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General Motors Turbo-Hydramatic Transmissions (Part II)


General Motors Turbo-Hydramatic Transmissions (Part II)Our coverage of the discontinued history of the Turbo-Hydramatic transmission series continues today. The THM was a singular solution to two different automatic transmissions used by Oldsmobile, Cadillac and Buick in 1963. Turbo-Hydramatic came at a time of modernizing automatics, which before the mid-60s were considered inefficient and less than smooth.

The THM400 replaced Buick’s Hydra-Matic and Dynaflow in 1964 and established itself as a smooth and reliable transmission. It proved useful in a variety of luxury and heavy-duty applications and easily ignored weight and torque. Before long, it became the transmission of choice for various small manufacturers outside of GM. However, no matter how excellent the THM400 was, it found itself squeezed by a trend towards greater fuel efficiency. It was also a bit heavy for widespread use in smaller or lighter passenger cars. GM needed more Turbo-Hydramatics!

It wasn’t long before the Turbo-Hydramatic line expanded. In 1969 a new version appeared, the THM350. The three-speed was developed specifically to take over from two older GM gearboxes. The first was the Super Turbine 300, a two-speed engine found in Buick, Pontiac, and Oldsmobile models from 1964 through 1969. Each company had its own brand name, but the Super Turbine underneath was always the same.

The THM350 also replaced the Powerglide, another two-speed automatic. Found primarily on Chevrolet vehicles, the Powerglide continued life for a few years after ceding most of its functions to the 350. It remained in production until 1973. Although the new 350 carried the fancy Turbo-Hydramatic branding , it was actually developed by the dedicated engineers at Powerglide. The THM350 was often referred to as a three-speed Powerglide among connoisseurs, and the team that worked on it internally at GM also used that name.

General Motors Turbo-Hydramatic Transmissions (Part II)

The THM350 used the same torque converter as the heavier THM400, although it skipped the Switch-Pitch stator we last heard about. Aside from its converter, it worked more like the Powerglide. It appeared during the 1969 model year in GM’s lineup and replaced the aforementioned Powerglide and Super Turbine 300. A notable development on the THM350 came during the 1972 model year when an air-cooled version arrived. The air-cooled variant was used on the smaller Chevrolet Vega and Nova.

General Motors Turbo-Hydramatic Transmissions (Part II)

The THM350 also found its way into early ’70s GM trucks and was mated to four-wheel drive when needed. For four-wheel drive, the transmission used an iron adapter that attached it directly to the transfer case, just like the THM400. Some examples of rugged versions of the THM350 were confusingly called THM375-B.

Another development of the 350 came late in the 1979 model year, in the form of a new lock-up torque converter. This version was branded THM350-C and was the final development of the THM350. It remained in production until 1984 for passenger cars (discontinued in favor of the 700R4) and lived in various GM trucks and vans until 1986.

There was an interesting product practice in two of GM’s divisions when it came to transmissions in the late ’60s: Buick and Chevrolet both limited transmission choice. Two-speed Powerglides were supplied with most full-size vehicles in Chevrolet and Buick lots if they had a smaller engine. If a customer wanted to take advantage of the new Powerglide-esque THM350 and its three speeds, they would need to buy a bigger V8. This contrasted with Pontiac and Oldsmobile (which offered nearly identical cars, of course), as they paired the three-speed with any available engine.

The second development of the THM400 was ready for the 1971 model year. Dubbed the THM375, the lower number was an indicator of its status just below the 400. It was mechanically very similar to the THM400 but intended for lighter use. The 375 used a smaller set of shafts and yoke and had one less friction plate in the clutches. It used the THM400 bolt pattern but had a longer output shaft.

GM’s lesser full-size models got the THM375 when they were redesigned for 1971. The fourth-generation Buick LeSabre and seventh-generation Oldsmobile 88 received the 375, most often paired with the trusty Buick 350 ( 5.7 liters) V8. The 375 was rather short-lived and only existed until 1976.

