2000 blazer 4 dr, 140k, 4 button nv236 tc.
Started this morning, and it was in 4lo. I was hunting last night, and figured I might have bumped it when I was getting out of the truck. Nope. TC wont shift out of 4lo. It will go into 4hi, or auto for a few seconds, then the 4 lo light blinks and it goes right back to 4 lo. Cant get the TC to go into neutral, either. Removed tccm, cleaned off a little bit of corrosion, shined up the contactors, didn't help. Disconnected battery for 30 minutes, no help. After trying to shift a couple of times after starting, it just quits trying, it seems. No clicking from tccm, no clunk from the tc, it just doesn't do anything unless I turn it off and restart. Truck is in neutral, of course, when attempting this.

There is no S4wd light, either.

I removed the dash switch and tried to ohm it out, but could not get ANY resistance from any of the pins. (although I could have been doing it wrong?)

Does anyone have...

A- pictures of the back of their 4 button switch, and what I should be reading and from where?

or B-any possible other idea of what Could be causing this?

Just seems odd that I can hear the tccm, and the TC TRYING to change range/modes.

Thanks all, I really hope I can sort this out soon. In the mean time, I'm driving a Miata, fearing the eventual snowfall in WI.

 

*moved to steering, suspension and drivetrain*

 

why I can't get ANY readings out of the switch? there has to be SOME continuity (ie-resistance...) SOMEWHERE in that switch, right?

I've got a new one coming tonight that I'll compare it to, and post results.

 

update: was tinkering and looking around tonight at the truck. Confirmed that the vacuum solenoid works, (duh, as it's stuck in 4wd...) vacuum going in the bottom hose from engine, coming out the top going to the actuator.

Cleaned connections and applied dielectric grease on TCCM, disconnected battery, no help. I can TRY to change out of 4 low a few times after starting the truck. Tccm clicks, TC motor sounds like it shifts, and sometimes 2hi or auto will stay lit solid for a minute before dropping back on it's own to 4low.

I have one question though: with the truck off, front end off the ground, my front wheels spin free. Truck on (4lo engaged) the fronts spin opposite of each other as they should I think?

Here's the question: shouldn't the front wheels NOT spin at all if the TC is in 4low, rear parking brake on, in neutral, if all 4 wheels are supposed to then be connected? I'm not a 4wd guru, so I could be wrong. Or...am I in for a lesson on planetary gear operation...? (driveshaft stationary, hence wheels turning in opposite directions)

 

despite being unable to test the switch, i bought another one. something didnt seem right about the tc willingly changing into 4hi/auto, then dropping right back to 4hi.

why did it ALWAYS drop right back to 4low, despite successfully shifting?

sure enough, the switch did it. despite not being touched, something inside shorted, and the switch was always telling the tccm and encoder that i wanted 4 low, and it kept defaulting to 4 low.

there arent a lot of threads about this problem that include a resolution, so i hope this helps someone. a possibly expensive problem had a very cheap fix!

 

Thanks for following up for future reference. It seems this isn't all that common of a problem, glad you got it figured out!