Electronic automatic transmissions have gained popularity in recent years because of the flexibility that on-board electronics can provide in the area of powertrain management. Electronic transmission controls, for example, have done away with the rough, imprecise shifting patterns inherent in early mechanically controlled transmissions. With the continued refinement of electronically controlled automatic transmissions, an automatic can usually be made more fuel-efficient for average daily driving than its manual transmission counterpart.

TORQUE CONVERTER BASICS
The concept of using a torque converter, oil pump, planetary gear sets, clutches, bands and a computer-controlled hydraulic valve body to transmit torque and to change gear ratios is relatively simple. To illustrate, the torque converter allows the engine to idle in gear with the vehicle stopped and multiplies engine torque during the initial stages of acceleration. In addition to an electronically controlled clutch that prevents the torque converter from slipping during cruise conditions, the primary parts of the torque converter are the impellor, stator and turbine.

Each of these parts has a set of curved vanes that accelerates and controls the flow of oil in the torque converter housing. The impellor, which is driven by the engine, uses centrifugal force to push oil into the turbine. The turbine, which is attached to the transmission input shaft, recei...