NVG246 (AutoTrac) Questions

[WVtrucker]

Hello all,

New to the forums here and so far very impressed by the content. I know this has been discussed to some extent already, but I have a couple of questions about the NP246 tcase in my '99 seirra reg cab long box 75K miles.

 

So to start off, last year I develeped a terrible vibration in the driveline which was later diagnosed as a bad pinion bearing in the front differential and bad bearings on the front side of the transfer case. This was all pretty obvious to me by the amount of slop in the pinion and the transfer case. I had the front diff rebuilt and later replaced the transfer case thinking this would solve the problem. I did this because the working theory at the time was that after loosing an oil seal in the front diff on a long trip that I cooked it and that in turn wiped out the tcase. Well about 3 months later I start noticing it coming back again. I check the play in everything and it was back in the tcase again. This time I pulled the front driveshaft out to avoid eating up the pinion bearing again. So this is where I stand. I need to do something here and I am really begining to think that the culprit in this is that the driveshaft is really crooked. It came from the factory with two huge wieghts on one end on the same side meanint (to me anyway) that it is not straight and that is what it took to get it "balanced". I am going to pick it up from a driveline shop this afternoon so we will see what they say about it.

 

Has anyone else had this kind of dreadful experience with the running gear in this these trucks?

 

Also with the truck in 2wd should you be able to rotate the front driveshaft independantly of the rear (by hand)?

 

Has anyone swapped out their "Auto Crap" (as i have heard it playfully refered to LOL)?

 

 

Any insight is greatly appreciated

 

Thanks,

 

Mike

[ness5575]

i have not any of these problems so i cant be of any help. but I was curious how expensive it was to get the front diff rebuild?

[Larry]

Yes, the AutoCrap can be shed. A manual 241 or 261 can easily be swapped in. For the most part it is a direct bolt in. Trans output shaft spline count, mounting holes, etc. You will need to change the front drive shaft and place a resistors in the encoder motor weather pack connector that will be left disconnected down at the transfercase and place a resistor at the control head in the dash to prevent any ghost “Service 4wd” messages.

 

I cannot remember the OHM resistors you will need but you can easily figure that out by testing the resistance at the encoder motor and control head then go to Radio Shack and buy them to build your own spout connectors. The hardest part is reworking the center console (if you have on) so the lever will stick out and look natural. I have a friend that built a Pikes Peak Hill climb truck last year that did this. 4 Wheel & Off-road also did something similar to this on the Ultimate Avalanche.

[concours]

...yeah, what he said! Does the front drive shaft rotate freely by hand when in 2WD mode? My mechanical shift trucks did, would anyone with a properly functioning autotrac kindly slip under the truck and give the front drive shaft a spin? My '01 Burban 2500 with the 246 case has developed a (new) funny noise like the front DS is turning all the time.... also, when shifted into 4HI, the amber pilot does not come on.

[vhato]

...yeah, what he said! Does the front drive shaft rotate freely by hand when in 2WD mode? My mechanical shift trucks did, would anyone with a properly functioning autotrac kindly slip under the truck and give the front drive shaft a spin? My '01 Burban 2500 with the 246 case has developed a (new) funny noise like the front DS is turning all the time.... also, when shifted into 4HI, the amber pilot does not come on.

 

<{POST_SNAPBACK}>

 

 

 

 

The NVG246 ALWAYS has a little clutch engagement on the front. When the rear driveshaft spins the front is spinning to. When the rear is locked the front is locked.

 

Also, when in park there should be a decent (it seems excessive) amount of "play" when turning the driveshaft by hand from lock to lock. It will seem like alot.

 

All AutroTrac front driveshafts I have seen have 1 weight near front diff pinion side. Nothing wrong with more than that, however you said your driveshaft is not straight and that will wear out pinion and transfer case bearings quickly.

 

The AutoTrac (NVG246) from 99-2000 and pre July 2001 were finiky. They need transfer case flushes almost ever 30,000 miles or they may develop a "clunk" when coming to a complete stop or accelerating from a stop. A new Nickle-Plated Slip Yoke is available as a band-aid fix so you don't hear it, but it doesn't SOLVE the problem. Only use GM Auto-Trac II fluid. Auto-Trac I should be avoided at all costs (It shouldn't be available)

 

Let us know what comes of the bent front driveshaft.

[vhato]

...yeah, what he said! Does the front drive shaft rotate freely by hand when in 2WD mode? My mechanical shift trucks did, would anyone with a properly functioning autotrac kindly slip under the truck and give the front drive shaft a spin? My '01 Burban 2500 with the 246 case has developed a (new) funny noise like the front DS is turning all the time.... also, when shifted into 4HI, the amber pilot does not come on.

