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Quadra-Trac II

Launched in 1999 on the Jeep Grand Cherokee, the Quadra-Trac II transfer case normally sent most of the power and torque to the rear wheels. The moment a wheel lost traction, a gerotor pump applied hydraulic pressure to a multi-disc clutch pack, sending power to the front axle.

The gerotor pump used a rotor driven by the front drive shaft and the case by the rear drive shaft, creating a pressurized oil flow 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 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 therefore proportional to wheel speed difference rather than torque difference. By avoiding the need to pre-load the differential to assure torque transfer (so that normal use causes wear), the Vari-Lok could be virtually wear-free. Again, compared to systems using a viscous coupling (which also sense speed), Vari-Lok differentials responded more quickly and smoothly; and the pump response could be precisely tuned to driving conditions, enabling the use of this advanced system in the front axle as well as the rear.

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