Description

The Turbo Hydra-Matic 325-4L transmission, see image, is a fully automatic front wheel drive unit consisting of a four element torque converter with converter clutch, three compound planetary gear sets and an overdrive unit. Five multiple disc clutches, two roller clutches and a band provide the friction elements required to obtain the desired function of the compound planetary gear sets and the overdrive unit. The combination of the compound planetary gear sets and the overdrive unit provides four forward ratios and one reverse. Changing of the gear ratios is fully automatic in relation to vehicle speed and engine torque.

The torque converter couples the engine to the overdrive unit and planetary gears through oil and hydraulically provides torque multiplication. It consists of a pump or driven member, a turbine or driven member and a stator assembly. With the engine running, the converter pump acts as a centrifugal pump, picking up oil at its center and discharging it at the rim located between the blades. The shape of the converter pump blades causes the oil to leave the pump spinning in a clockwise direction towards the turbine blades. As the oil strikes the turbine blades, it creates a force which enables the turbine to turn. After the oil has imparted its force to the turbine, it follows the contour of the turbine shell and blades and leaves the turbine in a counterclockwise direction, or opposite engine rotation. If this oil is allowed to enter the inner section of the converter pump, it will hinder the ability of the pump to deliver oil with any force due to the opposing rotation of pump blades to oil flow. To prevent this from happening, a stator assembly is added to redirect the oil returning from the turbine and change its rotation back to that of the converter pump blades. The stator is located between the pump and turbine and is mounted on a one-way roller clutch which allows it to rotate clockwise, but not counterclockwise. The clockwise flow of oil is used to assist the engine in turning the converter pump. This increases the force of the oil driving the turbine and results in the multiplication of torque from the engine. As turbine and vehicle speed increase, the stator becomes inactive and torque multiplication ceases. At this point, the converter is acting as a fluid coupling, since the converter pump and turbine are turning at approximately the same speed.

The converter clutch provides a direct mechanical cooling of the engine to the transmission. This mechanical coupling prevents the slippage that occurs in conventional torque converters and results in improved fuel economy. The application and release of the converter clutch is determined by a series of controls and by drive range selection.

The hydraulic system in this transmission is pressurized by a gear-type pump to provide the working pressures required to operate the friction elements and automatic controls.