Turbo-Hydramatic 125

Turbo-Hydramatic 125

A Hydra-Matic 3T40 transmission, produced between 1979 and 2001, at the Ypsilanti Automotive Heritage Museum
Overview
Manufacturer General Motors
Production 1980–2002
Body and chassis
Class 3-speed transverse automatic transmission
Related Turbo-Hydramatic
Chronology
Successor 4T40-E/4T45-E
4T60-E/4T65-E
4T80-E

The Turbo-Hydramatic 125 was the first in a line of automatic transmissions from General Motors designed for transverse engine application. Introduced in 1980, the line evolved into today's 4T40/45/65/80 line.

Turbo-Hydramatic 125

The 3-speed Turbo-Hydramatic 125 was introduced in 1980 and produced through 2002. It carried over some parts from the light-duty Turbo-Hydramatic 200 and 350, but was generally a new design.

The 125 was renamed 3T40 later, following GM's new naming convention. The 4T40-E replaced it starting in 1995.

Applications:

Turbo-Hydramatic 125 C

This was a later model of the 125 that debuted for 1982 and included a torque converter clutch or TCC. The TCC acts similar to a clutch in a manual transmission vehicle, in that at speeds above 50 MPH a combination of engine vacuum- and temperature-sensing valves, and a dedicated brake switch - and in later models an ECU - activated a solenoid which engaged a clutch physically locking the impeller and turbine inside the torque converter together causing engine power to be transferred directly to the transaxle. The C in Turbo-Hydramatic 125 C denoted the use of a TCC. The solenoid that controls this is notorious for failure due to overheated plastic componentry, and problems indicated by an engine stall when stopping from speeds greater than 55 MPH. This occurs because the clutch does not disengage, and as the affected vehicle's speed nears zero miles per hour, the engine is forced to decelerate to zero rpm.

Turbo-Hydramatic 125 C AWD

For the 1988 model year, the Pontiac Division of GM utilized a THM 125C modified to accommodate a full-time All-Wheel-Drive planetary gear transfer case and output housing to send 40% of the output torque to the rear wheels in the 3.1 liter powered 6000STE. While a similar design was used for the later 4T65E, the three speed design was utilized for only two model years, after which all-wheel drive was discontinued as an option for the carline.

Turbo-Hydramatic 440-T4

A breakdown of THM440T4 is: Turbo HydraMatic Model 440 - Transversely Mounted - 4 Speed Fully Automatic transaxle.

A 4-speed transaxle which replaced the 125, was created for the 1985–1986 GM C platform (FWD)/GM H platform (FWD) cars, the Turbo-Hydramatic 440-T4. Virtually no internal parts were shared with the 125, and overdrive was achieved by departing radically from the standard single Simpson compound planetary gearset's common sungear layout. This transaxle was first used in GM's 1985 Buick Park Avenue, Oldsmobile 98, and Cadillac Deville. In 1986, after switching to FWD, the Buick LeSabre, Oldsmobile Delta 88, and later, the Pontiac Bonneville also used this transaxle. The 440-T4 was later renamed the 4T60 under GM's new transmission naming convention. A heavy-duty version of the 4T60 called the 4T60-HD was used in the 1989-1990 Pontiac Grand Prix with the turbocharged engine. GM started adding electronic controls to the 4T60 in 1990 with the Cadillac Allante carline's unique F7, and after 1991 with the corporate 4T60-E.

One unique feature of this transmission is its use of both a modulator valve and a throttle valve (TV) cable. Most transmissions either use one or the other to control shift points and shift smoothness. On the Turbo 440-T4, the TV cable exclusively controls the shift points, while the modulator valve controls the shift feel. This feature makes it very easy for a do-it-yourself mechanic to adjust the transmission shift points with minimum effect on the shift feel.

The 440-T4 was originally slated to be used with downsized C body cars for 1984, however, issues with failures due to lack of fluid lubrication to critical clutch train parts in the relatively complex and compact design, led to introduction of transverse FWD 4-speed automatics being delayed as an option until 1984 for the 1985 model year.

Applications:

See also

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