Most General Motors' transmission and transaxles are based on the Simpson gear train. The turbo hydramatic (THM) 375, 400, and 475 transmissions are heavy-duty transmissions generally used in full-sized RWD cars and trucks. The 425 is based on the same design as the others but had been modified for use in FWD vehicles with a longitudinally placed engine. The 3L80 (THM 400) was introduced in 1964 and has been used by all divisions of General Motors, as well as by many different manufacturers, such as Jaguar and Rolls-Royce.

The THM 250, 250C, 350, 350C, and 375B transmissions are 3-speed units. The 250 and 250C are light-duty, the 350 and 350C are medium-duty, and the 375B is a heavy-duty version of the 350. On THM transmissions and transaxles, the suffix C means the unit is equipped with a lock-up torque converter.

The primary difference between a 250 and 350 transmission is a 250 uses a band as the second gear holding device, and a 350 uses a band plus a multiple-disc clutch and a one-way roller clutch as holding devices during second gear operation.

The 200, 200C, 200-4R, 325, and 325-4L are light- to medium- duty transmissions. The 200, 200C, and 325 are 3-speed units and the 200-4R and 325-4L have four forward speeds. The 200 series are used in RWD vehicles and the 325 series are used in FWD models. The 325 replaced the 425 in FWD Buick Riviera, Cadillac Eldorado, and Oldsmobile Toronado models.

The 325 and 425 transmissions are not classified as transaxles because the differential and final drive gears are not built into the transmission case, rather a separate final drive unit is bolted to the transmission to drive the front wheels.

The turbine shaft of the torque converter in a 325 and 425 transmission is splined to a drive sprocket. This drive sprocket rotates with the turbine shaft and drives a multiple-link chain that connects the drive sprocket to a driven sprocket. The driven sprocket is connected to the transmission's input shaft. This arrangement allows engine torque to take a 90-degree turn before it reaches the gear train. After the input shaft, the internal components are identical to a 200 or 3L80 transmission.

The 3T40 (THM 125 and 125C) are 3-speed automatic transaxles designed for light-duty use. The 3T40 housing is a one-piece aluminum casting with the case over and oil pan bolted to it. The bell housing is integral with the transaxle case. The torque converter does not directly drive the transaxle, rather the turbine shaft drives two sprockets and a chain, which transfers engine torque to the gear train. This transfer is necessary because the gear train is positioned behind and below the centerline of the crankshaft.

Most General Motor's transmissions and transaxles are fitted with gear-type oil pumps. Exceptions to this are late-model 4L30/4L80 (THM 200-4R) and 4L60 (THM 700-R4) transmissions, which use a variable displacement vane-type oil pump, which is mounted to the valve body and is driven indirectly, through a drive shaft, by the torque converter.

Although there are differences between the various models of General Motors' transmissions with a Simpson gear train, most use three multiple-disc clutches, one band, and a single one-way roller clutch to provide the various gear ratios. Each clutch is applied hydraulically and released by several small coil springs. Two exceptions to this should be noted. The 350/350C has four multiple-disc clutches instead of three. The 200/200C uses a waved spring for clutch release in place of small coil springs.

Power Flow The power flow through General Motors' transmissions with a Simpson gear train is similar to that of the power flow through a Chrysler Torqueflite transmission. There are some slight variations within the different model types, but these are dependent upon the operating characteristics of each model. The chart below shows the clutch and band applications for one of General Motors' most commonly used transaxle, the 3T40.

Add-On Overdrive The 4L30/4L80 and 325-4L use the same multiple-disc clutches as the 200 but also have two additional clutches - the overrun clutch and fourth clutch. Each of these clutches is applied hydraulically. The forward, direct, and fourth clutches are released by several small coil return springs and the drive and low/reverse clutches are released by wave plate springs.

The 4L60 is a fully automatic transmission consisting of two planetary gearsets, five multiple-disc clutches, one sprag clutch, one roller clutch, and a band. (Refer to the clutch and band application chart below for more details). These provide four forward gears, including an overdrive. The five multiple-disc clutches are applied hydraulically and released by several small coil springs. The 4L60 has a one-piece case casting that incorporates the bell housing. The extension housing is a separate casting bolted to the rear of the case.