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In 1946, the Allison Division of General Motors initiated heavy duty transmission operations at its headquarters in Indianapolis, Indiana. Since that time, Allison has become a world leader in the design, development, and manufacture of heavy duty automatics for the world truck and bus market. This paper traces the history of this effort and discusses key innovations and events at Allison over the past 50 years. Included are discussions starting with Allison's history, its first bus transmission in 1946, and first automatic transmission for on-highway trucks in 1954. This is followed by the development of both the second and third generation automatics and the innovations that are incorporated in these generations. The effect of these innovations is expressed in terms of customer benefits and market growth. Lastly, a view of future trends in automatic transmissions is provided.

The continuing improvement and development of off-highway working vehicles has demanded corresponding continuing advancements in transmission technology. As a result Detroit Diesel Allison will introduce, in 1973, the Allison CLBT 750 fully automatic transmission for service in the off-highway market of 20 to 35-ton trucks and 15 to 20-yard scrapers. The CLBT 750 transmission was designed as a five-speed unit with automatic shift modes for second through fifth ranges. Design activity on the transmission included development of a hydraulic controls system which would provide optimum shifting for all conditions of engine and vehicle speeds and loads while ensuring that those shifts, both up and down, are smooth, positive, and safe. The design activity also included development of a minimum size, high capacity, highly responsive retarder to ensure safe descent of steep grades.

The need for continuous and smooth power flow during shifting of power shift and automatic transmissions has long been recognized. In addition, over the years, vehicle users have demanded continuing transmission improvement in such areas as shift quality, clutch durability, overspeed protection, and performance. The factors-clutch size, clutch coefficient variation, gear ratio step, and clutch apply pressure characteristics-which influence shift quality (or smoothness) have been identified. Compensation for the first three of these can be achieved by controlling the fourth, the clutch apply pressure characteristics, by means described as clutch capacity modulation. A fifth factor which has an influence on shift smoothness in automatic transmissions is the proper scheduling of range shifts with respect to vehicle speed, engine speed, and engine throttle position. Several methods of shift point modulation have been developed, each with its own advantages and disadvantages.