The History of Automotive Automatic Transmissions in the 1970s

Automobile transmissions through the 1970's were designed for economy more than speed, and as companies began producing front-wheel drive cars they became much more complex. There are three main types of drive-trains used on automobiles: front-wheel drive, rear-wheel drive, and four-wheel drive. Each has continuously evolved, and the 1970's saw dramatic improvement over the previous decades.

Top gears and overdrives

As the 1970's opened, it was customary for a sedan to have a two- or three-speed automatic transmission. Around 1973, emissions and mileage became important, and manufacturers began to design overdrive transmissions which had a high top gear for highway cruising. Many cars became four-speed automatics by the end of the decade, or had an overdrive gear. The lower RPM's of the engine would consume less gasoline, and therefore produce less emissions at speed. The only issue was that the heat of the smaller automatic clutch bands of these higher gears would be too much for the fluid to maintain, and the transmissions would overheat. Cars were then equipped with transmission coolers, which would send fluid from the unit into channels incorporated into the radiator. The engine coolant and transmission fluid would both circulate into the "two line" radiator.

Torque Clutch Converters

A novel idea from General Motors in the mid-1970's was the torque clutch converter, or "TCC". It is also referred to as a "lockup" converter clutch. This clutch, in the front of the rear-wheel drive transmission, received a signal at speed to clamp shut. This created a solid connection from the engine to the rear drive shaft, as an automatic transmission is a fluid system (the friction of the fluid is the driving mechanism). This was thought by many to improve mileage and emissions output. Many popular GM transmissions of that era have the TCC lockup, such as Turbo-Hydromatic models after 1978.

Part-time four wheelers

During the 1970's, there was a major rush on the SUV's of the time, such as Blazers, Broncos and Scouts. These 4x4's were fitted with a different kind of drive-train than their predecessors. Unlocking hubs and vacuum-controlled transfer cases would save the transmission from alternating on-highway, off-road abuses, where the gear ratios and forces were constantly changing. Coupled with overdrives, transmission failure was all too common when the heat buildup overcame the hardware. Major advancements to four-wheel drive transmissions in this decade were for economy, and the SUV market shrank until the vehicles did in the 1980's.

Front-drive cars take over

As the decade came to a close, manufacturers saw the triumph of a new design: the front-wheel drive car. Pioneered by the Asian car market, the smaller hatchbacks of the 1970's were among the most unique cars ever built. Their transmissions, ingenious and compact, could be placed onto the side of a motor and drive one or both front wheels. This would leave the rear axle free, and pull the car instead of pushing it, which changed the dynamics that rear-wheeled cars employ. Front-wheel drive cars used a belt/fluid system, like rear axles, but would not need to turn the force 90 degrees to get to the drive wheel through a differential. The constant-velocity axle joints would become the standard for cars in the following years.

After the 1970's

The 1970's automatic transmissions were an experiment in economy, and were designed for smaller, lighter cars. As the 1980's saw computerized engines, the transmissions of the era began to add more gears and more companies adopted the TCC lockup technology. Three- and four-geared transmissions became four- and five-, and the new, smaller 4x4's became popular for on-the-fly transfer cases pioneered in the late 1970's. The breakneck speed of a 1960's High and Low automatic transmission gave way in the 1970's to smoother, quieter highway cruising. It prepared the 1980's for CV technology, and much of the transmission design done in the period is still used in cars today.