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Transmission 101

What you don’t know can cost you and your customer.
Thursday, July 26, 2012 - 13:42
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Like many of you, I have the ASE A7 (Automatic Transmission/Transaxle) certification in my wallet. And, like many of you, I never have had a transmission laying in pieces on my workbench.

Let’s face it. If there is a problem with the transmission requiring more work than can be accomplished in the car, we tend to either send it out to a specialty shop or replace the failed unit with a good used or reconditioned one. But with advancements in transmission technology, the challenge is properly diagnosing the problem to begin with. Is an engine management failure, an electronic transmission control failure or an internal component failure causing the customer’s complaint?

The First Automatic
General Motors gets the credit for being the first OEM to install a fully automatic transmission in a production vehicle. The “Hydra-Matic” was a true, automatic four-speed and came as an option in the 1940 Oldsmobile line. Internally, the new transmission was very similar to a previous GM attempt at automating the powertrain called the Automatic Safety Transmission.

One important difference between the semi-automatic AST and the new transmission was the inclusion of a fluid coupling in place of a conventional clutch. Oliver Kelly, the lead designer and in whose name the patent was filed, called the coupling a “fluid turbo clutch.” It had a unique feature incorporated to minimize the car’s tendency to creep forward at idle. The coupling’s impeller was driven by the front planetary gear set rather than directly by the engine. There was no Park position on the early units. Instead, a parking pawl engaged when the car was in Reverse with the engine off.

Shortly after its debut, GM found itself shifting from civilian production to war-time production. Among its projects was the M-5 Stuart tank, powered by two Cadillac V8s (one per track) mated to Hydra-Matic transmissions. This gave GM bragging rights after the war, advertising their transmissions as “battle tested.”

It also became apparent to GM’s competitors that not offering an automatic in their model lines was a serious competitive disadvantage. Even though the Hydra-Matic shifted hard, reduced performance and gas mileage suffered, the consumer didn’t care. And while some manufacturers developed their own automatics, many smaller companies of the times opted to buy GM’s for use in their own products. By 1952, the Detroit transmission division had built more than 2 million units, making the Hydra-Matic arguably one of the most successful transmissions of all time.

The Technology Of Automatics
Beginning in the 1980s, transmissions began heading the way of other vehicle systems, turning over their control to computers and solenoids. The goal then, as it is today, was to improve efficiency and increase fuel economy.

Gasoline engines produce maximum torque in a fairly narrow rpm range. To keep the engine in that range, engineers are adding additional gear ratios. Multispeed transmissions are now in production incorporating six-, seven- and even eight-forward speeds.

Constant variable transmissions (CVTs) are popular with many Japanese automakers, and they too continue to evolve. The Nissan XTRONIC CVT transmission, for example, has a unique supplementary two-speed planetary sub transmission that operates in series with the CVT variator. This innovation increases the ratio spread over a typical CVT, and allows lower rpm at cruising speeds while still maintaining low speed drivability. Size and mass are also reduced and the manufacturer claims a 30 percent reduction in internal friction with a corresponding increase in fuel economy of 10 percent.

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