np 126 transfer case

1978chevy4x4

Original poster
Member
Dec 30, 2012
14
Hey guys. New here to GMT. Just got a 2002 Bravada a few days ago. I had no awd, did lots of research and found I needed to change tcase fluid before I did much at all so I did. The old fluid came out blue, looked clean and free of foreign material and looked to be proper amount. Replaced with auto trac II. Prior to this I wanted to see if I needed a sensor or what mught be going on. I jacked up both ends on stands and started her up and put it in gear. The case engaged the front axle as it should (to my surprise) but when reistance was applied to the front wheels with a 2x4 they would stop. I am assuming I need to rebuild it because the cluch pack is gone. I would just like some input on if my assumption is correct and if so where can I find rebuild kit because I havent had much luck.

Thanks guys
 

jeffro312

Member
Oct 4, 2012
442
East Haven,Ct
1978chevy4x4 said:
Hey guys. New here to GMT. Just got a 2002 Bravada a few days ago. I had no awd, did lots of research and found I needed to change tcase fluid before I did much at all so I did. The old fluid came out blue, looked clean and free of foreign material and looked to be proper amount. Replaced with auto trac II. Prior to this I wanted to see if I needed a sensor or what mught be going on. I jacked up both ends on stands and started her up and put it in gear. The case engaged the front axle as it should (to my surprise) but when reistance was applied to the front wheels with a 2x4 they would stop. I am assuming I need to rebuild it because the cluch pack is gone. I would just like some input on if my assumption is correct and if so where can I find rebuild kit because I havent had much luck.

Thanks guys

Hey I'm in same boat with no awd. There may be other causes to no awd

Tccm which is module that controls transfer case could be fried

Encoder motor could be malfunctioned

Or a wire shorted somewhere.

I haven't had time to look at mine I did a donut in snow just to get the dummy light to come on. Brought it to shop and my buddy said he did t have alot of time but whatever code he got pointed to wiring issue of some sort water getting inside or a connector or wire


Still been working to much to sorry about it


Will so truck drives OK without it in few inches of snow
 

jaguarjoe

Member
Nov 22, 2012
73
For my $0.02, I'd rip out all that selector switch/pushbutton 4wd/awd crap and put in a manual T/C like the old np231 or np242. No surprises at 80mph on the highway, no erroneous blinking lights, they just did their job, that's all.
 

1978chevy4x4

Original poster
Member
Dec 30, 2012
14
jaguarjoe said:
For my $0.02, I'd rip out all that selector switch/pushbutton 4wd/awd crap and put in a manual T/C like the old np231 or np242. No surprises at 80mph on the highway, no erroneous blinking lights, they just did their job, that's all.

If I could just hook up a toggle switch and go I would. Given the 126 is a single speed tc its either in or out of gear and as I understand the cluches are applied in varying amounts depending on speed. So in theory if I could get the clutch pack situation figured out an on/off switch would work. Im sure someone has done this or atleast attempted it. I really need to find a service manual for this t-case so I can dig in with some confidence. Anybody got some inight on the np 126? They seem do be fairly rare tough to find parts for.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Dec 5, 2011
577
Central Pennsylvania
I had major TCase issues in my '02 bravada. Sometimes wouldn't disengage, sometimes wouldn't engage. It took a total of 3 fluid changes (with over 1000 miles between each flush) to clean the crap out of my tcase and get it to work as it should. I also think the tech pulled the servo out and checked it, but didn't change it. There's a TSB that calls for TWO back to back Tcase fluid changes with some driving in between - so it's a known issue. Since my 3 flush job, I haven't had a single issue. I bought the car with 104,000 on it with no tcase fluid change and now I've got 120,000+. The AWD (SmartTrak) in our cars is actually pretty good when it works. I wouldn't mess with a single switch or anything without at least following the TSB to the letter, first. As for modifying the AWD - I cringe to think about it. From the factory our AWD is supposed to bias some torque to the front (that's why it will spin the tires when off the ground) when it senses a slip condition (front/rear) it engages the clutch pack and splits torque between the front and rear (maybe 50/50 maybe something else, don't know). But it all happens pretty quickly and fairly unnoticeably when it's working as designed. Here in PA we got 8 inches of snow over the holidays. I never plow my drive and it's pretty slick and at a decent grade. UPS/FedEx can't make it up our drive, but our bravadas(we have 2) have no issues whatsoever, even going around the house through the snow covered grass up the hill to unload groceries is no issue. She feels like my old 4x4 when it's engaged.
 

