Hydramatic (also known as Hydra-Matic) was an automatic transmission developed by both General Motors's Cadillac and Oldsmobile divisions. Introduced in 1939 for the 1940 model year vehicles, the Hydramatic was the first fully automatic mass-produced transmission developed for passenger automobile use.

History

During the 1930s automakers sought to reduce or eliminate the need to shift gears. At the time, synchronized gear shifting was still a novelty (and confined to higher gears in most cases), and shifting a manual gearbox required more care than most drivers cared to exert. The exception here was Cadillac's break-through Syncro-Mesh fully-synchronized manual transmission, designed by Cadillac engineer Earl A. Thompson and introduced in in the fall of 1928.

Cadillac, under Thompson, began working on a 'shiftless' transmission in 1932, and a new department within Cadillac Engineering was created, headed by Thompson and including engineers Ernest Seaholm, Ed Cole, Owen Nacker and Oliver Kelley. During 1934, the Cadillac transmission group had developed a step-ratio gearbox that would shift automatically under full torque. This same group of engineers was then moved into GM Central Research, building pilot transmission units during 1935-36 which were then handed to Oldsmobile for testing.

The Automatic Safety Transmission (AST) was a tangent outgrowth of this work. The AST was a semi-automatic transmission using planetary gears and a conventional friction clutch, requiring the driver to use the clutch to shift into or out of gear, but not between the two forward gears. Oldsmobile offered the AST from 1937 to 1939, while Buick offered it only in 1938. The results were not quite what GM Research had in mind.

The HydraMatic was designed to combine hydraulic operation of a planetary gearbox (allowing much shifting to be automated) with a fluid coupling instead of a friction clutch, eliminating the need for de-clutching. The transmission would have four forward speeds plus reverse, providing a broad range of torque multiplication. It incorporated a parking pawl which was engaged when the shift selector was placed in reverse with the engine off. There was no separate Park position as found with modern transmissions.

The result, dubbed "Hydra-Matic Drive," went into production in May 1939 for the 1940 model year. The first Oldsmobiles so equipped were shipped in October 1939. Oldsmobile was chosen to introduce the H-M for two reasons: economies of scale, and to protect the reputation of Cadillac in case of market failure (an unnecessary precaution in that Cadillac developed most of the design). Advertising proclaimed it "the greatest advance since the self-starter."

In 1940, the Hydra-Matic added 57 dollars to the car's price, rising to 100 dollars for 1941. In 1941, it also became an option on Cadillacs for 125 dollars. Almost 200,000 had been sold by the time passenger car production was halted for wartime production in February 1942.