Hey Blazer lovers! I have a real stumper. First, let me say I have spent the better part of two weeks digging through old posts and I have gleaned quite a bit of information regarding the 4wd system of my 2001 Blazer ZR2, my particular machine came equipped with an NV236 transfer case 4 button selectable Auto4wd, 4hi, 4lo, and 2hi.

The issue I have has me stumped, my Blazer won’t go into either Auto4wd, 4hi or 4lo (I know you’ve heard this all before) but the indicator lights on the dash switch indicate that I should be. I push the Auto4 button it flashes a few times and then illuminates solidly, the same happens for 4hi, 4lo flashes but never stays lit and goes back to the previous mode choice. I have verified I’m not in 4wd by backing into my ice-covered driveway and having my daughter film me spinning my back tires and the fronts only move fwd with the momentum of the car while in 4hi &/or Auto4. So my assumption is that 4wd isn’t engaging.

Here’s where it gets interesting (aggravating/confusing), I have replaced all the vacuum lines in the engine compartment, I have inspected very closely the vacuum actuator (bellows) it appears pristine, I have replaced (whether they were bad or not) the vacuum actuator switch attached to the middle of the firewall above the distributor and the electronic switch on the actuator cable where it attaches to the front diff. I’ve cleaned and greased the TCCM harness (it looked great but I figured...why not). In doing all of the aforementioned I had the Blazer up on stands all 4 wheels hanging free I started her up, hit the Auto4 button and heard the magical clunk of the front diff. engaging. I switched back to 2wd and the front diff. disengaged and the front wheels start slowing (I thought hooray). Unfortunately, when I dropped her to the ground and pulled her back onto the ice with 4hi still engaged and had not switched it since all 4 wheels were spinning while on stands, I still spun only the rear wheels on the ice. The fact that she won't go into 4lo is a symptom that I have no clue about.

So Blazer lovers help me out, is there a chance that when she’s hanging in the air the angle of the axles changes slightly and the shift fork fully seats; but when she’s on the ground and the angle changes back the shift fork doesn’t move far enough to engage the front diff? Perhaps after 230,000 miles, it’s time for a front diff. overhaul, there is a very light tapping coming from there. Should I consider replacing the actuator cable even though it appears to be moving the required amount? I’m totally confused about what is going on. Please help!!!

BTWswartlkk I really want your input on this

 

If the t case is engaging, then I would think the problem is in the disconnect in the passenger tube.

Maybe it's possible the gear coupling is not engaging fully? With no load, it might be enough to turn the wheel, but with the weight of the truck, it slips.

To inspect the coupling, you'll have to take the passenger knuckle off, remove the cv, and remove the tube from the diff

 

I will put that on the research list. Any chance you happen to know what that group of parts is? I'm looking for it online but can't find it. Part numbers?

 

Are there better versions of this out there than Dorman?

 

Look on gm parts direct, they will have exploded parts diagrams, with part numbers. You can either buy from them (a little expensive in my experience) or use that part number to search eBay, Amazon, rock auto, etc.

 

I would connect a hand vacuum pump to the vacuum line that goes to the actuator under the battery tray that pulls on the cable that engages the front axle. If it holds vacuum it is good, if it leaks down, it is bad.

There is also the situation in which the coupling gear on the front axle is worn so that the passenger side front axle is not engaging with the front differential. If this is the case, then like if the vacuum actuator that pulls on the front axle cable is bad, only one side of the front differential is connected to a wheel, the drivers side, and the passenger side is disconnected with the passenger side spinning. This happens because the front differential is an "open" type and with the drivers side wheel on the ground, the output on the passenger side will just spin freely.

 

Keep the info coming!