Auto 4WD System

BootHill · November 22, 2004

According to the dealer tech, Andrew, 80-90% of all GM vehicles when in Auto 4WD constantly locks in and out when on solid pavement. Mine, a 2002 GMC Yukon XL K-2500, didn't used to, but does now. The Service Rep took me out in one and the Tech in another (a 2002 and 2001 model, respectively) and neither of them had the behavior, but Andrew still insisted that the majority of them do behave that way.

The other thing that the Tech told me was that the transfercase splits the torque between the front and rear axle at different rates when it is in Auto 4WD. From the other things I had read it sounded like the transfercase was either engaged 50/50, or it wasn't engaged. (I understand that the Denali has a different transfercase than mine has and that it does do a torque split.) What is the truth there for my transfercase?

Thanks everyone for your replies. I just want to find out the truth.

Butch 02 Denali · November 22, 2004

According to the dealer tech, Andrew, 80-90% of all GM vehicles when in Auto 4WD constantly locks in and out when on solid pavement. Mine, a 2002 GMC Yukon XL K-2500, didn't used to, but does now. The Service Rep took me out in one and the Tech in another (a 2002 and 2001 model, respectively) and neither of them had the behavior, but Andrew still insisted that the majority of them do behave that way.

The other thing that the Tech told me was that the transfercase splits the torque between the front and rear axle at different rates when it is in Auto 4WD. From the other things I had read it sounded like the transfercase was either engaged 50/50, or it wasn't engaged. (I understand that the Denali has a different transfercase than mine has and that it does do a torque split.) What is the truth there for my transfercase?

Thanks everyone for your replies. I just want to find out the truth.

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Here's alink to a description of Autotrac. It is 50-50 system, and is either in normal 2wd or normal 4wd if it detects slipping. It does not have the capability, as you mentioned, of transferring the power front or rear as the Denali NVG149 vicious coupling transfer case does.

http://www.gmc.com/yukon/autotrac_popup.jsp

Butch 02 Sierra Denali

redvett · November 22, 2004

According to the dealer tech, Andrew, 80-90% of all GM vehicles when in Auto 4WD constantly locks in and out when on solid pavement. Mine, a 2002 GMC Yukon XL K-2500, didn't used to, but does now. The Service Rep took me out in one and the Tech in another (a 2002 and 2001 model, respectively) and neither of them had the behavior, but Andrew still insisted that the majority of them do behave that way.

The other thing that the Tech told me was that the transfercase splits the torque between the front and rear axle at different rates when it is in Auto 4WD. From the other things I had read it sounded like the transfercase was either engaged 50/50, or it wasn't engaged. (I understand that the Denali has a different transfercase than mine has and that it does do a torque split.) What is the truth there for my transfercase?

Thanks everyone for your replies. I just want to find out the truth.

<{POST_SNAPBACK}>

Here's alink to a description of Autotrac. It is 50-50 system, and is either in normal 2wd or normal 4wd if it detects slipping. It does not have the capability, as you mentioned, of transferring the power front or rear as the Denali NVG149 vicious coupling transfer case does.

http://www.gmc.com/yukon/autotrac_popup.jsp

Butch 02 Sierra Denali

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Something the autotrac article didnt mention is that while in autotrac the front axle is engaged and a loss of 1.5-20 mpg plus the wear on your front axle when 4 wheel drive isnt needed.

budhayes3 · November 22, 2004

Personally, I never use the auto feature of my transfer case. I either have it in 2wd or 4wd if necessary.

jctb · November 23, 2004

BootHill,

I have a 2000 Tahoe and a 2002 Silverado Z-71. Both have autotrac and it works smoothly and silently on both. If it is locking in and out on dry pavement then something is wrong with it.

JCTB

sonjaab · November 23, 2004

GUYS....I had Auto-trac in a 97 1/2 ton excab. and in a 01 2500 ld excab.

Worked like a charm up here in snow and ice land.

Yes, could feel it kicking in and out on occasion. But going from a foot of snow to

dry pavement within a mile it sure got a work out!

Especially hauling a Western plow rig on the nose !

Wish my 04 had it...........

the blur · November 23, 2004

I have the denali. there is nothing special about the transfer case. it's full time AWD ONLY. It's perfectly smooth, rides nice for the wife.

however, I doubt you can call it vicous coupling. I think it's electronic coupling. not entirely sure...

it's terrible in the snow. well not terrible, but my 95 safari van AWD is BETTER.

I alreadly got stuck in 8" of snow. had to get TOWED out !!! imagine 1 front wheel slightly spinning, and 1 rear wheel spinning. it was laughable.

IMO the open diff in the rear and front defeat the purpose of AWD. so no off roading for me. they don't even offer a limited slip rear in the denali.

at least you can choose true 4 wheel with your transfer case.

BootHill · November 23, 2004

I have the denali. there is nothing special about the transfer case. it's full time AWD ONLY. It's perfectly smooth, rides nice for the wife.

however, I doubt you can call it vicous coupling. I think it's electronic coupling. not entirely sure...

it's terrible in the snow. well not terrible, but my 95 safari van AWD is BETTER.

I alreadly got stuck in 8" of snow. had to get TOWED out !!! imagine 1 front wheel slightly spinning, and 1 rear wheel spinning. it was laughable.

