Here are a list of Jeep 4wd Transfer Case Systems that are used on the Jeep Vehicles and a brief description on how these systems work.
(Information from various Jeep Model owners manuals).
Command Trac - Is a part time transfer case. 2Hi, 4Hi & 4Low Originally used on the XJ Cherokee. Ratio changes can be operated shift-on-the-fly. Command-Trac should only be driven in 4WD on low-traction surfaces due to the front and rear axles being locked together (no differential action in the transfer case). Driving in 4WD on dry pavement causes excessive wheel and drivetrain wear. Four-wheel modes are most commonly used for wet/slick surfaces or extreme weather conditions (rain, snow, etc.) (4H), towing (N), and off-road activities (4L).
Command Trac II - Uses the same features and mechanical operation as Command Trac but uses a console mounted switch to control the Part Time transfer case as opposed to a floor mounted lever.
Rock Trac - Rock-Trac is similar to Command-Trac but uses the New Venture Gear and adds locking differentials and 4:1 low gear ratio. On 2007 models an electronic front sway bar disconnect was added and the locking differentials switched from air-actuation to electronic actuation. This can be found on the current Jeep Wranglers
Select Trac - This system has the same settings as Command-Trac but instead of locking the front and rear drive shafts together in the "4Hi" setting the drive shafts were coupled together using a viscous-type coupler that works the same way as a viscous limited-slip differential does; which allowed the vehicle to be driven in "4Hi" on dry pavement.
Select Trac II - This system is similar to Selec-Trac I but uses a console mounted switch to control the MP3022 transfer case as opposed to a floor mounted lever. Selec-trac II also differs with its lack of a part-time 4WD setting
Quadra Trac - This is a Full Time 4wd system. This system included a differential to shift torque between front and rear which could be locked with vacuum. This system offered only 4WD Hi modes in full-time or part-time (locked) modes, while the 1339 offered a full-time and part-time low range of 2.57.
Quadra Trac I - Quadra-Trac I was introduced in 2004 and works similar to the Quadra-Trac II system, but eliminates the "4 Lo" and "N" or neutral modes. By excluding these modes it creates a system that requires no driver input.
Quadra Trac II - This system was introduced in 1999 and it employs a two-speed chain-driven transfer case featuring three modes of operation, "4-All Time", "N" or neutral, and "4-Lo". In "4-All Time", torque is applied to the rear wheels under normal driving conditions. If the rear axle starts rotating at a significantly higher rate than the front axle, hydraulic pressure builds up in the gerotor and causes the clutch pack to progressively transfer torque to the front axle until both axles return to the same speed. "N" or neutral mode is intended for towing the vehicle. In "4-Lo", the front and rear axles are locked together through a 2.72 gear reduction ratio. This can be found on the Jeep Cherokee & Commander.
Quadra Drive - The Quadra-Drive system was introduced in 1999 and is based on the Quadra-Trac II system but adds limited slip differentials to the front and rear axles to create a four-wheel drive system capable of not only directing torque to the axle with best traction but to the individual wheel on an axle with best traction.
Quadra Drive II - Quadra-Drive II uses the New Venture Gear NV245 transfer case mated to front and rear axles containing electronic limited slip differentials.
Freedom Drive I - Freedom Drive I is a light duty full-time electronically controlled all wheel drive system with a locking mode to set the front/rear torque split for especially slippery conditions in the Jeeps. This can be found in the Jeep Patriot.
Freedom Drive II - Freedom Drive II uses the same hardware as the FDI system but adds a continuously variable transmission to simulate the benefits of a 19:1 low-range transfer case for off-road use. This function is an alternate program in the CVT and is not a transfer case function. FDII also adds hill descent control system, off-road tuned traction control, and electronic stability program. The Patriot with FDII also feature longer suspension travel, skid plates, tow hooks, and a full-size spare tire. This enables the FDII-equipped Patriot to wear the "Trail Rated" badge from Jeep.
Proactive automatic all wheel drive System - In normal conditions, 100% of torque is transferred to the rear axle. This system is sold under the name of Quadra-Trac I, however it is not a full-time all wheel drive, as on the other Grand Cherokees. This is a heavy duty system designed to handle larger power and output like from the Jeep Grand Cherokee SRT8.
Jeep Brake Traction Control System - or BTCS is a traction control system that uses the vehicle brake system to redirect torque from a slipping wheel or wheels to the wheel or wheels with the most amount of traction.
Jeep Electronic Limited Slip Differential System - These systems are alternatives to a traditional limited-slip differential. The systems various chassis sensors such as speed sensors, anti-lock braking system (ABS) sensors, accelerometers, and microcomputers to electronically monitor wheel slip and vehicle motion. When the chassis control system determines a wheel is slipping the computer applies the brakes to that wheel. A significant difference between the limited-slip differential systems listed above and this brake based system is the brake based systems do not inherently send the greater torque to the slower wheel.
Jeep Vari-Loc Differentials -
Eaton EGerodisc - A hydraulically-operated electronically controlled limited slip differential that is capable of providing variable torque up to full axle lock. These can be found on the Jeep Grand Cherokee WK & Commander XK.
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