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<H1>Info - Driveline Characteristics for All-Wheel and Four-Wheel Drive Systems #01-04-18-001A - (10/24/2002)</H1>Driveline Characteristics For All-Wheel and Four-Wheel Drive Systems


2003 and Prior Light Duty Truck Models


with All-Wheel Drive (AWD) or Four-Wheel Drive (4WD)


This bulletin is being revised to update the model years, models and information. Please discard Corporate Bulletin Number 01-04-18-001 (Section 04 -- Driveline/Axle).


The purpose of this bulletin is to help explain the operating characteristics of 4WD/AWD systems.<A name=ss1-1181935><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1181935&pubid=297&evc=sm#ss1-1181935" target="_blank">
<H5>Definitions</A></H5>AWD vs. 4WD


The very basic difference between AWD and 4WD is the intended usage of the systems.


AWD is usually intended for on-road use in inclement weather conditions, while operating smoothly on dry pavement by allowing for a difference in speed between the front and rear axles while turning. These systems do not have low range gearing for the transfer case.


4WD is intended to be used in low traction or off-road situations. Part time systems do not allow for a difference in speed between the front and rear axles while turning. This system effectively locks the front and rear propeller shafts together. When turning, the tires must slip to allow for the different turning circle of the front and rear axles, which is why this is intended for low traction or off-road use. These systems have low range gearing for the transfer case.PART-TIME 4WD


Part time 4WD refers to vehicles equipped with a transfer case to split power between the front and rear axles of the vehicle. This traditionally is a 2-speed selectable transfer case that can be shifted into 2HI, 4HI, 4LO and usually a Neutral position. There is no device to allow for a difference in speed between the front and rear axles. Examples of vehicles with this style of transfer case would be a Silverado with a manual shift transfer case (a shift lever on the floor) (RPO NP2) and a Sonoma with a push button transfer case with a 2HI, 4HI and 4LO position (RPO NP1). The RPO codes for this style of transfer case are NP1 (NVG 233, 243, 263) or NP2 (NVG 231, 241,261, BW 4401, 4470). FULL-TIME 4WD


A second version of a 4WD transfer case is a full-time 4WD transfer case. This style of transfer case has an open differential to allow for different speeds between the front and rear axles and operates similar to an AWD system. This transfer case can be locked to operate like a 4WD transfer case and or uses a traction control system to assist in low traction situations. An example of this type of 4WD is the H2. RPO code is NR4.AWD


There are two different categories of AWD systems. The first category is true AWD. This type of transfer case divides torque to the front and rear axles at all times. This ratio can vary depending on the system, but is usually 30/70% front to rear split but can vary depending on traction conditions, up to 100% front or rear. This type of transfer case can have a viscous coupling along with a planetary gear set to allow for difference in speeds between the front and rear axle or an open type of planetary gear set differential. An example of a vehicle with this type of transfer case is a 2002 Denali. RPO codes for this type of transfer case are NP3 (NVG 149, BW 4473) or NR3 (BW 4481).


The second category is an on-demand AWD. This type of AWD basically delivers torque only to a primary driving axle until a slip event occurs. At that point, the system electronically or mechanically will apply torque to the axle with traction. Depending on the type of system, this can provide up to 100% of the torque to the axle with traction. These transfer cases use an electronically actuated clutch pack, a hydraulically actuated clutch pack, or a viscous coupling to allow for a difference in speed between the front and rear axles. An example of an elec
 
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