IMO the open diff in the rear and front defeat the purpose of AWD. so no off roading for me. they don't even offer a limited slip rear in the denali.

at least you can choose true 4 wheel with your transfer case.

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Even with 4WD, if the diffs are open, you will have the same situation, since the transfer case only distributes power to the axle's differential, not to each tire. I have a the factory limited slip "locking differential" but an open front diff, so I effectively have two rear and one front wheel with power in the worst situations, until the limited slip slips, then I have the same as you.

You could always get an aftermarket limited slip in both axles, or better yet, air or electric lockers in your Denali and it will go like a champ. The only thing I'm not sure if the Denali has is low-range (4-LO).

USA1Fan · November 23, 2004

I had an '03 Trailblazer LT, and it seemed pretty smooth on pavement, though it did give a minor "kick" when it engaged (lived north-northwest of Chicago and was driving partially snowcovered roads). I also didn't notice any mileage decrease over standard 2WD mode in the same conditions (I usually drive slower in weather where the roads are potentially bad, and that hurts mileage more than the autotrac did). Just the same, I always used only 2WD unless I had a reason to think I might need 4WD, for the same reason as redvett mentioned, ie.- potential needless wear on the front drive components.

I haven't had a reason to use autotrac, let alone 4WD (hi or lo) on my new '04 Silverado yet, so no clue how it behaves just yet.

BootHill · November 24, 2004

I am really surprised that 40% of you so far experience it locking in on solid pavement. Mine used to be so smooth, I knew it happened but didn't get any of the things I mentioned in the poll.

redvett · November 24, 2004

I had an '03 Trailblazer LT, and it seemed pretty smooth on pavement, though it did give a minor "kick" when it engaged (lived north-northwest of Chicago and was driving partially snowcovered roads). I also didn't notice any mileage decrease over standard 2WD mode in the same conditions (I usually drive slower in weather where the roads are potentially bad, and that hurts mileage more than the autotrac did). Just the same, I always used only 2WD unless I had a reason to think I might need 4WD, for the same reason as redvett mentioned, ie.- potential needless wear on the front drive components.

I haven't had a reason to use autotrac, let alone 4WD (hi or lo) on my new '04 Silverado yet, so no clue how it behaves just yet.

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In the LT models you have instant fuel mileage info. Just drive on level hiway and engage autotrac and watch your mileage go down.

Also you should engage autotrac at least once a month to rotate all moving parts to lubricate.

BootHill · November 24, 2004

Techs just reported to me that it is not normal for it to lock then unlock constantly on solid pavement. They gave me a bulletin from GM that I will have to look over. They said they found foreign particle build-up in the transfercase and have flushed it out. I'm supposed to let them know in 500 miles if it is still experiencing the problem.

Butch 02 Denali · November 24, 2004

I have the denali. there is nothing special about the transfer case. it's full time AWD ONLY. It's perfectly smooth, rides nice for the wife.

however, I doubt you can call it vicous coupling. I think it's electronic coupling. not entirely sure...

it's terrible in the snow. well not terrible, but my 95 safari van AWD is BETTER.

I alreadly got stuck in 8" of snow. had to get TOWED out !!! imagine 1 front wheel slightly spinning, and 1 rear wheel spinning. it was laughable.

IMO the open diff in the rear and front defeat the purpose of AWD. so no off roading for me. they don't even offer a limited slip rear in the denali.

at least you can choose true 4 wheel with your transfer case.

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Blur, Only the SIERRA Denali and the Chevy SS use the NVG149 vicious coulping. The Denali's and Escalades through 2002 used them too, but now use an open transfer case with the computer controlling the brakes through the ABS system to give "AWD". As you've found, its not as good!

Butch 02 Sierra Denali

Elwood 2000 · November 24, 2004

I have the Autotrac and honestly, I can't tell the difference between selecting that button or the "4hi" button. They both act the same, no matter what the traction situation is. I ususally just select "4hi" when I need it.

Blue-2001-Z71 · November 24, 2004

The differance between AUTO and 4HI on mine is very noticeable. AUTO is what I use for foul weather and light dirt roads since it allows the diffs to operate openly. Only use 4HI when off road or on a road socked in with snow, water, or mud. In 4HI the T-case locks up and forces the front and rear to turn at the same speed. I made this mistake once, made a U-turn on dry asphalt in 4HI and heard the tires howl and felt the axles start to bind.

AUTO on mine works great in rain for keeping the rear end in line when I take off from a stop or round a corner at speeds too fast for 2WD in the same conditions. I've never spun or lost grip in AUTO and am very pleased with how smooth and refined the system operates on the road.

I had a Jeep Grand Cherokee with a 5.2L and a "full-time 4WD" transfer case. Worthless is the only way to describe it when it came time to go off road. Trail rated my butt. Get it in deep sand and the left front and right rear would spin out, the opposite tires would just come to a halt in sand shallow enough to let my Z-71 roll through without a sweat in AUTO. No option to lock the front and rear together, but available 4LO. That did the same thing, just at a lower gear ratio.

Bottom line is I love it. This truck has the most flexible and adaptable drivetrain I've owned in some time. The only thing I'd change would be to add an air locker front and rear and wire them to that they both locked whenever 4HI or 4LO was selected.