GM 4T60 transmission

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.

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. After the 1995 introduction of the electronically controlled 4T40-E 4-speed, the 3T40 was gradually phased out.

Applications:

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.

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.

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