The All-Trac system had five main parts: the front differential, rear differential, center differential, the transmission, and the
transfer case. The
transmission, front differential, center differential and transfer case are all one large assembly and are connected to the
transversely mounted engine. The transmission
bell housing, front differential and center differential can be separated from the transfer case sub assembly. The transmission output shaft feeds power into the front/center differential assembly (the center differential is contained within the front differential). The center differential applied equal torque to both the front and rear wheels. For all manual transmission models except
Previa, and the 1988 and newer
Celica, there is a small button inside the vehicle labelled
Center Diff. Lock and when activated, the transfer case locks, resulting in the front and rear differential input shafts being driven at the same speed. The result of this is that if the vehicle is in a situation where traction is uneven or poor (i.e. muddy or snowy), the car can move with traction in either the front or rear wheels. 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. ==Model series==