 Neither the Previa nor the Celica have a lockable transfer case, but the viscous coupling can be temporarily disabled for testing purposes via a selector switch on the transfer case assembly.The vehicle should not be driven with the viscous coupling disabled as transfer case or transmission damage will occur.Some Celicas were equipped with a Torsen torque-biasing rear differential which apportions torque to the left and right wheels depending on available traction, up to a mechanically determined maximum torque bias ratio or TBR.