AWD Rocked In the Snow

[ToeJam]

I just spent four days in Northern Central Washington. Lots of snow and daytime temps were in the high teens, low twenties. The main roads were a good mix of ice and compact snow. The non-maintained roads I used to get to the cabin we were staying in were plowed, but not sanded and not traveled enough to pack the snow down (think sandy beach).

 

The AWD on the Denali was just awesome. The power transfer to the front was flawless and unnoticable. Point the truck and that's where it went. Still lots of power to slide the back out if I "needed", but very easy to control. I stopped on more than one steep hill just to see if I could make it slip. With good, gradual control the truck launched with no tire slip and accelerated pretty good!

 

The quadrasteer was also insane in the snow and ice. Truck just moved around like it was on ice skates. Even the anti-lock brakes did an amazing job and I'm pretty impressed with the stock BF Goodrich tires.

 

And as always, the XM RULED for the five hour trip. The first few hours of the trip home today was in -6 degrees F weather, and the entire truck performed like it was 80 degrees outside.

 

One thing I didn't like - When I was starting the truck up in the morning (say 10 degrees F), the engine was very slow to turn over. It always started the first time, but was slow to turn and had a couple of belt squeaks. Is this normal for cold weather? I'm sure the squeaks are, but the slow turning bothers me....would a new battery fix the problem?

[HillHound]

The AWD on the Denali was just awesome.

I agree. My wife's Trailblazer has it (AutoTrac that is) and I've been impressed with it too. Sometimes we even use it if we have to drive in heavy rain.

 

I love driving in deep snow!

[Lance Link]

I drove 120 miles through a blizzard a couple weeks ago and the Yukon was unflappable. Almost like I was driving on dry pavement. I had it in 4Hi most of the time but was making great time. I love the XM as well.

[JayMan]

One thing I didn't like - When I was starting the truck up in the morning (say 10 degrees F), the engine was very slow to turn over. It always started the first time, but was slow to turn and had a couple of belt squeaks. Is this normal for cold weather? I'm sure the squeaks are, but the slow turning bothers me....would a new battery fix the problem?

 

edit by Jerry

No, slow cranking is NOT normal. They usually crank quite briskly, even at -20 F. Since the truck is so new, I would get the dealer to do a load test while cranking. I would also check for the following:

 

1. Poor connections

2. Starter problem

3. Bad battery (You get a dud once in awhile)

4. Parasitic drains

5. Grade of motor oil

 

If you were using something like a regular mineral oil 10W-30, that could really thicken and affect starting performance. A regular 5W-30 oil should still be ok at 10 F.

 

I like to use Mobil 1 0W-30 in winter. It dipped to -34 F this morning and no problems.

 

I've had quite a few vehicles with serpentine belts and they always seem to squeak or chirp if started cold and/or damp. Nature of the beast?

[GnatGoSplat]

Did you experience any wheel slip? I've always been curious how fast the AWD transfers power from the wheels that slip to the wheels with grip.

[Butch 02 Denali]

Here's a nice description of the AWD transfer case and it's operation. The AWD system is a 100% biasing system, that means it can transfer 100% of drive to either the front or the back. It is not the same as Auto trac.

 

 

 

NVG 149 AWD Transfer Case Description and Operation

 

The NVG 149 RPO NP3 is a single speed, single mode transfer case. The mode is full-time all wheel drive. It has a planetary differential gear set that splits the torque, normally 38 percent to the front wheels and 62 percent to the rear wheels.

 

