All-Trac

All-Trac was a proprietary full-time symmetric four wheel drive system used on a variety of Toyota badged models from 1988 to 2000. It was considered a revolutionary advance for four wheel drive automobiles into the mainstream consumer market and its electronic/vacuum controlled locking center differential was rare in a passenger car. The system originated in Japan under the GT-Four moniker in 1986, but was not released in the U.S. until 1988 under the All-Trac moniker.

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 front differential distributes the power to the front wheels, while the center differential distributes power to the transfer case that will send power to the rear differential. In most of the models, when the transfer case is not locked the majority of the power is routed to the front wheels as would be in a Toyota FWD automobile.

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 power distribution to be 50/50 between the front and rear of the vehicle. The wheels themselves are not always going the same speed because there are still the open front and open rear differentials allowing for differences in wheels speed from left to right. The final result of this is that if the vehicle is in a situation where traction is uneven or poor (i.e. muddy or snowy), all four wheels will have power transmitted to them, allowing the vehicle to use the wheels that still have traction to move the car. However, due to the open rear and front differentials, if the road conditions are very slick the wheels with the least amount of traction will still spin at a faster rate then those that do have traction.

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Wikipedia