GM / General Motors: 99 Silverado with ATC automatic transfer case doesn't engage to 4WD. Need help.

"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

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!
 

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.
 

"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

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.

 

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


"SDNomad" <com> wrote in message
news:google.com... 

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.

"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