Consumers have long had the need for vehicles that could ferry loads of passengers anywhere, anytime. These days, crossovers and big SUVs are the default choice for that particular job. But back before every soccer mom in America decided that "going commando" meant driving Civics dressed up as Hummers, consumers had the option of buying four-wheel-drive boxes with plenty of room for the whole family.
The Astro Van has used two different kinds of all-wheel-drive system. Older vans used what we now consider "conventional" AWD; these systems use a "viscous coupling center differential" containing a silicone fluid. When plates inside the VCCD move closer together, this "shear thickening" fluid routes a certain amount of power from the default axle -- the rear axle, in this case -- to the supplemental axle -- the front -- to gain extra traction. These VCCD systems can theoretically tolerate slightly different gear ratios in the front and rear, but it's never advisable.
In 1999, GM took what might seem to be a step backward by installing a New Process 136 model transfer case in place of the old VCCD. The NP136 was functionally identical to a typical four-wheel-drive transfer case, locking the front and rear axles together with a chain drive instead of a shear-thickening fluid. When the rear wheels slip, clutches inside the NP136 engage the front axle, which then spins at the same speed as the rear axle. So technically, the Astro used "part-time four-wheel drive," not all-wheel drive. Under no circumstances can you use different gears front and rear with a FT-4WD system; in fact, even installing shorter or taller tires on the rear can break the Astro's transfer case.