General Motors Turbo-Hydramatic Transmissions (Part II)

In other parts of the Chevrolet lineup where the 375 or 400 were not used, models still used the Powerglide until 1973. In 1974 this two-speed was finally replaced entirely by THM, with the introduction of the 250. The new THM250 was a derivative of the 350 and essentially continued in the tradition of Powerglide engineering. The main difference between the 350 and 250 was the removal of the intermediate clutch pack. A lighter transmission, it was generally used with smaller engines. It was never mated to an engine larger than an inline-six, found in the Nova and Camaro for 1974-1975.

The THM250 also proved to be a very short-lived gearbox and was replaced after only three years by the THM200. The 200 was intended for even less horsepower than the 250 and arrived in 1976. Keep the 250 in mind though, we’ll get to that in a bit. The THM200 arrived shortly after the 1973 oil crisis, when efficiency had suddenly become the main concern of every car manufacturer in the world. The 200 was again a derivative of the THM350 but emphasized the use of lighter materials. Aluminum alloys were used on the internal components, instead of heavier metals.

General Motors Turbo-Hydramatic Transmissions (Part II)

For 1976, the 200 was implemented on the Chevette’s new T-body platform and similar. The Chevette needs Rare Rides Icons coverage, because the T platform underneath was truly global and important. The 200 also found its way into older X-body cars like the Chevrolet Nova and Oldsmobile Omega. It was even used in a truck, Chevrolet’s Captive Import LUV, and the truck’s true identity as the Isuzu P’up.

Even though it was a lightweight transmission, that didn’t stop GM from using it in bigger, more powerful cars than the Chevette. For example, the 200 was used with Oldsmobile’s terrible 5.7 liter diesel, among other applications in GM’s 60 degree V6, the 2.3 liter Vega 4 and with the Isuzu 1.4 in the Chevette . It was used in cars as large as the full-size Caprice.

It turned out that the THM200 wasn’t exactly reliable, especially when used in heavier, more powerful cars. When tasked with moving a V8 Caprice around all day, the 200 didn’t prove to have great longevity. Mechanical and hydraulic problems were common and led to premature transmission failure.

General Motors Turbo-Hydramatic Transmissions (Part II)

The problems with the 200 were widespread enough to lead to a class action lawsuit. GM maintained its innocence and said the 200 was a family of transmissions and not just one. This was a valid statement considering various bells be completely different transmissions. The lawsuit was filed against GM in March 1979 but was not settled until 1986. GM decided to settle and put $17 million in escrow to cover repairs, with an additional $2.5 million in reserves cash in case there are more claims than expected. Repairs at the time were set at $500 a piece ($1,318 adjusted).

The THM200 was so bad that GM offered a bit of an exit, and for the 1979 model year offered the previously discontinued THM250-C as an option in place of the THM200. The 200 was upgraded to the THM200-C for 1979 with a locking converter like the rest of the line. It continued its production until 1987.

General Motors Turbo-Hydramatic Transmissions (Part II)

Let’s go back to the THM400 for a moment. As GM used various other Turbo-Hydramatic transmissions, the big old 400 was phased out of passenger cars around 1980, in favor of the lighter, more efficient units described above. It continued to be used in GM’s line of full-size C/K trucks, as well as G-series vans like the G10 and Vandura. It remained in this usage until the 1990 model year, when it was renamed the 3L80.

The name indicated three forward gears, a fore-aft position, and a high drag rating of 80 invented by GM. GM renamed the entire THM line in 1987 under the new methodology and moved away from the Turbo-Hydramatic naming.

General Motors Turbo-Hydramatic Transmissions (Part II)

By the early to mid-1980s, it was apparent that vehicles needed another forward gear to cope with the pressures of fuel economy and better performance. That’s where we’ll pick up next time, as the Turbo-Hydramatic line has entered the brave new world: fourth gear.

[Images: GM]

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