1978chevy4x4

Original poster
Member
Dec 30, 2012
14
TequilaWarrio said:
I had major TCase issues in my '02 bravada. Sometimes wouldn't disengage, sometimes wouldn't engage. It took a total of 3 fluid changes (with over 1000 miles between each flush) to clean the crap out of my tcase and get it to work as it should. I also think the tech pulled the servo out and checked it, but didn't change it. There's a TSB that calls for TWO back to back Tcase fluid changes with some driving in between - so it's a known issue. Since my 3 flush job, I haven't had a single issue. I bought the car with 104,000 on it with no tcase fluid change and now I've got 120,000+. The AWD (SmartTrak) in our cars is actually pretty good when it works. I wouldn't mess with a single switch or anything without at least following the TSB to the letter, first. As for modifying the AWD - I cringe to think about it. From the factory our AWD is supposed to bias some torque to the front (that's why it will spin the tires when off the ground) when it senses a slip condition (front/rear) it engages the clutch pack and splits torque between the front and rear (maybe 50/50 maybe something else, don't know). But it all happens pretty quickly and fairly unnoticeably when it's working as designed. Here in PA we got 8 inches of snow over the holidays. I never plow my drive and it's pretty slick and at a decent grade. UPS/FedEx can't make it up our drive, but our bravadas(we have 2) have no issues whatsoever, even going around the house through the snow covered grass up the hill to unload groceries is no issue. She feels like my old 4x4 when it's engaged.

Thanks for the input! I havent done anything yet as far as major over hauling goes and I was going to change it atleast one more time anyway because it seemed to make sense. Also I removed my encoder motor the other day to do a bench test. I couldn't get it to do anything. Took the cover off the motor and it smelled and looked like at some point it had almost had a melt down. So Im definitely going to replace that after a couple more fluid changes. Got another question, do you know if the encoder actually shifts the transfer case or does it just control lockup in the clutchpack? From what I have been able to find and read it only controls the clutch lockup and the t-case is always in "gear" waiting for the clutches to lockup. Is this theory correct?
 

BRomanJr

Member
Dec 9, 2011
371
1978chevy4x4 said:
Thanks for the input! I havent done anything yet as far as major over hauling goes and I was going to change it atleast one more time anyway because it seemed to make sense. Also I removed my encoder motor the other day to do a bench test. I couldn't get it to do anything. Took the cover off the motor and it smelled and looked like at some point it had almost had a melt down. So Im definitely going to replace that after a couple more fluid changes. Got another question, do you know if the encoder actually shifts the transfer case or does it just control lockup in the clutchpack? From what I have been able to find and read it only controls the clutch lockup and the t-case is always in "gear" waiting for the clutches to lockup. Is this theory correct?

YES, that is the correct theory of operation. The clutch is the only thing the encoder operates and it always have a small amount of engagement until rear wheel "slip" is detected. It has also been discussed that the TCCM "learns" the clutch pack and corrects for pack wear and fluid degradation up to a point.
 

jeffro312

Member
Oct 4, 2012
442
East Haven,Ct
BRomanJr said:
YES, that is the correct theory of operation. The clutch is the only thing the encoder operates and it always have a small amount of engagement until rear wheel "slip" is detected. It has also been discussed that the TCCM "learns" the clutch pack and corrects for pack wear and fluid degradation up to a point.

So I'm just gunna chime in with a question on subject sorry.....

If tccm learns this how would one go about resetting tccm?? If it learned the fluid decoration. Then we replace fluid a couple times does it automatically learn this new fluid....