 

<{POST_SNAPBACK}>

 

 

 

 

The NVG246 ALWAYS has a little clutch engagement on the front. When the rear driveshaft spins the front is spinning to. When the rear is locked the front is locked.

 

Also, when in park there should be a decent (it seems excessive) amount of "play" when turning the driveshaft by hand from lock to lock. It will seem like alot.

 

All AutroTrac front driveshafts I have seen have 1 weight near front diff pinion side. Nothing wrong with more than that, however you said your driveshaft is not straight and that will wear out pinion and transfer case bearings quickly.

 

The AutoTrac (NVG246) from 99-2000 and pre July 2001 were finiky. They need transfer case flushes almost ever 30,000 miles or they may develop a "clunk" when coming to a complete stop or accelerating from a stop. A new Nickle-Plated Slip Yoke is available as a band-aid fix so you don't hear it, but it doesn't SOLVE the problem. Only use GM Auto-Trac II fluid. Auto-Trac I should be avoided at all costs (It shouldn't be available)

 

Let us know what comes of the bent front driveshaft.

 

Conours,

The Amber light is burned out. You can replace the whole switch panel for $40.00 from http://www.GMPartsDirect.com if it bothers you.

 

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[concours]

vhato, It was my understanding that the case had front shaft engagement as you describe when in AUTO 4WD mode, but in 2HI, it was like the conventional systems before it. That is, front axle disconnect NOT engaged and transfer case NOT turning the drive shaft. I just came back from a 65 mile trip, the T-case and front diff. are both too hot to touch. Can someone please check for shaft rotation by hand while parked in 2HI?

[Blue-2001-Z71]

I once checked this with my truck in Park 2Hi and the shaft was indeed locked up. Not sure if this is supposed to be, but that's how it is.

[vhato]

I once checked this with my truck in Park 2Hi and the shaft was indeed locked up.  Not sure if this is supposed to be, but that's how it is.

 

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Yep, at ALL times the front driveshaft is engaged. 4HI/4Lo are 100% clutch engagement Auto mode determines clutch engagement based on computer info and changes in real time. 2Hi has very little clutch engagement.

[concours]

DAAAAEEEEEUUUUUM. This is hoakie $hit. Thanks for all the replies. Is there someone who sells a scanner for this stuff? I bought a windows based program for my software for my VW TDI that is so complete and affordable, it's the bomb.

[vhato]

DAAAAEEEEEUUUUUM. This is hoakie $hit. Thanks for all the replies. Is there someone who sells a scanner for this stuff? I bought a windows based program for my software for my VW TDI that is so complete and affordable, it's the bomb.

 

<{POST_SNAPBACK}>

 

 

 

I use AutoTAP and/or EFILiveV6. AutoTAP CAN work with all vehicles after 1996 while EFILive is specific (and highly controllable) to GM's LS based vehicles such as 1999-2005 Fullsize trucks/SUV's and 1997+ Camaro/Firebird Corvette.

[WVtrucker]

Well, just to update everyone on what has happened since the last post here is the saga.

 

I went and got the driveshaft back and the man says it is straight as an arrow and balanced just fine.

 

So I took it home and put it up on jackstands and with a stethoscope I isolated the vibration to the tcase and tranny tail shaft area. Even with both driveshafts out it still shook like mad!!

 

At that point I gave up and decided to talk to some tranny shops and they were as puzzeled as I was if not more.

 

Then I decided well lets give the dealership a shot at it as they are supposed to be the "experts" I took the mechanic down the road and the only comment he made the whole time was "This is gonna be a fun one to get into." 2 hours after dropping it off they call me and try to tell me that the problem is a weak leaf spring in rear on the drivers side and that it will be $1400 to change the springs out before they can go any farther. I proceded to tell the man that the problem was not the leaf spring because it is present when the rear is not even ingauged.

At that point I told them not to touch it again and went and picked up my truck.

 

So after all that time and effort I finally came up with the best solution to the problem.... I traded the thing in on a Ford F-150. The Ford is not half the truck that the GMC was but at least it isn't trying to shake itself apart on the highway.

 

If GM wants me to come back to them again they are gonna have to do some serious work on these trucks. The potential is there to smoke the competition but they cannot just depend on this new line of fuel efficeint high power engines carry them through for long if people are having to rebuild the driveline all the time along with all the other engineering blunders they have made.

 

Well just my two cents. Please don't hate me because I bought a Ford but I just don't have time to screw with a vehicle that is not dependable at this point in my life.

 

Mike