General Motors celebrated the sixtieth anniversary of the automatic transmission

Friday 25th June 1999

General Motors celebrated the sixtieth anniversary of the automatic transmission. Alfred P. Sloan, GM’s first chairman, had been more than impressed when he took his inaugural ride in a 1940 Oldsmobile equipped with brand new Hydramatic transmission. ‘For fifteen years, I have felt that the gearshift lever had no place in a really modern car,’ Sloan wrote at the time. ‘I feel very strongly that it is only a matter of time when every car must have this kind of a transmission.’ The Hydramatic transmission had no torque converter. Instead, it used a two-element fluid coupling along with three separate planetary gearsets. Together, this mechanism created four forward speeds along with reverse. The standards were 3.82:1, 2.63:1, 1.45:1, and 1.00:1 when used in passenger cars. They were somewhat different in truck applications.The Hydramatic transmission had twin pumps to keep pressure in its hydraulic system. These units also circulated lubricant around the internal components. The front pump drew power from the fluid coupling, which pressurized it from the moment the driver started the engine. The rear pump was turned by the transmission’s output shaft, which minimized its pressure but simplified the design.When starting out, power ran from the Hydramatic’s front planetary gearset to the fluid coupling, then in to the rear gear, reverse gears, and output shaft. This caused a great deal of initial slippage, which actually enhanced performance when the vehicle was first starting out.Unfortunately, many Hydramatics had trouble moving across the 2-3 shift range, due to lack of synchronicity between the two bands and double clutches that completed the task. This sent many vehicles to the shop for frequent adjustments, which only the most experienced mechanics could perform properly. Another limitation of the transmission was that it had no parking gear – drivers had to engage the emergency brake to keep their car from rolling away. On the other hand, its internal architecture allowed those with dead batteries to roll, or “pop” start, their vehicles. Quirks aside, the Hydramatic transmission did its duty as the first of a long line of innovative GM transmissions. Its designers did the hard job: they proved the concept would work. It would be up to their successors to apply the finishing touches.

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