I like many of you have an issue with my 4wd. It started last winter, of course on the days i needed it, the 4wd wouldn't kick in and the "4wd Service" light would come on. But the next day, i would try it and it would pop right in no problem.

A couple of weeks ago it started to act up again, but this time it just won't go into 4wd at all. Though a few days ago i tried it in my driveway and now it is stuck in 4wd and wont come out. I checked the button area (i have Auto) and that seemed to be working fine, and the TCCM has a little bit of corrosion on it, but nothing that i think would stop it from working (i still hear the click). Anyone have any input?

One thing i was also wondering was when i hit the 2wd button there is a "whiny" sound coming from underneath my car. I have traced it to a black cylinder thing. Sounds like a motor inside. Anyone know what that is?

 

I think you are talking about the encoder motor. It is kind of common problem. Also the corrosion on the TCCM is pretty common too. The wires were so corroded on mine that they were starting to pull out from the plug.

 

Corrosion can definitely stop it from working. Clean the contacts very well and see if that helps. Coat them with some dielectric grease to keep the corrosion at bay.

Where is this "black cylinder thing"?

 

The "black cylinder" is right under the middle of the truck. It looks like it is part of the drive shaft. I am going to hop some pictures of it up.

 

Check the vacuum solenoid that is on top of the distributor, it will have one vacuum line going in and another going out, it will also have a electrical plug on it. One line should always be sucking air, once you hit 4wd it will signal the solenoid to open up and it will let the vacuum through to the other hose. Then that house will contract the vacuum diaphragm under the battery tray which in turn will pull a wire on the differential and lock the 4wd, which in turns triggers a switch that will signal the computer that 4wd is engaged.
So the solenoid could be bad like in my case and is not opening up or just not closing. You can test the electrical connection to it by hooking a bulb to it or a meter and once you hit 4wd on your front panel it should light up and stay lit. To fix the solenoid you can buy one or just beat up a bit.This worked for me.
Also check for the diaphragm under the battery tray to be holding vacuum and for the wire that it pulls to be getting stuck. Hopefully that helps.

 

Mine had the same noise coming from underneath, and it would only make the noise if 4wd low, with 4wd high it would not make a noise. And it turned out to be the solenoid that I talked about earlier.

 

So do the hoses come off? I can't seem to take the hoses off, to check if both hoses were sucking.

 

Yeah they come right off. They are probably dry rotted but they do come off. Testing this will really point you in the right direction. It is round and half of the solenoid will be like plastic black color and the bottom half is just like metal. There will also be like a small orifice on top of it and nothing goes there.
To test the diaphragm under the battery tray squeeze it with you hand and plug the side of the hose from the solenoid and see if that holds the vacuum. This will let you know whether the diaphragm or the hose is bad. Then you can move to whether the cable that it pulls is stuck. Let me know what you find out.

 

Thanks for your help, this is what i have so far.

The solenoid on top seems to be working; the hose on the bottom is sucking while the hose on top is getting suction. So to test the diaphragm, do you have the battery some where else or hook it up to another car, because i might be missing something on how to check the diaphragm if it is under the battery tray.

Also i just noticed some corrosion on the TCCM wire part, in the orange box. Can you take those out and clean them? There are a couple that have stuff on the top and bottom (orange, grey/black and a purple one).

Sorry i seem like a total newbie. Old cars and trucks are easier to work on, not as much electrical stuff.

 

if you wanna bypass having to mess with the vacuum switch on the the firewall, and bypass the vacuum actuator under the battery you should really consider this setup. I did it to my blazer and mounted it inside by my left knee.
http://diamond.site90.com/html/actuator.html