99 Silverado with ATC automatic transfer case doesn't engage to 4WD. Need help.

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I have a newer body style 99 silverado with a automatic transfer case that is not locking into four wheel drive. The front axle locks in because I placed it in 4WD and put the front end up in the air and
checked. But when I put the rear axle in the air, while in 4WD and motor running in gear, the transfer case does not turn the front drive shaft. I can hear the encoder motor inside the transfer case running when I push the 4WD and 2WD selector buttons so I assume it is shifting from 2WD to 4WD high to 4WD low to 4WD auto, etc. but the transfer case doesn't lock up. I don't know if the encoder motor is what shifts the vehicle into low gear when you push the 4WD low button, but it does go into low when you select 4WD low. It is easy to tell that it is going into low because it just creeps along. Would a front or rear speed sensor cause the transfer case not to lock into 4WD? Whatever makes the transfer case lock in or regulates it locking in, is not working. I do not have a 4WD/AWD service indicator light on. And all the indicator lights on the 4WD and 2WD selector buttons say the trasfer case is shifting from 2wd high to 4wd high to 4wd low, etc. My only indication that things are not right is that the transfer case is not turn the front axle. Any ideas? Thank you!
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"SDNomad" wrote

Your clutch pack that modulates power to the front drive shaft is probably burnt up. Check your fluid...it should be a nice bright blue color. If it's black...you can almost guarantee that the clutch pack is burnt up. The transfer case is shifting into 4hi and applying the clutch, but if the clutch discs are worn...you will not have any torque to the front drive shaft.
Fairly common problem with these trucks. Often what causes it is a side bearing problem in the front differential. I also suspect that way too many folks believe that you should be running around in the Auto4HI position even in dry conditions. Not a good idea.
Ian
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This is the first time I have attempted to engage the 4WD since I used it last winter during a snow fall. I don't drive with it or auto 4WD on unless I need it. I will check the fluid and let you know what I find. And how do you check the side bearings in the front differential. Do you pull the differential cover and remove the front axle shafts? Thank you!

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I called a 3 local transmission shops and all said there are no clutches in this transfer case. It works just like a manual transmission with a clutch folk and an electric motor that engages the gears. So I think the clutch being burnt up is out as far as a possibility.

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"SDNomad" wrote

Is that right? Is that what "you" think? I'll tell you one thing that you have learned.....those three transmission shops don't have a clue what they are talking about, and you would be wise to stay away from them.
"If" as you originally said, you have the automatic transfer case that has a button on the dash that says 'Auto4HI' then yes, that t/case has a clutch pack in it that modulates torque to the front diff. I've only rebuilt about 5 of them in the last three months.
Ian
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YOu are absolutely right and they were wrong. I looked the transfer case (NVG 246) up online and it has what is called a wet clutch. I called all 3 tranny shops this morning and said I had this problem and how much would it be to replace the clutch pack and all 3 told me no clutch. I have a set of 4 GM service manuals for this truck but the manuals just tell me to use the "transmission unit repair manual (TURM)" to repair the ATC. Now I have to find out where to find one of those. Is it a terribly difficult repair to do? I have rebuilt motors, manual clutches, and one manual transfer case on a 97 Dodge in the past.

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Ian;
What is the cause of the failures ?
I've been told running mis-matched tires can contribute.
Any input appreciated, I'm starting to see those TC's on units in for transmission repair, it's only a matter of time till I get a 'crispy' one...........
G

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"Gary Glaenzer" wrote

Cause is usually incorrect preload on the front diff side bearings. Weird eh? The added pressure of the front diff takes out the clutch pack in the t/case.
In most cases, we overhaul the front diff, and rebuild the clutch pack in the t/case. If you are lucky...most t/cases only need to have the clutch pack rebuilt. But if they get really hot, they can often end up scattering clutch material and bearings throughout the t/case. The bearing on the clutch pressure plate is prone to just disintegrate half of the balls within the ball bearing and spread them throughout the t/case.
GM actually has a bulletin that instructs you to (depending on symptoms and diagnostics) basically overhaul the front diff and rebuild the clutch in the t/case.
By the way, yes....mismatched tires can cause the problem. Basically anything that can cause driveline windup is the death knell for these t/cases. They actually run 5% torque to the front diff even when in 2wd, so it doesn't take much to fry them.
It will be a nice job once all these trucks go off warranty. I suspect that GM will do a lot of "goodwill" repairs on these things once word gets around about how flimsy they are.
Another "recommendation" from GM is to install synthetic diff fluid in the front diffs after the overhaul (along with the updated white vent that can handle synthetic).
From what I can see, a lot of these front diffs were incorrectly assembled at the factory....or...the preload specs were incorrect. I say the latter because preload specs used to be an actual torque right at the side bearing adjusters....but I've noticed that in the last few years, it's been changed to a combined turning torque measurement at the pinion. IE: pinion preload is 20-30 inch/lbs of turning torque....side bearing preload is adjusted so that you end up with about 50 inch/lbs of turning torque at the pinion.
A good clue that something is starting to go wrong in those t/cases is the fluid will be nice and black and smell like burnt clutch......which I'm sure you are very familiar with.
Ian
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Thanks for the information
G

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Ian, Thank you for your help. What is meant by preloading the front end side bearing? Do you mean the torque on the side bearings? What do you do correctly preload the front diff? By the way it sounds, I am going to replace the front diff side bearings to be safe. Thank you

