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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > 4x4 Drivetrain

4x4 Drivetrain 4x4 Repairs & Problems

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  #1  
Old 02-11-2011, 05:59 PM
JohnnyBra JohnnyBra is offline
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2006 Chevy TrailBlazer LT EXT
Imperial Blue Metallic 4.2L I6 4X4
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Exclamation Stuck in 4hi - Mother In Law broke Wifes Trailblazer

Mother-in-law was watching our kids today as we had to do some errands. So along with the kids, she got the keys to my wifes 2006 Trailblazer EXT LT (equipped with all the kids car seats).

She had to use 4hi today to get up our driveway after taking the kids to the bus stop. When leaving our house for hers, she tried shifting back to 2hi... The 2hi light flashed a few seconds and then 4hi. She called me about the issue, but said she just left the switch in 2hi and eventually by the time she got to her house, it switched.


When I went to get the car (and kids) back from her, I of course had to try the issue myself. I switched to 4hi without issue doing about 20mph.. Then I tried going back to 2hi. The light flashes for about 4-5 seconds, then 4hi lights back up. I've tried this rolling, stopped, in park, in neutral, in reverse, rolling slowly in gear.. No difference. 4hi stays lit up and is definitely engaged as I can feel the wheels pushing/pulling in curves.

I even tried shifting to 4lo in neutral and it does the same thing.. the 4lo flashes a few seconds, then 4hi lights back up. Putting it into auto, it flashes about 5 seconds and then Auto stays lit... but you can tell the front wheels are still engaged in 4hi. Switching now from Auto to 2hi, the 2hi light flashes a few seconds, then 4hi lights back up >_<


Doing this in the driveway, in park with the engine running, the moment I switch from 4hi to 2hi I can hear a soft click from what sounds like the transfer case, 3 times.. the 3rd click the 4hi light turns back on solid.

Have been searching the forum but haven't found anything quite like this. Now her car sits in the driveway with 4hi stuck engaged and I'm a little confused about what to try next. It's a bit muddy from the bad weather, so I'm thinking I'll try a car wash to make sure there's no mud in the system. Saw many posts about Transfer Case oil maintenance, and I must admit she has 62k miles and I've never changed the oil (had 30k miles when we bought the vehicle so pretty sure no one else has done it.) I'll go ahead and get fresh transfer oil so I can do that, but at a bit of a loss where else to look from here. Not sure if this model has a encoder motor or vacuum actuator or what, as first look underneath I see a big plastic skidpan in the way of everything.
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  #2  
Old 02-11-2011, 06:34 PM
TechnilubeCom TechnilubeCom is offline
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I'm curious about this too. This sounds very much like what happens to my 02, although eventually with some playing or coming back the next day and starting in 2WD mode it does seem to go back. I flashed the computer per one of the suggestions here that the 02 had programming issues and would drop into sleep mode. Also changed all the fluids. I can hear both motors engage, when it works properly. And since it does work when engage, I tend to think it's not mechanical anywhere and some sensor acting up.
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  #3  
Old 02-12-2011, 02:43 PM
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the roadie the roadie is offline
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Sounds a lot like a wimpy encoder motor or a transfer case whose mode-changing shaft needs more force than the motor can give it.

Definitely start by changing the fluid, and make note of how much comes out and its color. Too little fluid coming out is a bad thing.

Our system uses no vacuum, and has an encoder motor to change modes, and a front axle actuator (also electric) to engage a splined disconnect on the passenger side of the oil pan.

More reading:

http://www.offroadtb.com/articles/te...y-of-operation

http://www.offroadtb.com/articles/te...4wd-disconnect
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Old 02-12-2011, 05:40 PM
JohnnyBra JohnnyBra is offline
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Thanks for the info The Roadie!


Took it over to my dad's today and we put it up on the ramps. Removed the encoder motor and drained the transfer case oil. With the encoder off, powered on and attempted a shift and the motor made a clicking noise but no more.

Disassembled the Encoder Motor assy and got into the planetary gear sets.. They seemed dry and a bit dirty, so added a little air tool oil and seemed to free it up a bit, but locked in either direction after about 8 turns.. We THOUGHT this was normal (learned our mistake in a bit) so reassembeled and put it back on the vehicle. Powered up and it seemed to be working again, indicator showing shift going to 2hi and sounded like it too.

