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Discussion Starter #1
Hi All

Trying to figure out my 4wd issues and could really use some help. I’ve been searching the forum pretty extensively but to no avail.
Here’s the skinny:
2003 envoy xl slt
When I turn the switch out of 2wd to auto or 4hi, the encoder motor hums for maybe 5 seconds, stops, and hums again for another 5 seconds then stops again. It sounds like the motor is trying to rotate the shift shaft but can’t. After doing this back and forth a few times, the motor gets warm. The lights on the selector switch flash in the selection I set it to but then reverts back to 2wd. I never hear the normal sounds from the transfer case when the gearing is engaged.
So I took the motor off and tried rotating the shaft manually but can’t get it to switch to 4hi or 4lo (I can get it to neutral but that’s it). I have a pretty good idea how it should feel from working on my Avalanche TC when I replaced that motor. If I rotate the front drive shaft back and forth while turning the spline shaft, I can feel where it starts to grab but it never engages and goes into gear.
I tried the selector switch while the truck was in motion as well with the same result.

Thanks in advance for any advice
 

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2002 chevy trailblazer_lt
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So I'm assuming you already know to get from 2Hi to 4Lo you would turn the shaft CCW and about 45 degrees is the most it would take to be in 4Lo. Maybe less. I will confess I have never done that but I have removed a position sensor from the shift motor and bench tested, setting the various positions by the voltages they return to the TCCM. That's where I get that 45 degrees from....

It just occured to me while proofreading this post you mentioned rotating the front driveshaft while trying to shift it manually. At that point is the front driveshaft even connected to anything in the transfer case, or are you just spinning some clutch discs with no application pressure on them. Maybe you would need to turn the rear driveshaft.

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And not having actually done it I suspect getting to
4Hi or 4lo could take quite a bit of strength if it is possible at all. The shift motors have a boatload of gear reduction in them. I suspect it is for good reason. A4WD is pretty close to the 2Hi position but does you no good to be there anyway as without a working shift motor there will be no engagement when slippage is detected as the shift motor is required for that.

Have you tested the brake circuit in the shift motor? If the brake is not releasing that in itself might stop the motor from shifting, but I would expect it to set a DTC for that.
 

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Discussion Starter #3
Thanks TJ. When I rotate the front drive shaft, the rear rotates just a little, until the backlash is taken up in the drive line. I haven’t tried it yet with the rear wheels off the ground in neutral, but having tried to engage it while in drive and rolling, I suspect the result will be the same.
I haven’t tested the brake circuit, I’ll look for a way to test that.
Is it safe to remove the motor and move the selector switch? That might tell me if the brake circuit is working correctly.
On my avalanche, it didn’t take much force to rotate the shaft to the other positions so I suspect this would be similar.
The service 4wd light is off btw
For a4wd, doesn’t it still engage the TC but disconnect the front axles via the axle disconnect by the front right wheel?
 

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For a4wd, doesn’t it still engage the TC but disconnect the front axles via the axle disconnect by the front right wheel?
A4WD works like this, the front axle disconnect locks and remains there. The shift motor moves to a position that applies a very slight amount of pressure on the clutch pack that drives the front driveshaft. The TCCM monitors the rear and front wheel speeds watching for slippage seen by a faster speed of the rears vs the fronts. In the event of slippage the shift motor applies more force to the clutch pack, sending more torque to the front wheels. When the system detects no slippage it releases the pressure on the the clutch pack. It is generally observed that this process is not gentle. Engages with a bang which is why it is widely recommended to avoid the A4WD (auto) setting entirely
 

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Is it safe to remove the motor and move the selector switch?
I wouldn't do that. There is a magazine article that descibes using two 9 volt batteries to position a shift motor for installation. One for the brake circuit and a second to energize the motor. I would likely do this instead. Let me see if I can link the article...look around pages 24,25,26. The say there is a test part harness but with a wiring diagram I'm sure it can be done without the special harness by using some jumpers.

 

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2004 gmc envoy_sle
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I was able to unlock the tc brake with a 9v batt but it had trouble turning the motor. Needed a 12v motorcycle batt for that. I was able to turn tc shaft to 4lo, then cw to neutral, 2Hi, then "something." Neither A4WD nor 4Hi are "detent" positions, feels more like it's spring loaded. Try turning tc shaft with all 4 wheels off the ground, trans in neutral, may have to turn rear driveshaft.
 
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