The Allison 500 Series transmission is an automatic transmission that is manufactured by General Motors. It is used in medium duty vehicles. For example, regular, dump and small concrete trucks; they are also used in delivery trucks and some buses.
The AT500 Allisons does not have a "park" feature built in to them like an automatic transmission that you would find in a passenger vehicle. The park is customarily achieved through an air brake system used in the truck for its regular braking system.
Some of the units also contain a power take off (PTO) option that allows the connection of a separate system where it draws its power directly from the transmission. Lets say you have a dump truck, the hydraulic system that powers the piston to lift the bed is powered through the PTO.
Besides being very popular in its home market of the U.S. it is also popular in Canada and Mexico. In Central America, most notably in Panama, Nicaragua and Guatemala; In South America, most notably in Argentina, Peru, Colombia, Chile, and Venezuela. It is also used in most major markets around the world.
There are several different versions of the Allison 500 Series Transmission