My awd stopped working. At first was like stuck in awd. Now I have just rear wheel drive back tires spin spin. Fronts never grab. Did one fluid change 600-700 miles ago.. will more fluid changes likely have any effect?? Also to op. How did u bench test encoder motor is it as simple as running 12 volts to its connector ?
 
Dec 5, 2011
577
Central Pennsylvania
1978chevy4x4 said:
Thanks for the input! I havent done anything yet as far as major over hauling goes and I was going to change it atleast one more time anyway because it seemed to make sense. Also I removed my encoder motor the other day to do a bench test. I couldn't get it to do anything. Took the cover off the motor and it smelled and looked like at some point it had almost had a melt down. So Im definitely going to replace that after a couple more fluid changes. Got another question, do you know if the encoder actually shifts the transfer case or does it just control lockup in the clutchpack? From what I have been able to find and read it only controls the clutch lockup and the t-case is always in "gear" waiting for the clutches to lockup. Is this theory correct?

1978chevy4x4 - as someone else already pointed out you are essentially correct. I presume from your screen name that you have or had an old Chevy 4x4 - probably one with an NP205 to NP207 or something similar. Our Tcase is NOTHING like that. We don't have a low, or a neutral. The clutch pack essentially separates/joins the front and rear based on what the encoder motor does. There is a feedback circuit in the encoder that tells the TCCM "yep, I'm engaged" or "Nope, not engaged". That's how the TCCM tells what's going on vs what's been instructed. If you're motor's bad, that could explain alot if not all of it. If it fails in the engaged position, it will stay engaged until some outside force moves (or removes) the encoder motor. For some reason, the TCCM doesn't see this as a problem (mine never complained). However, if the TCCM commands lockup, and it DOESN'T occur, you should get a "Service AWD" light. What's interesting is that the TCCM is supposed to learn the clutch and fluid behavior and adjust accordingly. After replacing your encoder motor, you should reset the TCCM. I'm not sure how to do this but there ought to be a howto somewhere on gmtn. When all is working well there is some slight bias toward the front (about 5% someone once told me). This is normal for most AWD systems.
 

1978chevy4x4

Original poster
Member
Dec 30, 2012
14
jeffro312 said:
So I'm just gunna chime in with a question on subject sorry.....

If tccm learns this how would one go about resetting tccm?? If it learned the fluid decoration. Then we replace fluid a couple times does it automatically learn this new fluid....

My awd stopped working. At first was like stuck in awd. Now I have just rear wheel drive back tires spin spin. Fronts never grab. Did one fluid change 600-700 miles ago.. will more fluid changes likely have any effect?? Also to op. How did u bench test encoder motor is it as simple as running 12 volts to its connector ?

jeffro- from what I can gather from the wiring diagram I downloaded 12v should ignite some sort of movement in the motor. I tried every combination of pins possible and never got anything. and like i said pull the cover on the motor and that may tell the whole story. It did for me anyway.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
1978chevy4x4 said:
...I tried every combination of pins possible ...
I just have to mention (as an engineer) that guessing like this can sometimes do more harm than good. Especially when the encoder motor includes a motor and a potentiometer sensor, and under the right conditions, a straight-up 12V and ground applied to the sensor and its wiper can damage it. Similar risk to the other tool I wish everybody would just discard nowadays - the 12V test light. A meter (on the voltage function) is designed to not interact with the circuit it's measuring. A test light by design, must inject a certain amount of current with 12V behind it, to light up the light. This can damage some sensors and feedback circuits. I would only (reluctantly) use a test light on a LIGHTING circuit.
 

1978chevy4x4

Original poster
Member
Dec 30, 2012
14
Just an update guys. I installed a new encoder motor on my bravada, which was a piece of cake, and still no awd. Then "bing" the lightbulb came on, pulled the atc fuse for 30 sec or so and tried again, AWD works perfectly. Thanks for the help guys. This site and its resources are a lifesaver.
 

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,856
Tampa Bay Area
Last edited:

Forum Statistics

Threads
23,400
Posts
638,838
Members
18,614
Latest member
dakis

Members Online