Ok guys, I've been racking my brain trying to figure this problem out and I cannot afford to keep throwing money at this.

My 02 4 door Blazer (auto, 3 button [NP1] 4x4) has been giving me grief.) I previously had an issue with the HVAC blend door actuator not working correctly, and was advised to swap the vacuum actuator switch (Dorman P/N 600-500) since it was sucking fluid up to the vacuum reservoir.

After doing so this saturday, I also wondered if it would fix the issue of not being able to go into/come out of 4wd (4hi or 4lo). Well, the problem erratically persists.

Before I touched anything, I was having problems with the Blazer sticking in 4wd. It would eventually come out, though. Saturday morning, however, it was still stuck in 4Hi from the day before. I tried backing up, lo and high speed shifting, turning and shifting, etc. Nothing would let the car come out of 4wd. (It had been stuck in 4HI this whole time)

I looked at several things, such as the front diff vacuum actuator, the vacuum lines, the TCCM, and the TC motor. I came to the conclusion that the motor was to blame as it would attempt to shift, but couldn't.

I thought it's either a weak motor, or stuck T-case. (I should mention here that while i was unable to go back into 2Hi, I was able ro go from 4hi to 4lo.) I also reasoned that since this is a low mileage california blazer (79000 miles) its got to be a case of sticking components, or bad motor. I ended up following this video as it was (at the time) EXACTLY what was happening

https://youtu.be/mkrSMKA_EBc

I replaced the motor and much to my chagrin, i had the EXACT same problems. 125 bucks wasted.

Now, after having installed this motor and trying a few times to reset the computer (unplugging the battery five 5 min) I was able to shift from 4Hi, to 2Hi!

I was quite happy, but my celebration was too soon as I then attempted to shift BACK to 4Hi, but was unable to do so. I could only shift from 2Hi to 4lo. I can hear the motor kicking the tcase between those two gears, but it wont stop on 4hi.

After messing with this thing for an hour or so, I end up with one of two problems:

1.) Either the Blazer is stuck in 4HI and will only shift between 4HI and 4LO

Or

2.) I will only be able to shift between 2HI and 4LO, and will not shift/stop at 4HI.

Im starting to suspect a bad TCCM, but I cant afford to throw money at this thing. I need to know what my next step should be. I am thinking about going to the junkyard tomorrow after work and grabbing a 4wd dash switch, and an TCCM. Should i grab anything else while im out there? Is there a test procedure i can try?

 

How are you determining if it is in 4WD or not? 2 separate things need to happen to engage 4WD.. 1) Transfer case goes into 4WD. 2) Front Axle locks.

The vacuum actuator switch only deals with locking in the front axle once the transfer case engages.

When entering a 4WD HI from 2WD with the vehicle going down the road, does the status light in the 4WD button blink and then return to 2WD after a period of time? Have you inspected the TCCM connectors for corrosion?

If you want to test the 4WD switch assembly, there are specs and instructions in the self-help part of this web site.

 

When selecting 4HI, whether rolling at 5mph or standing still, pushing the button would send signal down to the T-case motor where I would hear it attempt to shift, but was unsuccessful in actually shifting it. The 4HI light would blink for 1/2 a second, then go back to either 4LO or 2HI.

I will tentatively update this thread with the following information:

I just came back from Pull And Save, and brough with me two TCCM's. One from an 02 and one from an 04. After taking the 04 apart, and making sure there was no evidence of either burnt or singed soldering on the board, i installed it and Voilá! I am able to shift between all three positions.

I checked for corrosion at the terminal pins on the original TCCM, but there was none to be found. Even the board looks ok, but for some reason, it looks like it went bad.

 

Glad to hear it is fixed.