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"SDNomad" wrote

Yeah, I'd say that you could do it. The only problem is that you need a special tool to properly preload the clutch pack and measure the gap to determine what shim you will end up using with the new clutch fibers and discs. I have noticed that with very few exceptions, the original shim that was installed, usually ends up being the right size. So you could take a chance.....and just use the original shim....or you could take your clutch pack down to the nearest dealer and ask them to measure it up for you....determine the correct shim....and you can take it from there. If you lived in my city.....I'd do it for a bottle of wine....(grin)
Because you are only doing the clutch (assuming that the clutch pack hasn't really gotten hot) there is very little to disassembling these t/cases. And they are fairly simple to get out of the truck.
If you decide you want to do the repair...let me know and I'll hook you up with more repair details.
Ian
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I found a manual for the transfer case so I am going to attempt to service the thing myself. Is the clutch pack an expensive part? I am wondering if I would be better off looking for a used or rebuilt ATC so I can get back on the road faster.
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"SDNomad" wrote

If you can find a good place that rebuilds them....yep, I'd say that you will be on the road a whole lot quicker. I'm not sure what the clutch discs and fibers cost. I do up estimates, but I never really bother looking at the individual prices much. You could call a dealer and ask....9 fibers and 10 steels if I recall correctly. Plus the shim kit. Oh....and you are supposed to replace all the t/case bolts that hold the two halves together as the case is magnesium and the bolts and washers are special to prevent wierd corrosion problems so they cannot be reused.
Ian
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BTW, don't dismiss the idea that the dash switch is defective, this problem exists in probably about 90% of these trucks running from the time the dash switch was used to present. Mine was faulty, and my two buddies as well. I believe if you look deep into this, the switch instructs the transfer case shift and then confirms it. Therefore when it goes faulty it would be possible for a half shift causing damage.Mine was giving me the check transfer case warning but nothing was stored in the history. Mine made weird grinding noises and locked the differential before the switch was fixed, which is not a hard fix. Do yoursef a favour and pull the switch (ten minutes) and remove the back off the switch and check the solder joints, I will bet that they are cracked and faulty. Just a thought as this seems to be misdiagnosed all too often from what I have seen.

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Okay, in '99 the NVG 246 has clutches. There were two styles. The old body style truck used a preload clutch. The new body style used the non preload clutch. They have different parts and are not interchangeable. So if getting a rebuilt or parts make sure you get for the correct clutch pack. If you want to see if it is the clutch pack if you have the vehicle with the rear tires off of the ground, in transmission in neutral, remove the encoder motor, turn the shaft coming out of the transfer case to apply the clutch. Then try turning the front drive shaft by hand. If you can turn the front drive shaft fairly easily then you know it is the clutch. If you can not turn the driveshaft then it could be the switch etc. But you said that you could hear the encoder motor. As Ian said you have to set the preload on the clutch pack with a special tool. Take to a dealer and have them measure. Just be sure not to remove the clutch disc once it has been measured. Also while apart replace the apply bearing in the apply plate. Old bearings were plastic and they break. This could also be why your 4WD does not work.
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I took the encoder motor off and shifted the transfer case manually into 4WD. I could still turn the front drive shaft, although it was stiffer to turn. I would think I shouldn't have been able to turn it at all if the clutch plates were good. I also ran the encoder motor and it worked fine. So I would say the transfer case is definitely bad.
Is there any way to tell if the bearings in the front diff need replacing without complete dissassembly?
I also did some reading up on overhauling the front differential and I think that one is over my head. And there must be 20 different specialty tools you need to do the job.
Thank you Ian and everyone for your help.
snipped-for-privacy@comcast.net (Warren Townsend) wrote in message

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"SDNomad"

Your method works. I have a hoist.....so I generally just lift up the truck, shift into 4hi and hold the brakes lightly while spinning up the wheels. If the front wheels stop moving.....it's fairly obvious that the clutch pack is not transferring power. Or, we use the scan tool to check front and rear driveshaft speeds...they should stay the same....if not, the clutch pack is toast.

No real surefire method without disassembly....but I would take the drain plug out and check the magnet and the color of the fluid. Often, when the bearings are toast, there will be all sorts of metal chips/flakes on the magnet and the diff fluid will be black. This is "not" a foolproof method for determining if the bearings are a problem.

This is not a job for a DIY'er. Believe me, there are plenty of "technicians" who can't properly overhaul and set-up a differential.
Ian
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Ian, I purchased a 246 transfer case out of a 2002 Chevy with 20k for $450 to put in. I figured if I was buy all the clutch parts, shim kit, and seal kit, I would have $350 in the rebuild and it would still have 125k on the bearings, gears, and other moving parts. My truck has 125,000 miles on it.
I called around on replacing the pinion gear bearing and side bearings in the front diff and they qouted me around $750-$800. If it comes to that I think I will do the same thing and pick up a low milage front axle for $400 to $500. I think I would be better off as far as expense and better all around equipment when finished.
I'll let you know how it's going once I get the other transfer case in. I'll check the diff at that time also.
Thank you for your help, Doug
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Not even a resourcefull one with unlimited time? Just wondering - I don't have one of these trucks so I don't need too. Like I remember my father describing how long it took him to get his rear diff lined up right, how you use grease to see the mesh pattern of the ring and pinion.
-The Lonely Grease Monkey 1985' K5 305CUI TH700R4 NP208 KJ's successor
"Ignorance is preferable to error; and he is less remote from the truth who believes nothing, then he who believes what is a wrong." - Thomas Jefferson
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"Lonely G-Monkey" wrote

Sure, anyone can actually do one. It might take a while and a whole lot of ruined bearings, crush sleeves, and pinion seals...but you can do it.
Ian
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