NEED HELP Drivetrain locked up after hard stop

stanmile

Original poster
Member
Dec 31, 2012
6
2007 Trailblazer, 4.2L, 4X4, 3.43 non-locking rear end, 285K miles

This has happened twice in the last 6 months. There are no apparent bad sounds such as whining or grinding when normally driving and I haven’t used AWD or 4Hi in over a year. After making a hard stop, once for a changing traffic light and once to avoid hitting another vehicle pulling out of a parking space in a parking lot, the drivetrain completely locked up. In other words, I stabbed the brakes and then when I tried to move forward afterwards, the truck would not move forward. It felt like the trans was straining and trying to move the truck, but something in the drivetrain was locked preventing the truck from moving forward. All I had to do to unlock it was to move the shifter to reverse and then back to drive and all seemed normal. I did not let the truck roll backwards when in reverse, still had brakes applied, so this is not a brake issue. I changed the rear diff fluid about 15K ago and noticed a little slop in the spider gears but didn’t see any broken teeth in the ring or pinion. I suppose that it might be possible that the spider gears are getting jammed, but I don’t think that would cause a total lockup since the truck was pointed straight and the carrier should still have been able to turn regardless of the spider gears. There are no obvious signs of problems with the driveshaft. I haven’t opened the rear differential yet since this happened, but I was wondering if an issue in the transfer case could also cause this? I kind of hate having to start tearing things apart without knowing the general root cause. I suppose that it could also be a trans issue too, but think that’s unlikely. I have tried to intentionally recreate this in my driveway but have had no success. Anyone have any ideas or come across this type of thing before?
 

mrrsm

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Oct 22, 2015
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I'm not the expert in the 4X4 category of mechanical problems ...but the Transfer Case in your vehicle seems to be a better place to start the investigation. This GMT Nation Link might give you some additional insights and if you scope out a generic Google search with the string "Trailblazer Transfer Case Rebuilt"... you should be able to locate them for around $285.00 if you can find someone with a 100% rating on eBay.

You don't mention the mileage...Oooops...No... There it is... 285,000 Miles... or how the vehicle has been used (Towing, Off Road, Mountainous Terrain, etc...) But if they demand regular Gear Oil Changes and yours is bone dry with enough driving to reach the Moon 240,000 away and get you 45,000 Miles closer to the Earth on your return trip... imagine the Wear and Tear on that Transfer Case since 2007... You get the idea. These are the things that can go sideways with so much re-directed torque through any type of Gear Box:

https://gmtnation.com/forums/threads/2004-bravada-stuck-in-awd-permanently-fixed.17695/#post-546069

If you decide to do this work yourself... Watch this video around the time they remove the Transfer Case...for a few Tricks of the Trade to save you some aggravation... Naturally...you can ignore all of the other information about having to lift the SUV Body off of the Frame to get to the Engine and Transmission... :>)


And on behalf of Your Fellow Members and your Readers... Because these problems involve people who want very much to "cut to the chase" and get their repairs done "STAT!"... it is easy to rapidly pour out the post onto the pages ...with all of the data about what the perceived issues are in a solid "Wall of Words...". So if you think of it...Take pity on us...and put in some 1.5" to 2" Paragraph Breaks along the way... it makes it so much easier to "Eat the Elephant... One Bite at a Time"... :>)
 
Last edited:

stanmile

Original poster
Member
Dec 31, 2012
6
Thanks for the reply Mr RSM, but I'm a bit confused. I watched the video, and didn't see anything regarding the transfer case (the unit attached to the back of the trans), but rather they showed removal of the front differential. I don't think my issue is with the front differential.

I'm the 2nd owner of the TB, bought it with 50K, and have been religious about changing all fluids. I never towed with the truck and mostly flat highway miles.

Last night, I actually changed the front axle oil and transfer case fluid. Both were full, and the transfer case looked clean and blue, didn't see any metal fragments. Also, popped the rear differential cover. The spider gears in the rear axle, along with the pin, are definitely worn and need replacement, possibly along with the carrier. I have yet to remove the carrier to inspect the bearings, but I didn't see anything in the rear axle that would cause it to bind and lock up, ring and pinion look good with no excessive backlash. By the way, dealer had replaced the carrier along with bearing at 80K.

So, I'm wondering if the transfer case internals would be a feasible source of the lockup I experienced. Remember, simply shifting to reverse freed it up. Or could it be the trans? By the way, a rebuilt transfer case is about $1000. I've done quite a bit of online searching for rebuilding a TB transfer case, but not much luck. Found some information regarding other vehicles, but I think the TB unit is a bit more complex.
Thanks
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
I would say to try and find where your lockup is located, Securely set front and rear up on jack stands and disconnect the driveshaft. Put the tranny in neutral and see what will not spin, the diff or driveshaft. If it's the diff, you're done. If it's the driveshaft, try making the transfer case shift to neutral by turning and holding the switch past 4lo. It should definitely spin. If not, then i'd say it's the transfer case. It it now spins, then it's the tranny.
 

stanmile

Original poster
Member
Dec 31, 2012
6
Thanks Mooseman, but unfortunately it's only locked up on me twice in the last 6 months while I was on the road and I can't recreate it.
 

Reprise

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Jul 22, 2015
2,724
The shop manual will have the procedure for rebuilding it (and I've seen articles online on doing it.) It's a little involved, but when I look at the rebuild kit on Amazon, it doesn't have a million parts. Pretty sure you'll need a bearing press, though.

If you'd rather replace than rebuild, you can get one way cheaper than $1000 from a JY, pick-n-pull, eBay, etc. This is a time where that 3.42 helps you, because that was the most commonly installed gear, especially as the run went on. On the TC, there should be a metal plate with a couple of things stamped on it. At the bottom is the gear ratio - since your rear is 3.42, the TC has to match.

BTW, this is the option I'd use, as the rebuild kit looks to cost $160-180 or so; you can get a used case for $200-$350 or so. For the extra $, I'll go with the plug-n-play option and save a lot of frustration teaching myself how to rebuild it)


The other thing on that plate will be the mfg / model #. It's most likely a NP226 (NP stands for 'New Process', a company that manufactured xfr cases for GM and Mopar; they're in most of GM's truck lines (or at least they were back in the '90's, when the 360s were being sold).
 

Bow_Tied

Member
Dec 21, 2014
453
London, ON
I know you don't think it is brakes, but that is what it sounds like to me. A line collapsing internally trapping the pressure for a very short time until it bleeds off. JMO; good luck with the troubleshooting.
 

stanmile

Original poster
Member
Dec 31, 2012
6
I know you don't think it is brakes, but that is what it sounds like to me. A line collapsing internally trapping the pressure for a very short time until it bleeds off. JMO; good luck with the troubleshooting.
 

stanmile

Original poster
Member
Dec 31, 2012
6
Yeah, it sure does sound like a brake issue...you've actually got me second guessing myself. That was the first thing that I thought the first time it locked up, 2nd thought was tranny. I know it only happened twice, but it didn't feel like the brakes were holding it back. The first time it happened, I was in a bit of a panic mode and it took me at least 30 seconds before I cycled it through the gears thinking it was the trans. When I put it back in drive, it was freed up. The 2nd time it happened, I immediately put it in reverse, back to drive, and I was free. Kind of odd.

I think I'm going to rebuild the rear axle and then drive it for a while, otherwise I'm going to be chasing my tail on this one. Don't want to just drop in another transfer case without knowing for sure.
 

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