Questions About The Awd System On Our Trucks

[diggerdan11]

So I have a couple questions that have been bugging me about the AWD system in our SS's. I have been looking all over and cant find the information.

 

I know that Obviously AWD means power is going to all 4wheels All The Time. unlike Auto 4wd where when it slips it locks into 4wd

 

Question 1

 

What is tha power ratio (how much power is going to the back and how much to the front)? I know its not Symetrical (50/50) System like Subaru has. So is it 45/55, 40/60, 35/65 what?

 

Question 2

 

Is it and adabtive System? For exampe I know that cars like say the Mitsubishi Evo or the Audi S4 can send power to whatever wheels needs it the most. Does the AWD system on our trucks adapt and send power where its needed or does it always stay the same?

 

Any Help would be greatly appriciated

 

One of the main reason I ask is because people are always asking me about my Truck. Most people think its 2wd and I have to say no its AWD and then I have to try to explain to them exactly what AWD means and why its different than 4wd or other setups. One of my co-workers has a newer Ford Escape and it has some goofy 4WD system, where it is in 2wd until it slips and then it locks into 4wd, but you cant lock it into 4wd yourself.

[2bfast]

So I have a couple questions that have been bugging me about the AWD system in our SS's. I have been looking all over and cant find the information.

 

I know that Obviously AWD means power is going to all 4wheels All The Time. unlike Auto 4wd where when it slips it locks into 4wd

 

Question 1

 

What is tha power ratio (how much power is going to the back and how much to the front)? I know its not Symetrical (50/50) System like Subaru has. So is it 45/55, 40/60, 35/65 what?

 

30/70 front to rear

Question 2

 

Is it and adabtive System? For exampe I know that cars like say the Mitsubishi Evo or the Audi S4 can send power to whatever wheels needs it the most. Does the AWD system on our trucks adapt and send power where its needed or does it always stay the same?

 

Power is generated to the wheels at all times

Edited January 19, 2011 by 2BFAST (see edit history)

[04CHASE]

NVG 149 Transfer Case Description and Operation

 

 

 

The 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

 

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.

 

© Copyright General Motors Corporation. All Rights Reserved.

 

Document ID # 873908

2003 Chevrolet/Geo Chevy K Silverado - 4WD

 

 

******************

 

Transfer Case Viscous Coupling Test

Tools Required

J 45382 Transfer Case Tester

 

 

 

 

To determine if the viscous coupling is operating correctly, use the following test procedure. Testing the viscous coupling operation by removing a propeller shaft and driving the vehicle will cause the viscous coupling to fail. The test will determine if the viscous coupling is faulty from a totally locked up failure, or if the viscous coupling fluid has leaked out from being severely humped.

 

 

Important

The Viscous Coupling Test is very sensitive to the temperature of the transfer case. If the transfer case is very hot, the time to turn will be faster. If the transfer case is cold, the time to turn will be slower.

 

 

1. Operate the vehicle for 24 km (15 mi), or until normal operating temperatures are reached.

2. Set the parking brakes for the rear wheels.

3. Position the transmission in NEUTRAL.

4. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.

5. Remove the front propeller shaft. Refer to Propeller Shaft Replacement - Front in Propeller Shaft.

6. Install the J 45382 into the transfer case front output shaft.

7. Using a torque wrench at 11 N·m (100 lb in) on the J 45382 , turn the front output shaft 90 degrees.

8. Note the length of time it takes to turn the front output shaft the 90 degrees.

* If the front output shaft turns slow, and takes at least 10 seconds, the viscous coupling is operating correctly.

* If the front output shaft will not turn, the viscous coupling or the planetary carrier assembly could be locked-up.

* If the front output shaft turns easily, and the 11 N·m (100 lb in) cannot be obtained, the viscous coupling is faulty.

 

 

© Copyright General Motors Corporation. All Rights Reserved.

[diggerdan11]

Oh WOW thanks Chase!!!!!

 

That Answered my question perfectly

 

That write up should be Pinned in the AWD section so everyone can read that!!!

[smoothdime]

Oh WOW thanks Chase!!!!!

 

That Answered my question perfectly

 

That write up should be Pinned in the AWD section so everyone can read that!!!

 

Agreed!!! best explanation I've ever seen

[T-Bag]

bump for new members

[GLEN02777]

Not sure if this is my issue but this is the problem I'm have with my SS. when driving for about 30 min than pulling into a parking space I notes that there is some kind of binding going on an can't pinpoint where..

[Downeast Johnny]

Not sure if this is my issue but this is the problem I'm have with my SS. when driving for about 30 min than pulling into a parking space I notes that there is some kind of binding going on an can't pinpoint where..

 

Is your binding happening under extreme steering input? Quick guess is possible front hub probs. T- cases seem to be pretty durable, hubs do go out.

[demonte1997]

This is a great thread. Huge thanks to CHASE for the NVG information. It's a perfect description of how the system works.

 

My experience with this transfer case in my 2002 SD below if it is helpful to anyone:

 

My transfer case is completely shot. The viscous coupling on it is done. When I take tight turns with my truck, it will lurch a little and it almost feels like the rear wheels are dragging because they are fighting with the fronts (the viscous coupling is not allowing for slippage between the front and rear so they are all trying to rotate at the same speed). This may or may not be the same issue GSILVERADOSS is experiencing. In addition to issues with tight turns, my truck makes a rhythmic clunking sound when I'm on the throttle that has been pinpointed likely to the chain smacking the inside of the case OR the viscous coupling just rattling around in there. That said, it's actually quite amazing how stout this units are. The damn thing has held up like that for the 6,000 miles I've owned it and it has about 176,000 on it now.