This manual provides comprehensive information for the Allison MTB 643(R) automatic transmission. Engineered for heavy-duty applications, this transmission is designed to be compatible with a wide range of diesel engines, from 5.5L to 13.0L, and capable of handling up to 300 horsepower. Its robust construction and 4-speed forward, 1-reverse gear configuration make it suitable for demanding tasks in buses, school buses, motorhomes, specialty vehicles, medium-duty trucks, and fire apparatus. The MTB 643(R) features electronic shift control with hydraulic backup and an integrated cooling system, ensuring reliable performance and durability in challenging operational environments. The purpose of this Service and Repair Manual is to equip owners, mechanics, and technicians with the knowledge necessary for the proper maintenance and repair of the Allison MTB 643(R) transmission. This English-language manual details essential procedures, including scheduled maintenance intervals, fluid capacity and type specifications, filter replacement guidelines, and diagnostic capabilities. It covers technical data such as torque capacity and fluid specifications, along with information on its electronic control systems. By following the instructions and specifications within this manual, users can ensure the optimal performance, longevity, and reliability of their Allison MTB 643(R) transmission.