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Launched in 1999 on the Jeep Grand Cherokee, the Quadra-Trac II transfer case normally sent most power to the rear wheels. When a wheel lost traction, a gerotor pump increased hydraulic pressure on a multi-disc clutch pack, sending power to the front axle.
The pump used a rotor driven by the front drive shaft and the case by the rear drive shaft, increasing pressure to the clutch pack in proportion to their speed variation. Clutch discs were alternately splined to the front and rear drive shafts. Because Quadra-Trac II could immediately pressurize the clutch pack, it had a faster response than a normal viscous coupling. (Quadra-Trac II's low range had a 2.72:1 torque ratio and fully locked the center differential).
A similar pump was incorporated in the Vari-Lok differential; power transfer was proportional to wheel speed difference rather than torque difference. By avoiding the need to pre-load the differential to assure torque transfer (which causes wear in normal use), Vari-Lok could be virtually wear-free. Again, Vari-Lok differentials were faster and smoother than viscous systems, and the pump response could be precisely tuned to driving conditions, enabling the use of this system in the front axle as well as the rear.
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