Before the Hydra-Matic was introduced in 1940, General Motors tested the waters and tested its technology with the Automatic Safety Transmission.

General Motors has a long history of technical innovation, but it has often been carried out in cautious, measured stages. The Hydra-Matic of 1940, the American auto industry’s first fully automatic transmission, is a good example. (See our feature on Hydra-Matic here.) For the GM transmission engineering staff led by Earl A. Thompson, Hydra-Matic was really two important technical advances in one: a hydraulically governed four-speed automatic gearbox, coupled to a two-element fluid (aka hydrodynamic) coupling. It was decided to roll out the gearbox unit to the public first, so in June of 1937 came the introduction of the Oldsmobile Automatic Safety Transmission, or AST.

As shown in the cutaway schematic above, the AST was essentially the gearbox portion of a Hydra-Matic, with four forward speeds via planetary gearsets and hydraulic servos operating a set of bands and clutches to perform the automatic gearshifts. The main difference was that instead of the Hydra-Matic’s fluid coupling, the AST used a conventional mechanical clutch assembly. The clutch pedal was required for starting and stopping, but beyond that the AST upshifted and downshifted on its own.

A throttle-operated, kickdown passing-gear feature was also included along with two shifting patterns. The driver could manually select low gear and then move the lever  into High at 20 mph, triggering a 1-2 upshift, or simply leave the selector lever in High and allow full automatic shifting. When shifting itself, the AST skipped second gear, going directly from first to third, but reportedly the acceleration was similar using either method.

While AST was heralded as an Oldsmobile feature in 1937-39, selling for $80, for production scheduling reasons it was actually manufactured at the Buick transmission plant in Flint, Michigan. And it was offered as an extra-cost option at Buick, too, where it was advertised as the Self Shifter. By either name, Automatic Safety Transmission or Self Shifter, the transmission wasn’t a true automatic in the modern sense, but it led directly to the successful launch of the Hydra-Matic for 1940.