01 Sierra: 4x4 no Longer Works

[chevymotocross]

2001 GMC Sierra SLE 4x4 4.8. It has the push button 4x4 setup with Auto 4wd.

Owned the truck almost a year. 125,000 miles. 4x4 worked great last winter. Not working this year.

Couple days ago during snowstorm I selected AUTO 4wd and was about to start my journey. Truck was slippin and sliding and clearly not in 4wd. Just as I started to recognize the condition, the Service 4wd System light came on. I pulled over to a side road. Shut the truck off and restarted. Service 4wd light went away and this time I selected 4 HI. Note: The service 4wd light would never return again.

Still not working in 4hi even though the light is illuminated on 4hi. Ok great. This time I try 4 LO. Light is illuminated on 4 LO and the truck is clearly in low range because the truck is much slower but torquier. BUT, still no traction, front wheels not engaging.

I can hear the transfer case doing its motor thing and engaging between all the 2wd and 4wd options. I removed the front diff actuator and that is functioning fine (retracting and moving). I decided to put a new one in (Dorman) after removing the original because the plastic threads got kinda messed up.

Still no 4wd. I jack up the front of the truck. In 2hi each front wheel is independent from one another. I engage any of the 4wd options and yup, the diff is definitely engaging because now the front wheels react to one another when the truck is jacked up.

I had a friend carefully drive the truck while I observed the front drive shaft. It spins although I also notice it spins in 2wd so not sure if thats a good test. I read somewhere that these NP246 T cases will provide some output to the front shaft even in 2wd due to the clutch pack inside.

All 4 wheels on the ground if it’s in 2wd I can spin the front driveshaft. In the 4hi and lo modes it definitely locks in.

Ok on to the transfer case fluid. Drained it out. Black and BURNT like real burnt stunk up my garage. I replaced with new Auto Trak II and tried to cycle thru the modes a few times and drive. Still no 4x4.

Is it safe to say my T case is screwed? Is it possible the T case motor although it seems to function that it may be broken? I would think the selector lights would be blinking and not going into the different modes if that were the case?

Think I bought a lemon
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[jocko]

before throwing in the towel.. I had some rather unexplainable problems with my 01 silverado's auto-tcase. Some of them were similar to what you've experienced - but some were not, so this may be no help at all. My troubles started after I got the trans rebuilt. Took it back to the trans shop and they swore they didn't touch the t-case and were no help. Anyway, they DID apparently touch the harness... I found the t-case controller harness plugged in, but hanging by a thread to the t-case. Plugged it in all the way and all was good. I got lucky. Worth eliminating the easy stuff before you go down a more expensive road. Good luck!

[chevymotocross]

Originally Posted by jocko

before throwing in the towel.. Worth eliminating the easy stuff before you go down a more expensive road. Good luck!

Thanks Jocko.

I will double check the electrical and grounds although it *seems* like the electrical side of things are functioning.

When your 01’ had the issue, did it seem like the electrical side of the system was working? From what I have gathered, usually with this setup, if something on the electrical end isn’t working, the indicator lights will flash, or not light up at all.
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[jocko]

If I recall, my symptoms were the "service 4WD" light and not being able to get OUT of 4WD once in it. So, if it wouldn't go into 4WD when I selected it - I got the light. If I could get in to 4WD via pushbutton seleciton, I couldn't get out until I stopped the truck, shut it down, and restarted. Some similarities, but not many, to your issue.

[whtgmc68]

Sounds like what my 01 Sierra did and the clutches in the xfer case were smoked. It only had 69,000 miles on it. Cost me about $1,100 parts and labor bqcknin 2006.

[95 S_Trucker]

Black transfer case fluid means the clutches got burnt. Pump rub is a problem on the NP246 transfer cases. Keep that in mind if you get yours rebuilt or if you get a used one.

[chevymotocross]

Originally Posted by 95 S_Trucker

Black transfer case fluid means the clutches got burnt. Pump rub is a problem on the NP246 transfer cases. Keep that in mind if you get yours rebuilt or if you get a used one.