So we left for the dealership for fresh Transfer Case oil... Got there at 12:05 to find out they closed at noon....... Checked Advance, Napa, no one would claim to have any substitute, so we decided to put the old oil back in until the dealership opens monday.

Old oil is dark, almost black, and doesn't quite look like 2 quarts came out. We put it back to get on the road again, backed off the ramps and indeed we were in 2hi again So to test I put to 4hi.. Heard motor rotate, clicked into 4hi. No issue. Then went to 4lo. Shifted properly again, without issue.

Shifting out of 4lo back to 2hi, it didn't make it. Indicator lights flashed for 2hi for 10 seconds or so... then went out entirely and showed a red N lit up instead >_<....

But it was in fact in 2hi. Drove back on the ramps, took off encoder again, and went deeper this time. Removed all 4 planetary sets.. discovered that the deepest 2 sets had a few teeth missing. Those teeth were embedded in the outer wall of the planetary cup.. Using a sharp pick to remove the embedded teeth pieces, brake cleaner, then fresh tool oil, got the whole planetary set together and spinning freely in both directions with no 8 turn limit till it bound up again (like we discovered it was SUPPOSED to).

Reassembled, put on the trail blazer, the motor made one slight burp and was silent, no clicking, no nothing.. From the burnt electrics smell, pretty sure it's given up the ghost now. Started the Trailblazer and we're actually in neutral now. >_<

SOOOO, then removed the rear motor cover off the encoder and was able to free-spin the motor shaft by hand until we found 2hi. Indicators still don't say it, but pulled the #8 30A fuse to make sure nothing tries to move again.


Now we have 2hi.. totally disabled 4x4, and I'm looking for a new encoder motor assembly now. HOPEFULLY that and a couple quarts of Auto-Trak II will have my wife's car back on the road next week.... For now it's usable in 2hi (although the least bit of snow on our driveway and it's useless without 4x4)
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Old 02-12-2011, 06:00 PM
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the roadie the roadie is offline
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Mr. Bra, or may I call you Johnny, ...

no seriously, congratulations on a well-thought out experimental process, ending in success and some idea about the root cause. You have NO IDEA how many members would have asked exactly how many degrees to turn the shaft to get it back into 2HI, along with needing a picture-based how-to document to make sure they didn't mess something up.



Temporarily, ATF would have worked in the transfer case just fine, especially in 2HI. Auto Trak II fluid is ATF with some friction additives needed by the clutches inside.
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Old 02-12-2011, 09:11 PM
JohnnyBra JohnnyBra is offline
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Thank you My dad has always been a backyard mechanic and drug me along for the fun. Our basic philosophy is,

"It's already broke so it's no good the way it is... Let's tear in and see if we can find out why".

Now due to all the new age digital control modules and programmable circuits, we find it a bit more useful to check google first on newer cars.

Cleaning out the chewed up planetary gear teeth freed things up enough that the mechanism could move properly again, but it appears the motor is toast after it's meal of hardened teeth.. Assuming incorrect "almost there" positioning is what was keeping the indicator lights from getting the right reading from that sensor ring (Position sensor we assumed) on the top of the Encoder motor where it attaches to the Transfer Case. Hopefully replacing the whole unit and re-enabling power via the Fuse should have things online. The spline appeared to move without much force via turning the motor shaft by hand. Wasn't too hard to find 2hi by hand, and a quick start of the engine was easy to tell the transmission was engaging (though must admit first attempt we heard grindy gears and shut down quickly to try again).



Was a used vehicle when we bought it, so no idea if it was my wife, mother-in-law, or previous owner that did it in. Assuming shifting to 4hi (or out of 4hi) at higher speeds would throw extra strain on that encoder, making it eat it's own teeth...
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  #7  
Old 02-12-2011, 09:54 PM
TechnilubeCom TechnilubeCom is offline
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A mechanic I now know suggested this unit when I asked him a short time ago, said that is usually the issue in his experience. After reading this and the troubleshooting, think when I have the money I'll just get a new one and swap. :-)
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  #8  
Old 02-19-2011, 10:20 PM
JohnnyBra JohnnyBra is offline
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2006 Chevy TrailBlazer LT EXT
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As a follow up..

New encoder motor installed. 2 quarts of fresh Auto-Trac II fluid. Replaced the #8 30a fuse, and everything's working perfectly now

Thanks folks!
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