ATSG Tech Manual GM 8L90 - HYDRA-MATIC 8L90 Manual - The 8L90 transmission is a fully automatic eight speed rear wheel drive transmission that is available in both two and four wheel drive configurations. It features clutch to clutch shifting which provides eight forward and one reverse gear. It also uses a four element lockup torque converter, four planetary gear sets as well as five multiple disc and one mechanical clutch set. The 8L90 utilizes an off axis chain drive pump to maintain fluid volume and operating pressures.
The externally located TCM controls the changing of gear ratios through the use of solenoids and also controls transmission line pressure through the use of a variable force motor. The TCM will also control converter clutch operation.
The 8L90 has undergone some parts changes from it’s inception until the present.
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