The NVG 149 utilizes magnesium housings. Proper fasteners, brackets, and fill/drain plugs must be used to prevent galvanic corrosion. The planetary differential uses the carrier (6) as the input. The annulus gear (4) connects to the rear output shaft (5) and rear wheels. The sun gear (3) connects to the front output shaft (7) and front wheels through the chain (8) and sprockets. The viscous coupling (2) consists of a sealed housing filled with a high viscosity silicone fluid and thin steel plates alternately splined to the inner and outer drum. The inner drum is connected to the input shaft (1), and the outer drum to the sun gear (3). Whenever there is a speed difference between the front and rear wheels, the inner and outer plates of the viscous coupling spin relative to each other and the silicone fluid provides resistance. The resistance was tuned to be high enough to bias power quickly to the wheels with traction, and low enough to prevent binding in a tight turn on dry surfaces. This is the most common way the viscous coupling is activated, the shear mode. If the speed difference is high, the coupling can lock or hump. This "hump" occurs when the heat generated, expands the fluid inside the housing, changing the fluid dynamics between the plates. This results in pressure between the plates, forcing them into contact with each other, similar to a clutch pack. In the hump mode, the coupling can bias torque 100 percent to one axle, if required. Situations requiring this are extreme such as backing up a steep gravel grade or climbing over off-road obstacles. The viscous coupling is not serviceable; it must be replaced if defective. This is because each viscous coupling is calibrated for optimum vehicle performance for both the shear and hump modes. If the viscous coupling is in the "hump" mode too long, severe damage will occur.

 

To prevent damage to the viscous coupling, DO NOT:

 

Tow with only two wheels down

Drive without one propshaft

Drive with a "donut" spare tire for an extended period of time

Operate the vehicle on a hoist to diagnose the driveline components

Power Flow - No Wheels Slip

 

 

During normal operation, 100 percent torque is delivered to the input shaft (1) from the transmission. The torque is split to 62 percent to the rear output shaft (2) and 38 percent to the front output shaft (3), by the planetary differential. Because there is not a loss in traction or slip in the front or rear wheels, the viscous coupling is locked in place and there is no "shear" mode or "hump" mode involved.

 

Power Flow - Front Wheels Slip

 

 

When traction is lost at the front wheels, the viscous coupling works in conjunction with the differential to bias the torque more to the rear wheels. The rear torque goes higher than the 62 percent, and up to 100 percent torque to the rear output shaft (2). The torque at the front output shaft (3) is lowered from the 38 percent, to as low as 0 percent torque.

 

Power Flow - Rear Wheels Slip

 

 

When traction is lost at the rear wheels, the torque is biased to the front wheels. The torque to the front output shaft (3) goes higher from the 38 percent, up to 100 percent torque. The torque at the rear output shaft (2) is lowered from the 62 percent, to as low as 0 percent torque.

 

Customers may have concern that the transfer case is not operating properly because one set of tires spun for a brief period. It is normal for one set of tires to spin until the viscous coupling engages.

 

Turning off the traction control switch, if equipped, enhances the function of the viscous coupling. The viscous coupling, as described above, engages by heat. Allowing one set of tires to spin or slip for a brief period will generate heat in the viscous coupling. The engine speed should be kept at a constant speed during the brief spin of the tires. Pulsating the engine speed or hard acceleration will not allow the viscous coupling to operate properly.

 

 

Butch 02 Sierra Denali

[ToeJam]

Did you experience any wheel slip? I've always been curious how fast the AWD transfers power from the wheels that slip to the wheels with grip.

Well, it's hard to say since I was on snow and ice. But basically I never felt the front wheels "kick in". There was never a surge of power or any pulling on the steering wheel. The truck just kept moving and turned exactly where I wanted it to go. I also didn't loose any headway when climbing if the back started to slip. The power transfer was completely fluid and flawless.

[GnatGoSplat]

Hmmm, sounds good.

I've been debating whether or not to switch my truck to AWD from manual 4x4. I can pick up a low-mileage Escalade transfer case (NVG149, same as the SS and Denali) for $300 shipped. Although I like my 4x4, it's hard to anticipate just when I'm going to need it as sometimes snow/ice can be patchy. Not to mention it takes a few seconds to kick in and out. Auto 4x4 might work, but AWD sounds more seamless and quicker.

 

I've never used 4LO on my truck EVER, so losing 4LO won't be a big deal.

[PewterC3]

You never felt transfer because the power was already there. Under normal operation you've got 38% of the torque going to the front wheels already. That only changes if slip is detected. Don't compare it to "on demand" 4WD systems, they're not the same. Plus with 4WD systems you can still get the "ass trying to pass the front" syndrome in snow and ice, something you won't experience with AWD.