Torque Converter

It replaces the clutch of a manual gearbox. An impeller driven by the engine throws fluid at a turbine connected to the gearbox; a stator between them multiplies torque when pulling away. At cruising speed a lock-up clutch inside the converter connects engine and gearbox directly to stop slip and save fuel.

It replaces the clutch of a manual car and lets the car stand still in gear with the engine running. Because a torque converter always slips a little when it is not locked, it allows smooth starts and absorbs vibration, but costs some efficiency. Modern automatics lock the converter at low speeds to save fuel. A converter failure can contaminate the transmission with debris, so a fluid flush and a careful inspection usually follow its replacement.

The control unit engages the lock-up clutch through a solenoid. It monitors slip by comparing engine and turbine speed. A pump impeller driven by the engine throws fluid at a turbine connected to the gearbox. A stator between them redirects the fluid and multiplies torque at low speed. A lock-up clutch connects both sides at cruising speed.

Lock-up faults cause shudder at steady speed, higher revs on the motorway and codes such as P0740 or P0741. Shuddering at steady speed, slipping or overheating fluid can point to a lock-up clutch or converter problem.