A torque converter is the fluid coupling between an engine and transmission that allows the engine to idle at low speed without engaging the transmission. A torque converter uses an engine-driven turbine (fan) to push fluid through a matching turbine attached to the transmission. At low speeds, the engine turbine’s fluid simply passes through the transmission’s turbine without moving it, constantly recycling back through the drive turbine. Though this design does have a number of advantages over anything comparable, it is not completely free of inherent flaws.
Torque converter shudder fells like a slight to heavy vibration in the transmission, and is usually accompanied by an interruption in power transfer. It generally occurs at part throttle and light acceleration, between 15 and 50 miles per hour, just before the shift to one of the top gears where the converter reaches “lock-up.” Since it is temperature-related, it tends to happen on hot days, after idling in traffic for long periods of time.