How to Operate the 4WD?

[89Suburban]

Oh you also have a pin on a servo in the front diff to lock passenger side output shaft (lock the hub?). It's all automated though.

It's a plunger that moves a splined sleeve to connect the pass side shafts together in the diff.

Look here and skip to 5:15.

diff

 

[89Suburban]

 

[steiny93]

if auto was "not intended for 24/7 use" it would be clearly noted as such in the owners manual, that is a statement you created out of nowhere or some misconception.
you are confusing yourself, it would be impossible to apply power to the front and not reduce power to the rear and therefore load. (there is not 2 engines with 1 that suddently kicks in to the front)
less power applied to the rear = less stress on the rear, auto does not divide power 50/50 but it will reduce stress on the rear "on demand".
yes there is a mpg hit and Divided wear (on demand) instead of 100% wear on the rear end.
that's why you can't do a burnout in auto unless you have a nice horse power upgrade, because it detects wheel spin and applies the means to correct it.
and auto is safer than 2wd, not as safe as AWD but it is a 2nd best. I will gladly take a smidge mpg hit if it might save my life and divide the wear and not put my rear end into early retirement
you could hit a pot hole in 2wd and die, but auto "might" save your life and your rear end.
I have broken several rear ends in 2wd and have YET to break one in auto or awd, why? because it splits the wear when needed = reduced rear end wear
and all those rear ends I broke were guess what? g80's guess what is on my auto vehicle and awd vehicle? that's right the exact same g80
hmm I wonder why they have not broke yet? any guess
rant over you will never ever convince me that driving in 2wd is better than auto, it's impossible.

and in reference to my first line all you will find on the owners manual regarding "auto" is this


AUTO (Automatic Four-WheelDrive) : Use when road surface traction conditions are variable.When driving in AUTO, the front axle is engaged, and the vehicle's power is sent to the front and rear wheels automatically based on driving conditions. This setting provides slightly lower fuel economy than 2wd. Do not use AUTO mode,if equipped, to park on a steep grade with poor traction such as ice,snow, mud, or gravel. In AUTO mode only the rear wheels will hold the vehicle from sliding when parked. If parking on a steep grade,use 4hi to keep all four wheels engaged.
not a single iota about not for use 24/7.......

Not using auto 4wd has been covered many times; if you are in auto you have the front end components in motion; they are wearing
gm has been hit on this many times, they have even updated their public website to reflect it here: https://www.gmc.com/gmc-life/how-to/when-to-use-four-wheel-drive . Per the owners manual wording, it is using fuel, you are wearing components.

Regarding torque and breaking rear ends.
In auto 4wd all the torque is going to the rear end, no torque is shed until the rear end looses traction (auto isn't dividing any power before loose of traction).

Explain exactly how using auto 4wd reduces torque to the rear axle before the rear axle slips. After the rear axle slips the torque load doesn't matter, it'll be lower the before it broke loose.

Concerning the delay; absolutely their is a delay. Drive on ice once and you'll experience it. Rear tires on ice, front on pavement (like 6 months here in ND) the fronts are in constant chatter mode.

 

[Doubeleive]

Not using auto 4wd has been covered many times; if you are in auto you have the front end components in motion; they are wearing
gm has been hit on this many times, they have even updated their public website to reflect it here: https://www.gmc.com/gmc-life/how-to/when-to-use-four-wheel-drive . Per the owners manual wording, it is using fuel, you are wearing components.

Regarding torque and breaking rear ends.
In auto 4wd all the torque is going to the rear end, no torque is shed until the rear end looses traction (auto isn't dividing any power before loose of traction).

Explain exactly how using auto 4wd reduces torque to the rear axle before the rear axle slips. After the rear axle slips the torque load doesn't matter, it'll be lower the before it broke loose.

Concerning the delay; absolutely their is a delay. Drive on ice once and you'll experience it. Rear tires on ice, front on pavement (like 6 months here in ND) the fronts are in constant chatter mode.

EXACTLY AS NOTED PER YOUR LINK
"CAN BE USED ON ANY ROAD CONDITION WITHOUT RISK OF DAMAGE TO YOUR VEHICLE"
as for the rest of it, I am no expert but it works great for me and it say's "the clutches modulate torque forward to provide stability and enhance traction to the vehicle"
if that takes wheel slip so be it, but I would guess it takes extremely little slip otherwise this bish would be chirping around on me all day like it does in 2wd

and: If road conditions frequently alternate between "high- and low-traction areas" lol that's me I create low traction by pressing the big pedal

AUTOMATIC 4 HI
If road conditions frequently alternate between high- and low-traction areas, consider using the “AUTO” setting found on select GMC electronic transfer cases. This setting allows your GMC to automatically distribute torque to the front axle by anticipating the need for additional traction. Shifting into “auto” engages the front axle, but the transfer case sends power primarily to the rear wheels in normal conditions and the clutches modulate torque forward to provide stability and enhance traction to the vehicle. Although not always optimal for efficiency and wear of your vehicle 4wd driveline, AUTOMATIC 4 HI can be used on any road condition without risk of damaging your vehicle.