What usually causes burnt clutches in these transfer cases? Obviously using wrong fluid. Low fluid. Unserviced fluid. But as far as operation aren’t the clutches fully engaged in 4hi and 4lo? I would assume the clutches come into play during AUTO and during selection changes... just like shifting your gears on a manual transmission.
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[95 S_Trucker]

No, its more like the clutches in an automatic transmission.

They allow some slippage to prevent clunking/binding when in Auto 4wd. Sometimes they burn up on their own. The biggest reason they burn up is the fluid gets low. The fluid gets low when a tiny pinhole is rubbed through on the case(pump rub).

Is the rear of the transfer case covered in grime? And hard to find the origin of the grime? that's pump rub.

[chevymotocross]

Originally Posted by 95 S_Trucker

Is the rear of the transfer case covered in grime? And hard to find the origin of the grime? that's pump rub.

Nope, clean as a whistle!

Scanned it today.

Code C0374 - General System Malfunction

After 10 or more consecutive slips are uncorrected, this code is activated.

Says if transfer case is shifting normally... which mine is.... problem may be internal.

The burnt fluid doesnt lie. Sounds like I need a new T case.
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[hatzie]

The front axle has an electric actuator to engage the front axle. If that's not happy you won't get traction from your front wheels.
Black Smurf Blood isn't a good sign but I'd check the front actuator and make sure it's working.

[chevymotocross]

Originally Posted by hatzie

The front axle has an electric actuator to engage the front axle. If that's not happy you won't get traction from your front wheels.
Black Smurf Blood isn't a good sign but I'd check the front actuator and make sure it's working.

Thanks for the reply hatzie but as I had stated I already checked and confirmed for a functioning front actuator and replaced with a new one as well due to the plastic threads getting boogered up upon removal (plastic threading into metal housing).
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[hatzie]

Originally Posted by chevymotocross

Thanks for the reply hatzie but as I had stated I already checked and confirmed for a functioning front actuator and replaced with a new one as well due to the plastic threads getting boogered up upon removal (plastic threading into metal housing).
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Missed that.
If that's working properly then your clutches have gone to pieces on you.
I've never been a big fan of those xfer cases.

If he isn't in love with the NV243 can he swap it out to an NV263?
How nasty is the swap? As in how much other crap needs to be sourced to do it and will the PCM get terminally upset about it.

If it helps I have the 2001 Unit Repair Manuals in the Queue to scan...

[chevymotocross]

Originally Posted by hatzie

Missed that.
If that's working properly then your clutches have gone to pieces on you.
I've never been a big fan of those xfer cases.

If he isn't in love with the NV243 can he swap it out to an NV263?
How nasty is the swap? As in how much other crap needs to be sourced to do it and will the PCM get terminally upset about it.

If it helps I have the 2001 Unit Repair Manuals in the Queue to scan...

Thanks hatzie,

So i spoke with a transfer case specialty shop. The transfer case I have is an np246. He said typically what ends up happening with clutch burnout is either wrong fluid, low fluid, never serviced fluid, or a weak encoder motor. He said what sometimes happens is it seems to engage fine but the motor fails to maintain the pressure needed for the clutch pack. Overtime, depending on how often you use 4x4, the clutches burn up. There are other issues with these cases as well such as “pump rub”.

This is exactly why these new gadgets are fantastic on new vehicles but for folks trying to buy something on the used market or keep their vehicles for the long haul will run into these issues once the system gets old. I certainly never had these issues with my old np203 and 205 equipped trucks!

$800 for a reman t-case and $300 for a new encoder motor.
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[hatzie]

243 was a typo. I was thinking about both numbers and fouled up.

Can you swap the light duty Autotrac NV246 out to an NV261 or NV263?
If it won't upset the PCM and you don't have a console in the way the 261 has a smaller count of electronic hurdles... I would guess the only real hurdle to using the NV261 would be wiring in the front axle actuator.

The RPOs...
NP1 Transfer case, electronic shift (NVG 263)
NP2 Transfer case, floor-mounted shifter (NVG 261)
NP8 Transfer case, electronic Autotrac (NVG 246)