With the engine running and the torque converter empty of fluid, the input shaft will not turn. However, when the torque converter is filled with fluid, the shaft will not only turn, it will turn with enough force to drive the transmission internal components, which drive the vehicle. Therefore, the fluid in the torque converter makes the connection between the engine and the transmission.

Based on the simple three-element converter, there is no mechanical connection between the engine-driven portion of the converter and the transmission input shaft. Only the fluid in the torque converter couples the engine to the input shaft. The paragraphs on the following pages describe each component of the torque converter and explain how the hydraulic coupling is accomplished.