Truck stuck in A4WD - but actually it is 2HI need help

esbob

Original poster
Member
Oct 14, 2012
29
Background info:

2005 TrailBlazer LT with 4.2 i6, G80, 3.73's

Bought with 99k miles on it. 4 wheel drive switch worked, and 4 wheel drive did as well. Tested 4HI and 4LO worked awesome. At 100k miles I had the front and rear end fluids changed, and the transfer case drained and filled with GM AutoTrak II fluid. 2 days ago we had some crappy weather and my wife had the TB, I told her to put it in A4WD and leave it there. She did.

So today I got in, and turned the switch back to 2HI and the light for 2HI flashes a few times, then turns off and stays on A4WD. Does this no matter what selection I put it to. The service 4WD system light is NOT ON. I remember the last time I tested the 4WD system I could distinctly hear the two different sounds under the truck. This time there is just a very dull low noise under the truck like a motor trying to do something but not suceeding. The truck IS indeed stuck in 2HI anyway.

I did a lot of searching on this forum and found some useful information. I pulled the fuse under hood for TCCM for 30 seconds and replaced, pulled fuse under seat and put it back. Nothing. The neutral light on the TC switch does light up when I first start the truck if that means anything. I just had the truck at the dealer last week for them to program the new LGM.

Any help is appreciated, and snow is on the way. :sadcry:
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
Well, my advice for years has been to not use the A4WD mode except for very specific semi-icy conditions. If it's ice and snow enough to use 4HI, commit to 4HI and use it. If it's just wet, 2HI is fine.

That said, this issue probably has nothing to do with A4WD mode. The two noises you should hear are a high pitched <1 second whine during the 2HI->A4WD transition and vice versa, and the low pitched one during ANY mode transition. The high pitched one is the front axle splined disconnect actuator, and that one might be dead or frozen. Has it been cold? Two bolts and you can take that actuator off the disconnect assembly, and see if it works in mid-air or not.

If you're in A4WD mode and go to 4HI, what do the lights do and what noises do you hear?

If there's no wheel slippage, A4WD is indistinguishable from 2HI in normal driving. On a lift, you can see what the splined disconnect is doing.
 

esbob

Original poster
Member
Oct 14, 2012
29
I was hoping you'd respond! :smile:

I did not know this about the A4WD, that is my bad. Anytime previously when I switched from mode to mode I would hear both distinct noises you speak of, one from front of vehicle and one from under. Now when I switch to anything else there is just a very quiet click and idk which place it is coming from. The indicator light will flash on whatever selection I turn it to, for about 10-15 seconds and then the A4WD light will be lit again.

It HAS been colder, 35 or so day 20 or so at night.

There is snow on the roads and if I roll into the snow in 1st and give it gas the rear end kicks out bad and just spins, which is why I'm assuming just the rear is driving. It will spin like nuts, and I go back and you can just see the spinning in the snow from rear tires.
 

RayVoy

Member
Nov 20, 2011
939
esbob said:
There is snow on the roads and if I roll into the snow in 1st and give it gas the rear end kicks out bad and just spins,
If you move the selector into 4HI, do you get the expected 4x4 results, or does it act like your above description?
 

esbob

Original poster
Member
Oct 14, 2012
29
Same results. If I put the selector to 4HI, it will flash on 4HI for a 10 seconds or so, then it gives up. The selector will be pointing to 4HI but the lights indicate A4WD and the front is definetely not driving.
 

TangoBravo

Member
Dec 5, 2011
208
There are far more experianced people on here then I but, what Im thinkin is possible is while your wife was out driving around with it in A4WD she was also slipping the rear tires enough to engauge the system maybe many times in one day, maybe even engauging it hard enough to finally break something and now your stuck. My advise remove the axle and check your disconnect and motor.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Your splined disconnect may be full of dirty and congealed grease, preventing the engagement fork from moving. This can feed back to the actuator, which may also be full of congealed grease. Your symptoms are consistent with an actuator that isn't moving when commanded. Take a hair dryer to the actuator and see if it will free up. Listen down there as somebody else moves the mode switch. You can do this with the ignition on and the engine off. But normal driving should get the opil pan warm enough to get the splined disconnect moving, but it may not warm up the actuator at 20 degrees. Do you have access to a lift or a heated garage? How much are you wanting to dig around in there yourself, or are you going to find an experienced independent mechanic?

The front axle actuator is powered by a different fuse from the TCCM - the rear seat fuse 48. It also runs the "N" mode switch light, which is why seeing the red N at start-up is a good test of that fuse. But the power also snakes through a jumper in the front fuse block before going to the actuator. The wiring to the actuator could also be damaged or the ground is bad. Time to get cold and dirty, it seems - or pay somebody who isn't a goof around trailblazers. Or drive to San Diego with some beers and I'll help you fix it. :wootwoot:

ADDED: Upon reflection, and reading TangoBravo's post, the transfer case encoder motor may be at fault, too. The transition from A4WD to 4HI involves ONLY moving the encoder motor. If it fails to move, you'd also get those indications. I'd pull the encoder motor and try to turn the transfer case control shaft manually. If it doesn't move easily, then the TC is buggered internally. If it moves, then try to run the system with the encoder motor unmounted from the TC and see if it moves easily. It could be broken/jammed planetary gears inside the encoder motor.

