Hello again, everyone. As is always the case when I'm posting, this is about 4x4 issues which I've had for the past two years (and been able to work around, thanks to this forum, until now).

Background: Got the Blazer in 2008 and the 4WD system worked intermittently (this is the auto transfer case, by the way). First replaced the selector unit, then the vacuum switch on the firewall (as recommended by a dealership), to no avail. I was able to force 4WD to work by selecting 4WD and splicing the vacuum lines together (bypassing the vacuum switch), so that told me two things: first, the vacuum actuator, a common culprit, was not the problem; second, the problem was the switch not opening. I tested the new and old switches with a battery and confirmed that they do, in fact, open. Rather than deal with TCCM or wiring issues, I simply spliced the vacuum lines every time I wanted 4WD and returned them to their original state otherwise (a hassle involving popping the hood every time I needed 4WD). This worked fine for the past year.

Today we had our first real snow, so I went to do this method for the first time since last March. The car drove HORRIBLY. The steering wheel shook, there was considerable resistance in the driveline, etc. So I disabled 4WD and dealt with the conditions.

Just now, I put the car up on jack stands, did my 4WD jerry-rig, and observed all four wheels turning just fine. Took the car off the jack stands and the problem remains. I disabled the 4WD switch but left the vacuum splice (essentially, left the front differential active, but discontinued sending power to it) and it ran smoothly again.

So the question is this: Is my front differential blown or can there be another explanation? (I'd have confidence that it's blown if not for the fact that it works fine when active but not receiving power - I just don't know enough about differentials to know if that would be expected.)

I'm honestly getting tired of doing everything under the sun, short of getting raped at a stealership, to make this 4WD system do what it's supposed to, so this will honestly be the last time I ever post about 4WD issues. If I can't get them resolved permanently, I'm getting rid of the Blazer.

As usual, thanks in advance for any assistance.

 

I'm assuming all of your tires are the same size? You don't have bad u joints in the front shaft do you? Does it get worse if you turn the wheel while it's locked in?

 

Your initial problem could have been caused by corrosion at the TCCM connection.

I would also wonder if all of the tires are the same rolling diameter. If nothing has changed, tire wise, since the last time it worked ok, then the size is likely correct. But are they all aired up the same amount?

 

Tires are all OEM specification, and I'm meticulous about the care of my vehicles, so the air pressure is checked regularly.

Regarding it getting worse if I turn the wheel with 4WD engaged, I couldn't really tell you - I made one left turn with 4WD engaged before I decided I didn't like how it was feeling and shut it off. It felt like if I had continued, something would've gotten much worse.

I spoke with a local guy who's familiar with GM drivelines and he actually suspects U-joints, as well. Since the harshness doesn't occur unless I send power to the front differential, he thinks it may be the driveshaft connecting the transfer case to the front differential. I'm taking it to him next weekend to put it on his lift and take a look. It would be fantastic news for me if that's the problem.

As for corrosion on the connector, it's obviously possible, but I doubt it. Last I inspected it, it looked fine. But I'll be taking a closer look if I get the other issue resolved, as I had intended in wiring the vacuum switch to a manual toggle switch so I would no longer have to exit the car to engage 4WD.

I'll keep you updated. Thank you very much for the insight!