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The GM400 gearbox of the Silver Shadow:

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The GM400 gearbox, technical information



The General Motors GM400 (code name 'TH400', 'Turbo-Hydramatic') is a 3-speed automatic gearbox. It's made of a cast aluminum construction with some parts being made of cast iron, such as the center support for the transmission assembly.
The length of the gearbox is 24.37 inch (62 cm), and the weight (without fluid) is 135 pounds (61 Kilo). The internal set-up of the planetary gears is longitudinal, which means that the gears are fitted in line. The maximum torque output is 400 foot-pounds (542 Nm). The gearbox uses ATF (Automatic Transmission Fluid) that is cooled in an externally fitted heat exchanger.



The GM400 gearbox, manufactured by General Motors.



From the start of production of the Silver Shadow and Bentley T models (1965) the GM400 gearbox was fitted to all LHD examples. RHD Silver Shadow and T models before chassis number 4483 (1968) and all Phantom V and VI models up to 1978 were equipped with the 4-speed Hydra-Matic gearbox.
Until 1967 the torque convertor contains a variable blade pitch stator to reduce heavy drag in case the gearbox is not in 'Park' or 'Neutral' while the vehicle is stationary with the engine running. The pitch of the blades is controlled by an electrical solenoid, connected to a two-way connector plug. The solenoid switch is controlled by the throttle linkage, and is operated by the accelerator pedal.
From some moment in 1967, possibly when stock of variable blades was exhausted, fixed blades were applied.
The GM400 gearbox was fitted until model year 1992. Its successor was called '4L80': '4' denotes four forward gears, the letter 'L' denotes 'Longitude' and the number '80' denotes the maximum weight, load inclusive (8000lb, 3628kg).




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