 

[Doubeleive]

in "auto" all the time, no delay no chatter no problems at all, don't even need 4wd unless it gets really hairy
every once in a blue moon might get a traction control activation

 

[Doubeleive]

chatter would likely occur from either traction control or stabiltrac kicking in or ABS or both. Never had my differential chatter
I have had the vehicle "pause" from stabiltrac while it decides what to do when I give it a reason like driving up on a curb at speed so one or more wheels are off the ground.
that has nothing to do with the differential or transfer case other than the computer deciding how to correct it by applying a somewhat gentle power to move forward.
that can be disabled for the most part by holding down the traction control button for a few seconds so both turn off, abs requires pulling the fuse.
I have put all these things thru a workout....stabiltrac in particular is slow to decide what to do it will basically just cause a massive power drop then go. It's a potential life saver in many situations and a hinderance in others.

 

[Fless]

in "auto" all the time, no delay no chatter no problems at all, don't even need 4wd unless it gets really hairy
every once in a blue moon might get a traction control activation

Wouldn't you think that yours is a different transfer case and 4wd system? We're discussing the GMT800 drivetrain. Where does your description come from?

There is absolutely a slight delay in AUTO since the front diff actuator needs time to deploy.

 

[Doubeleive]

Wouldn't you think that yours is a different transfer case and 4wd system? We're discussing the GMT800 drivetrain. Where does your description come from?

There is absolutely a slight delay in AUTO since the front diff actuator needs time to deploy.

AUTO/2wd/4hi/4lo/awd should all operate the same from gmt800 to current, although I thought I read something about some of the T2's having some kind of option other than AWD when equipped but I could be wrong, might just be on the acadia

 

[89Suburban]

 

[Fless]

AUTO/2wd/4hi/4lo/awd should all operate the same from gmt800 to current, although I thought I read something about some of the T2's having some kind of option other than AWD when equipped but I could be wrong, might just be on the acadia

From the various descriptions the later versions of AUTO fully engage the diff, and the transfer case does the proportioning. Not so in the GMT800s; the front diff is not engaged in AUTO until commanded so; I've watched it change on my Tech 2 and experienced the delay of the front diff engagement.

Note the difference between driveline "wear" and driveline "damage".

 

[steiny93]

Note the difference between driveline "wear" and driveline "damage".

thank you

 

[noodlesandsam2]

My own rules - both with the 01 1500 ( 270K, no issues ) - Auto trac when it is raining, or snow and slippery and on the highway - press the button, never an issue. I switch back to 2WD as soon as it is not slippery - also I try not to do either switching when accelerating. 4W Hi - Deep snow, when Auto Trac is not sufficient. Again, I never stopped for 4Hi, but not when on the throttle.

Honestly, I don't think I ever used 4 Lo, unless it was to try it out in a parking lot. with the 1500 I ran dedicated snows, so traction was never an issue.

Same rules for the 2500 I just got, - he is at 156K, and the last owners must have been pretty gentle, it acts just like the 1500 did.

 

[Matthew Jeschke]

The clutch pack works with the encoder motor to pulse the front diff on and off for the auto function. This same technique is used on other vehicles, not just the GMT800. I believe Subaru uses a similar system, perhaps even the NP246 on some models as well. I've never had issue using it off road w/ delay or any of fore mentioned concerns. I venture that system would react within milliseconds (I used to design control systems as an engineer some automotive as well), faster than you'd even notice. I experienced no driveline failures or wear as previously brought up with my NP246 / GMT800 setup. My truck primarily exists off-road. I also tore down my functioning NP246 tx-case just to rebuild it, upgraded the clutches, put in case saver, etc as preventative maintenance because I spend most of my time in remote mountains were a breakdown off-road can be quite catastrophic otherwise and rebuilding this is somewhat trivial. Long story short, the clutches in both cases I tore down were like new so I don't know what the previous mention is of driveline wear, it seems to be a very well designed system. I've literally went through every drivetrain system on the truck when I built mine and found virtually no where anywhere in the system when I bought the truck. Both front and rear diffs, suspension, 4L60E, Tx-case, & engine. The GMT800 is a VERY well built truck (aside from interior).

My original case was tore down at something like 250,000 miles and the clutch pack didn't show any wear. Literally the auto 4wd clutches maybe had a few thousands wear on the entire pack. Same for the working core I used while I rebuilt my original. I tore it down afterwards. I use my truck in much harder conditions than the average person and it's yet to let me down in 60,000 miles since I bought it.

Where these transfer cases fail is from pump rub. There is a small metal clip to protect the case from the pump. That little clip wears out and the pump eventually digs through the case. A case saver will for all intensive purposes eliminate that problem. Everywhere else the case is built like a beast. I don't know where it would fail. It's built up WAY stronger than the 4L60E (which I went through and built up too). Probably stronger than a 4L80E as well. It's SUPER beefy. I have a post on the forum about rebuilding the NP246.

