For the Celica, the original ST165 GT-Four of October 1986 had "Center Diff. Lock" with switch located near the hand brake, and starting from the facelift model of October 1987 came with transfer case contains a viscous coupling unit that provides 50/50 power distribution to the front and rear differentials at all times, as in the Previa All-Trac models. 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.