont know jack about the auto 4wd set up. Just wanted to chime in because I know how frustrating it is to have no one respond to a question on here.

 

I can't put it in 4wd.....

 

It was the four button switch. I pulled it and wiggled the wires and everything started working. I could hear the tccm respond when I pushed buttons and I could get all four buttons to respond and stay on.

BUT:

1. It got stuck in 4LO and was definitely geared low. It took me a half hour to get it out of 4lo.

2. It is not actually in 4wd. With the 4wd lit I jacked the front wheel leaving the rear ones on the ground. I could turn the front wheels by hand. I think it should be locked but my reading said that I should only have jacked the right wheel and tried to turn it. So I now have a different issue that I will have to look into.

For those of you who come across this thread with a 1999 and newer 4 button switch (with auto 4wd) a lot of the information for other models does not appear to be relevant. Someone can correct me if I am wrong but this is how I believe the system works.

The button on the dash will send a signal to the tccm module. When 4wd is required it will activate a switch on the firewall. One hose is connected to vaccum and the other goes to the vacuum diaphram under the battery. When the switch is activated, vacuum gets applied to the diaphram, which pull a cable that goes to the transfer case. There is a sensor where this cable enters the transfer case, but I could find no 3 hose vacuum switch like on earlier models, nor did I seen anywhere that would be used. I also did not see an encoder motor on this setup, but it must be there as that is how LO is activated. The encoder motor ONLY switches the vehicle into LOW and isn't used for 4hi.So the only components that I have seen on this setup are the four button switch, the tccm, the two wire/two hose switch in the middle of the firewall, the vacuum diaphram, the cable from the diaphram to the transfer case, and the sensor where the cable enters the transfer case. There is also an encoder motor for 4lo, and mine appears to be working.


Hopefully someone will correct any errors.

 

Na the wheels will still spin. However when its in two wheel the tires will spin the same way and when in 4wd they will spin in opposite directions. If they dont then could be a vac problem. I would check by the drive shaft. Should spin free in 2wheel and shouldn't in 4. If still no 4wd after determining transfer case engagement then move to vacuum and actuation from there. Also the vacuum actuator cable goes to the diff not the transfer case. The vacuum switch is on top driver side of the transfercase. You can not see it. You have to reach up on top of the case and feel around. Its there.

 

thanks for that information. I tried spinning just the right wheel as described here:

1999 jimmy 4x4 not working - Mombu the Chevrolet Forum

It definitely spun free in 2wd and was hard to turn in 4wd. I could still turn it though. It was harder to turn in 4lo than in 4hi. So it appears my system is working but is it possible that my 4wd is slipping. I think I read that there were disks in the tc? Worn? Wrong fluid?

 

Fluid in transfer case is thin. Not red. Not blue either though. Almost black or very dark purple

 

I still think you should check the front drive shaft. Not to be confused with cv shafts. Drive shaft comes from transfercase to front diff. CV shafts go from front wheels to diff. If you keep calling the front diff a transfer case its going to be very hard to diagnose your problem. Also the manual calls for tranny fluid in the transfercase and gear oil in the diff. If your confusing them and fill up your diff with transmission fluid you will have a problem. Spinning the drive shaft will tell you if you have a problem at the T-case or the diff. Dont jack it up. Leave tires on ground. Isolate the problem! try spinning the drive shaft in 2wheel....then engage 4wd and try to spin it. It should not spin in park. If it will not your problem is with the actuator going to the diff or the diff itself. I think your confusing the diff and transfer case. Your slippage is at the diff not the T case in your test. It sounds to me like the shift fork is not engaging the two shafts in the right side of the DIFF. Try manually pulling the cable at the actuator under the battery. It should move about 3/4 of an inch with some tension. If it will not move then your cable is jacked or the mech in the diff is jacked. The fluid in the transfer case which is behind the transmission is transmission fluid and should be on the thinner side. Fluid in the front diff which your front tires are connected too is gear oil and should be on the thicker side.

 

When I turn the right wheel in 2wd, the wheel turns freely and does not turn the front driveshaft between the tc and the diff, but when I put it in 4wd it is much harder to turn but I can do it with one hand, and the frt driveshaft turns too. I am thinking my clutch packs are burnt. Or there is not enough pressure on them. Or the fluid in my transfer case is wrong or burnt too.

edit: I was definitely looking at the transfer case fluid. Not the diff fluid. It was black or very dark purple.

I was able to move the actuator cable when I tested it previously.