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Jeep uses a variety of four-wheel drive and all-wheel drive systems on their various vehicles.

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Command-Trac

Introduced along with the Jeep Cherokee (XJ) in 1984, Command-Trac was standard on XJ models built with 4WD. This system offers a basic "shift-on-the-fly" transfer case, for manual ease and assist while engaging. Although Command-Trac does include 4x4 capabilities, it can only be driven under temporary usage; due to the axels locking and all four wheels rotating in unison giving out extra traction, therefore it cannot be driven on dry pavement under most circumstances. 4-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). Even though Command-Trac was created over 20 years ago, it is still readily available in Jeep vehicles (except Grand Cherokee and Jeep Commander) even now.
The Jeep Grand Cherokee is the only brand inside the company marque to totally discontinue this 4WD system from its lineup during mid-production inside a vehicle generation (unlike the XJ whose entire lineup was dropped rather than Command-Trac itself). The reason behind this was its poor sales along with the SE trim among consumers, an improved Selec-Trac and simplistic Quadra-Trac had the convience and comfort of "Full-Time" 4WD that Command-Trac obviously lacked for luxury SUVs; it was quickly dropped from the ZJ platform in 1996
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Selec-Trac

Selec-Trac also debuted with the compact Jeep Cherokee in 1984, borrowed from the AMC Eagle. It is a more common feature in the upscale version Jeep Wagoneer (XJ), and Dodge Durango models (1998-present). It has a shift-on-the-fly transfer-case like Command-Trac but unlike it, Selec-Trac has option of full-time 4WD (making 2WD somewhat of a novelty). Full-Time 4WD has the ability for the front and rear axels rotate to at different speeds, making driving on dry and wet surfaces possible year-round without shifting back to 2WD again; Yet all other 4x4 modes from Command-Trac are still available.
other Jeep vehicles used Selec-Trac like Jeep Grand Cherokee, before being discontinued on that platform in favor of the electronically-controlled Quadra-Drive II AWD system in 2005. Selec-Trac is currently only available for the Jeep Liberty lineup.
Selec-Trac uses the New Venture Gear NV242 transfer case.
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Quadra-Trac

The Quadra-Trac name is used on a variety of full-time 4WD systems. The first version was launched in 1973, with a new unrelated system used in the 1980s. Yet another system carried the name in the 1990s.
Quadra-Trac was the trade name for the Borg-Warner 1305 and 1339 gearcase. It was a chain-drive system introduced in 1973 on the Jeep SJ trucks behind the General Motors Turbo-Hydramatic 400 automatic transmission. It included a limited slip differential to shift torque between front and rear which could be locked with vacuum. The 1305 lacked a low-range, while the 1339 offered an optional 2.57 planetary gear.
The Jeep Quadra-Trac was differentiated from the open New Process Gear NP203 used by Dodge, General Motors, and Ford] in that it included a center limited-slip feature, in this case a clutch pack.
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The Borg-Warner system was replaced with a New Process Gear NP207-based chain-driven system in 1980.
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The NP207-based system was replaced with a New Venture Gear NV249-based chain-driven system in 1992.
The NV249 utilizes a "Viscous Coupler" to determine power transfer between the front and rear axles. The goal of this device is to provide smooth and efficient 4WD operation on dry surfaces--if a differential in speed occurs between the axles, heat buildup in the viscous coupler transfers power to the slower axle, providing some traction in off road conditions.
1993-1995 NV249 transfer cases used the viscous coupler to transfer power in both high and low ranges.
1995-1998 NV249 transfer cases had a low-lock capability, meaning a hard gear transferred power in 4LO.
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Quadra-Drive (WJ)

Quadra-Drive employed the New Venture Gear NV247 transfer case. This two-speed chain-driven transfer case uses a gerotor, a clutch pack coupled to a hydraulic pump, to transfer torque between the front and rear axles. The transfer case contains three modes, 4-All Time, Neutral, and 4-Lo. In 4-All Time, 100% of torque is sent to the rear axle in normal conditions. If the rear axle starts spinning at a 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. Neutral mode is intended for towing the vehicle. In 4-Lo, the front and rear axles are locked together and torque is split evenly between them through a 2.72 reduction gear ratio.
In Quadra-Drive, the NV247 transfer case is mated to front and rear axles containing Jeep's Vari-Lok differentials. Vari-Lok differentials also use a gerotor to transfer torque.
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Quadra-Trac II (WJ)

Quadra-Trac II also employed the NV247 transfer case but lacked Quadra-Drive's front and rear Vari-Lok differentials.
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Quadra-Trac I (WJ)

Quadra-Trac I consisted of the New Venture Gear NV147 transfer case, a version of the NV247 transfer case lacking neutral and low-range modes.
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Quadra-Drive II (WK, XK)

Quadra-Drive II consists of the New Venture Gear NV245 transfer case mated to front and rear electronic limited slip differential. It includes a Neutral mode and utilizes a 2.72 reduction gear ratio in low-range.
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Quadra-Trac II (WK, XK)

Quadra-Trac II consists of the New Venture Gear NV245 transfer case without Quadra-Drive's ELSDs.
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Quadra-Trac I (WK, XK)

Quadra-Trac I consists of the NV140 trasnfer case, a version of the NV245 trasnfer case lacking neutral and low-range modes.
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