 

[Fless]

The clutch pack works with the encoder motor to pulse the front diff on and off for the auto function. This same technique is used on other vehicles, not just the GMT800. I believe Subaru uses a similar system, perhaps even the NP246 on some models as well. I've never had issue using it off road w/ delay or any of fore mentioned concerns. I venture that system would react within milliseconds (I used to design control systems as an engineer some automotive as well), faster than you'd even notice. I experienced no driveline failures or wear as previously brought up with my NP246 / GMT800 setup. My truck primarily exists off-road. I also tore down my functioning NP246 tx-case just to rebuild it, upgraded the clutches, put in case saver, etc as preventative maintenance because I spend most of my time in remote mountains were a breakdown off-road can be quite catastrophic otherwise and rebuilding this is somewhat trivial. Long story short, the clutches in both cases I tore down were like new so I don't know what the previous mention is of driveline wear, it seems to be a very well designed system. I've literally went through every drivetrain system on the truck when I built mine and found virtually no where anywhere in the system when I bought the truck. Both front and rear diffs, suspension, 4L60E, Tx-case, & engine. The GMT800 is a VERY well built truck (aside from interior).

My original case was tore down at something like 250,000 miles and the clutch pack didn't show any wear. Literally the auto 4wd clutches maybe had a few thousands wear on the entire pack. Same for the working core I used while I rebuilt my original. I tore it down afterwards. I use my truck in much harder conditions than the average person and it's yet to let me down in 60,000 miles since I bought it.

Where these transfer cases fail is from pump rub. There is a small metal clip to protect the case from the pump. That little clip wears out and the pump eventually digs through the case. A case saver will for all intensive purposes eliminate that problem. Everywhere else the case is built like a beast. I don't know where it would fail. It's built up WAY stronger than the 4L60E (which I went through and built up too). Probably stronger than a 4L80E as well. It's SUPER beefy. I have a post on the forum about rebuilding the NP246.

So you have a faster actuator than what is shown here? Mine is just like the one he tests.

 

[Marky Dissod]

Where these transfer cases fail is from pump rub. There is a small metal clip to protect the case from the pump.
That little clip wears out and the pump eventually digs through the case. A case saver will for all intensive purposes eliminate that problem.
Everywhere else the case is built like a beast. I don't know where it would fail.
It's built up WAY stronger than the 4L60E (which I went through and built up too). Probably stronger than a 4L80E as well. It's SUPER beefy.
I have a post on the forum about rebuilding the NP246.

Where does one get this 'case saver'?
Where do I find your post about rebuilding the NP246?

 

[Matthew Jeschke]

There are a couple different styles.

This is the most affordable style HERE

It slides in and increases the surface area so the protector lasts longer, versus the factory design that is just a small clip going over one of the lugs on the pump half.

This is what I used. It's a new pump face HERE

Either does the trick fine. You can get generic pump half kits for cheaper too.

Honestly, if a person just dropped the case, separated it and changed that case saver, it would likely do the trick. I went the extra mile and replaced all the guts for couple hundred bucks, including upgrading to Raybestos GPZ clutches. If you change out the bearings though you'll have to redo front output shaft seal. It's a two way seal for which you'll need a tool (roughly $100 for the tool) or VERY careful hands to make sure the inner seal lines up with the outer and the shaft.

 

[2591tdj]

Greetings,

I am a complete newbie when it comes to 4WD controls. I bought a 2005 GMC Yukon XL over the weekend and I want to make sure I understand how to operate it properly.

I have a few questions:

1. Do I need to stop the vehicle to engage the 4WD Auto setting? Is it the same for the other settings?
2. Can I drive at highway speeds with the 4WD Auto setting on?
3. Any other insights or tips would be greatly appreciated.

Thank you in advance for your help!

I’ve seen comments on other threads saying that it’s okay to drive on hard surfaces in 4Hi. I made the mistake of forgetting to shift out of 4Hi in my 2005 Z71 Tahoe when I reached pavement and drove 45 miles, mostly at highway speeds but with several complete stops & 90 degree turns. As I reached my driveway the front diff started making a loud grinding, clanking noise. I had destroyed the front diff and had to have it replaced. The repairman said that driving on hard, dry pavement in 4Hi did the damage.

 

[Marky Dissod]

I’ve seen comments on other threads saying that it’s okay to drive on hard surfaces in 4Hi.
I made the mistake of forgetting to shift out of 4Hi in my 2005 Z71 Tahoe when I reached pavement and drove 45 miles, mostly at highway speeds but with several complete stops & 90 degree turns.
As I reached my driveway the front diff started making a loud grinding, clanking noise.
I had destroyed the front diff and had to have it replaced.
The repairman said that driving on hard, dry pavement in 4Hi did the damage.

This forum REALLY NEEDS the 'Helpful' smiley; none of the other 6 smiley options are as appropriate.

Although I do wonder if you could've gotten away with it if you'd never made any turns, still sucks that it happened to you.
Thanks for saying it to us.