General Motors introduced the AWD option in 1990.
From 1990 to 1998 it utilized a Borg-Warner 4472 transfer case with a viscous clutch. The BW4472 features a full-time viscous-coupled transfer case that will automatically transfers torque where it's needed. The viscous coupling acts as a torque-biasing device attached to the front and rear drive shafts in a lightweight magnesium housing mounted directly to the automatic transmission. One set of the discs turns with the front axle, the other set with the rear axle. The discs are surrounded by a high-viscosity silicone fluid that, when deformed, transfers power from the faster rotating discs (or the slipping wheels) to those rotating more slowly (the wheels with grip). A 35-65-percent front-to-rear power split is used under normal operating conditions, while up to 100 percent of the engine's torque can be transferred to the wheels with traction when needed. Road surfaces needn't be slippery for the AWD system to deliver a traction advantage. If climbing a hill or steep grade, for example, the differential will send more torque to the rear wheels to compensate for the extra weight. The AWD also accommodates for speed differences that naturally occur when a vehicle travels around a corner on dry pavement and adds superior cornering capabilities under all circumstances. The system also provides better durability than a rear-drive-only system because torque is continually shared between the front and rear axles.
Source: Truck Trend
From 1999 to 2005 it utilized a New Venture 136 transfer case. The NV136 transfer case is an automatic transfer case with two drive ranges. Shifting from rear wheel drive to four wheel drive is done automatically when the transfer case shift control module receives wheel rotating slip information from the speed sensors. The transfer case shift control module then engages the transfer case motor/encoder to position the transfer case from rear to four wheel drive. When the transfer case shift control module receives information that the wheel rotation is the same on both axles, the transfer case shift control module sends position information to the motor/encoder to put the transfer case back into rear wheel drive. The 2HI position is for normal driving with the rear wheels pushing the vehicle. The 4 wheel drive position is for driving through sand, snow, mud, gravel, or heavy rain at normal or slightly below normal speeds. The NV136 is manufactured at Syracuse New York by New Venture Gear a division of New Process Gear for General Motors.
Astro / Safari Van: 1990 - 2005
Envoy: 2002 - 2005
S10 Blazer & Pickup: 1983 - 2002
Syclone: 1991 - 1991
Trailblazer: 2002 - 2005
Typhoon: 1992 - 1993
Lock-Right locker PT 1935
3.73 all S10 ZR2 / Sonoma Highrider 1994-2005
Parts swap information
Using this information, one can see the following.
The steering knuckle/bearing assembly interface changed in 95.
The CV/diff interface changed in 97
The diff/drive shaft/transfer case interface changed in 99.
The drive shaft/trans case interface changed in 92.
Everything aside from the diff/drive shaft/transfer case interface changed in 2003.
The majority of the differential internal components remained unchange throughout production.
AWD as 2WD/RWD
Can the AWD be turned into 2WD/RWD? This question is often asked by those seeking to squeeze a few more MPG out of their vans, but it is also asked by those with damaged front differentials.
For those seeking more MPGs, don't bother it's not worth the headache. Check out GasSavers.org, EcoModder.com and other high MPG sites for tips on driving better. If the van is 1996 or later that has an ODB-II port, it is highly recommended to purchase a ScanGaugeII or similar product. Monitoring RPMs, Present MPG, Current Trip MPG, and the Throttle Position Sensor has resulted in 25% increases in MPG.
For those with damaged differentials, while generally not recommended, it is possible. Be forewarned though, done improperly can damage the wheel bearings, cause the front wheels to fall off, and damage the transfer case. Since there is no way to deactivate the transfer case in 1990-1998 vans, the possibility of damage can not be removed. It could be permanently locked together come time to put the AWD parts back in, thus requiring it to be rebuilt or replaced.
The biggest mistake to make is to just remove the half shafts and drive shaft and think that the van is good to go. The wheel bearings will definitely fall apart causing the front wheels to fall of and the transfer case will lock the clutch to keep the front and rear output shafts turning the same speed. Being constantly locked up causes the fluid in the transfer case to solidify locking the clutch together.
The wheel bearings are designed to have the half shaft's stub shaft and nut hold them together. Disassembling the half shaft and re-installing the stub shaft and nut will hold the bearings together. Keep the front wheels off the ground anytime these are not installed to avoid damaging the wheel bearings.
1990-1998 vans, there is nothing that can be done to prevent damage to the transfer case. It's the luck of the draw. Might be fine, might be damaged.
1999-2005 vans, the transfer case can be de-activated by pulling the 20 amp ATC fuse located in the engine compartment's fuse box. Failure to do so will cause the computer to activate the clutch with the possible results being a locked together clutch and a burnt out servo motor.
While several vans have accumulated thousands of miles with AWD components removed, doing so is done at ones own risk. If there is any question about a van's safety, please keep it off the road.
Source: http://www.astrosafarivans.org & http://www.astrosafari.com