acting on the rear unit ring gear had a tremendous mechanical advantage. Since the rear unit ring gear with its attached reverse unit sun gear and the reverse unit ring gear were both locked to the transmission case, the planet carriers and driveshaft could not turn. As such, it provided an effective driveshaft mounted parking brake to be used alone or supplementing the hand brake. The first-generation Hydramatic (not the Controlled-Coupling version that succeeded it in 1956) did not have a separate park position as found in modern automatic transmissions. The driver had to shut off the engine and then place the transmission in reverse in order to lock the driveline to prevent the car from moving. Also, the original Hydramatic required periodic band adjustments as a routine maintenance item that later versions did not. Early 1940 model Oldsmobiles with Hydra-Matic Drive could be started with the transmission selector lever in any position. The car would then start to move unless the transmission lever had been left in N, neutral. The all cast-iron Hydramatic was the heaviest automatic transmission ever produced for automobiles. The heaviest of them all was the Truck Hydra-Matic version offered by GM Truck and Coach Division in its line of light- and medium-duty trucks and conventional buses, as well as with its transverse-mounted gas L6 engined transit buses produced until 1963. That particular version weighed in at an incredible 655 pounds when equipped with the angle drive for the transit bus application, while the ¾ ton and up pickup truck model (HM270) still tipped the scale at a solid 435 pounds. When coupled to GMC's heavy V6 powerplant of 1960-1962, the powertrain weight was not too much lighter than the weight of the entire body of a ¾ ton P-2500 model pickup truck. Even its successor, the Controlled-Coupling Hydramatic was reviled by shop mechanics having to remove or reinstall such a unit, as they, too, were quite heavy when compared to other contemporary units. In the end, the true Hydramatic was rendered obsolete because of its cost, both in raw materials used as well as the machining needed. The successor, Turbo Hydramatic, was a much simpler, lighter, and cheaper, if less efficient, transmission. See also List of GM transmissions GM automatic transmission fluid history Notes External links General Motors Powertrain Division World's First Mass Produced Automatic Transmission (YouTube), Weber State University, Ogden Utah 2017 Automatic transmission