Ok guys I am NOT a certified mechanic I have learned all I know from working in my garage. This is all based from my experience with my Olds Bravada and information I have gathered over the years. If there is any info you find to be incorrect post the correct info and help me out lol.

The Oldsmobile Bravada's have the "Smart-Trak" system and have automatic transfer cases but they were changed through the years. Also the front axle and differential of the Bravada is different as well it is engaged full time there is no vacuum actuator attached to the diff (I know this to be on the 1st and 2nd gen, I'm not sure on the 3rd gen). A note to Bravi owners we DO NOT have 4WD these are ALL-WHEEL-DRIVE vehicles we do not have a 4LO option. Our transfer cases are similar to that of an AWD Audi, just because all 4 wheels can spin doesn't mean it's a 4wd.

All first gen Bravadas (1991 to 1994) were produced with Smart-Trak all-wheel drive. The Borg Warner 4472C (BW4472C) transfer case is used in these trucks. ALL WHEELS REMAIN ENGAGED FULL TIME. The viscous clutch of the BW4472C compensates for the varying speed of the front and rear axles, allowing the all-wheel drive to stay engaged at all times without damage, even on dry pavement. It transmits torque at a ratio of 35% front and 65% rear and directs power to the wheels with traction whenever slip is detected. This is a two-piece case, and it is chain-driven.

Second gen Bravadas were equipped with either the BW4472 or the New Process Gear 136 transfer case. The BW4472 was used in 1996-97. The second generation Bravada began production in 1996 (I don't belive there were any bravadas produced in 1995). After 1998, Bravadas with Smart-Trak use the New Process Gear 136 transfer case. This is an "active on demand" computer-controlled transfer case. The vehicle normally uses rear-wheel drive, and when wheel slippage is detected, an electrically actuated clutch engages the front axle. No driver input is needed to engage the all-wheel-drive system, the vehicle has sensors to detect the speed of each axle.

The third gens I am not 100% on and gathered most of the info on them from internet and gm tech sites ect. Third gen Bravadas, built on the new GMT360 platform, had more drive choices. All-wheel-drive Bravadas use the New Process Gear 126, a slightly smaller, but similar computer-controlled transfer case than the NP136 used in prior years. It functions the same way as the NP136. Other Bravadas use one of the two-speed New Process Gear transfer cases, the NP226, NP236 or NP246 (I think similar to a second gens NP236 with the 4-button). This setup allows the driver to operate the vehicle in rear-wheel drive only, in computer-controlled all-wheel drive or in driver-activated four-wheel drive. The driver may select between the 4HI, 2HI and 4LO modes using an electronic switch on the dashboard. The 2002 to 2004 Bravada could also be ordered as a rear-wheel drive vehicle without four-wheel drive capability.

Again I am NOT a certified mechanic but I hope this helps out. People seem to be in a "fog" on the Smart-Trak system and i hope this helps clear it up a bit.