I have a 98 model with 145k that I recently purchased, knowing that the 4x4 was not going into the 4 high mode. I changed the 3 button switch on the dash, no help. The truck would flash and go into 4 low, but in 4 high it would flash and go back to 2 high. I put in 2 low with the back end jacked up to see if it would try to pull itself off the jack with the front wheels, which it did. I checked the TCCM and the connections looked ok, it did click as it went into 4 low. I pulled the battery out, and found that whenever the motor was running the vacuum dash pot was pulled in, therefore keeping the right front wheel locked in, by way of the cable. I proceeded to check the vacuum switch on top of the TC, which was stuck open as I suspected. I have no idea how long it had been that way, but no wonder the gas mileage was poor. I changed it, and verified that there was vacuum going to the switch. After cranking it back up, the dashpot under the battery was in the normal "unlocked" position. When I tried the 4x4, it did what it had done earlier and went into 4 low only. The dashpot never pulled in. I am assuming that even though the TC is in the low position that the right front wheel is not engaged since the dashpot is not pulled in, however I have not checked this again.
I am assuming that the vacuum switch is not being actuated, therefore the dashpot cannot lock in the right front wheel. Can anyone give me some guidance on where to go next, or do I just need to be happy with 2x4? Thanks rabbb

 

As an update the TCCM does nothing when I try to select 4 Hi, and the lights on the selector switch does nothing.

 

do a search for 4x4 diagnostics
swartlkk did a great write up on this system and troubleshooting it.

 

Thanks, I will!

 

I did not find any TCCM codes. I pulled the electrical connections to the TC and they looked fine. I was trying to move between 2H/2L/4L and like before the TC would not engage in 4H. After a few times I got it stuck in 4L. I started to pull the encoder but looked at the posts first, and saw that someone was blowing the ATC fuse. Mine was blown and the second one also, so I assume I need to pull the encoder??? BTW the TC is stuck in 4L but I am only turning the back wheels, still not pulling in the fronts after I changed the Vac switch. I now have all four off the ground so I know for sure.

 

You are chasing two problems. One that is on the electrical circuit that is causing the fuse to blow and the other is with the vacuum system that engages the front axle.

On the electrical problem, once the encoder motor stops responding, test the resistance across the two larger pins in the encoder motor connector. If it measures as a really high resistance, the motor itself is shot and it needs to be replaced.

On the vacuum problem, check for vacuum with the engine running at the port on the t-case that runs back up to the engine. It should test near the same as when you have a vacuum gauge directly connected to the engine. If it doesn't, you have a leak in the lines between the t-case & the engine. If it reads close, then move to the vacuum actuator under the battery and test there. Again, it should be within a few in-hg of vacuum as compared with the engine manifold vacuum when in 4HI or 4LO. A simple vacuum gauge can make diagnosing the 4wd system much easier.

If you have proper vacuum at the actuator, but the actuator still isn't moving, inspect the actuator for tears in the diaphragm. If that is still sealed & pulls in, but doesn't move the cable, check to see if the cable moves by hand. It should be somewhat easy to move by hand and should move at least 1/2" (~3/4" of movement is typical).

 

Ok thanks, I will get back on it tomorrow night. I will have to look at the connector for the encoder again, I remember 7 pins but I will have to look for the bigger two. So the encoder should function if I took it off the TC but left it plugged in?
The vacuum was operating and pulling in the actuator just fine when the vacuum switch was stuck, then when I installed the new one it would not function correctly.
I take it that there is an arm of some sort that makes the vacuum switch work(pushes it up) when the encoder engages the TC?

 

i had this same problem with a 4 button system. all that it was is a faulty dash switch. put in a new switch and the weird "pushing 4hi but going into 4lo" problem went away.

 

Thanks, I failed to mention that that was the first thing I tried, actually one used , then one new. No such luck for me, I am glad for you, keep truckin!

 

Be careful operating the encoder motor off of the transfer case. It can over-run and cause problems with the encoder portion of the motor assembly. The 3-button NV233 encoder motor can be bench tested off the truck where you can watch the rotation and kill power before it over runs. I posted quite a few pictures of what it looks like inside of one of these things awhile ago.

The NV236 (4-button) encoder motor has a brake assembly to lock the sector shaft at specific positions and is much more difficult to bench test.