A tech with a GM Tech II tool can command movement of the encoder motor and the actuator independently, and diagnose it quickly.
 

RayVoy

Member
Nov 20, 2011
939
the roadie said:
ADDED: Upon reflection, the transfer case encoder motor may be at fault, too. The transition from A4WD to 4HI involves ONLY moving the encoder motor.
That's what I was thinking when I asked what happens with the change to 4HI.
RayVoy said:
If you move the selector into 4HI, do you get the expected 4x4 results, or does it act like your above description?
The problem sounds like a jamming/not responding encoder.
 

esbob

Original poster
Member
Oct 14, 2012
29
Thanks guys for your help, it is much appreciated.

I can probably park it inside at work tomorrow. I was thinking about that to see if it might free things up. When I turn the switch I can hear a click, something trying to move and the voltmeter moves from a little above 14 volts to a little under 14 volts. That will happen like 3 times and it gives up. If I take out the encoder motor to check it for functionality and I move the shaft on the TC by hand does it have to be turned back to a certain spot or will the motor figure that out on its own?

Another question or two. If the system was functioning properly, does it go back to 2HI when you shut the truck off? What I mean by this is, say I'm driving around in 4HI and I get somewhere, shut the truck off. Does it go into 2HI before shutting off, then go back to 4HI when restarting the truck or does it stay in 4HI the whole time?

Next question, does the TCCM know if the encoder motor isn't moving and thus not doing anything with the front actuator? Right now all I have is a slight click right under the truck.

Thanks again for everything guys. I'm going to try and do as much as I can myself, I'm fairly confident I can handle it with the help of you folks.
 

RayVoy

Member
Nov 20, 2011
939
esbob said:
If the system was functioning properly, does it go back to 2HI when you shut the truck off? What I mean by this is, say I'm driving around in 4HI and I get somewhere, shut the truck off. Does it go into 2HI before shutting off, then go back to 4HI when restarting the truck or does it stay in 4HI the whole time?
It will stay where ever the switch is sitting. A few times I have turned the truck off with it parked and still in 4HI, not wanting to restart, I have moved the switch to 2WD. The next morning, it will go to 2Wd on it's own, but it always seems to take a little longer than normal, probably a re-sync thing.


esbob said:
If I take out the encoder motor to check it for functionality and I move the shaft on the TC by hand does it have to be turned back to a certain spot or will the motor figure that out on its own?
It should be in 2HI to change, not sure about the answer to your question.

esbob said:
Next question, does the TCCM know if the encoder motor isn't moving and thus not doing anything with the front actuator? Right now all I have is a slight click right under the truck.
The TCCM commands the encoder motor into position. When in position, the TCCM commands the front actuator into engagement, when engaged, it returns a signal. If in 4x4 (HI, or LOW), and the TCCM has received all required signals, the TCCM commands the 4x4 clutch to compress, sending power forward. If in AWD, the clutch is only compressed when wheel slipeage is detected.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
What might also be happening is the encoder motor has a brake/locking solenoid that has to be activated to allow the motor to move. If the wiring to the brake is compromised, the encoder motor won't move even when commanded. That could also cause the symptoms. I keep forgetting the brake because I've never had one fail.

And the AWD systems with the NP126 transfer case (essentially full-time A4WD mode) have no brake because the encoder motor never goes into a locked-up 4HI mode, and there isn't a LO range. Having a safety brake on the encoder motor prevents the transfer case from jumping from HI to LO or vice versa with a wiring short on the motor power section. That would be catastrophic.
 

esbob

Original poster
Member
Oct 14, 2012
29
So tomorrow I'm thinking I'll pull the TB into my shop and pull the encoder motor to see what happens while someone else turns the switch. Then I can try to turn the shaft by hand. Does it matter where I turn the shaft or leave it provided I am able to turn it? I don't want to turn the shaft to 4HI when the encoder motor is on 2HI or something like that.
 

esbob

Original poster
Member
Oct 14, 2012
29
Very helpful, I saved that one. Based on what I read, the encoder motor knows what position it is based on feedback from the transfer case. So I can't screw up changing that motor is what I'm getting at right?
 

esbob

Original poster
Member
Oct 14, 2012
29
So this morning I dropped the encoder motor and tried to operate it while out of the t-case, all it did was click. Got a new one and put it in the t-case, torqued it down and it works! I'm so stoked to have the functionality back, especially since there is snow on the ground and some roads here suck.

It wasn't a bad job at all changing it, except for my skid plate adding a little extra time to the job. Thanks guys for the support!
 

esbob

Original poster
Member
Oct 14, 2012
29
PM sent.
 

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