The torque converter in your car or truck sits converts rotating force to the transmission and the drive wheels. It does this with a propeller that pumps transmission fluid against the impeller blades, which engage a slip-clutch that completes the connection to the transmission driving gears. Without a torque converter, there would be no smooth starts or gradual acceleration--the engine power would translate force (torque) directly to the transmission and to the rear drive wheels, causing tremendous shock and strain, and leading to broken driving parts.
Noises from the transmission can be hard to interpret. One such noise sounds like a light rattling that occurs when the car moves in a drive gear but goes away when the selector is put in neutral. This could indicate damaged needle bearings between the stator, impeller and turbine. Once the bearings give out, a definite grinding noise persists. A heavy knocking or clanging noise may indicate that the torque converter has broken loose at one or more of its mounting bolts. The noise will be constant and in synchronisation with the engine RPM.
High Speed Power Loss
Torque converters have a one-way clutch. If the clutch seizes and freezes the stator in position, the fluid cannot circulate properly between the impeller and turbine and causes high speed power loss. Under power, the engine may overheat from the friction turbulence inside the converter. A totally locked up clutch can cause the transmission to overheat, putting wear and stress on the transmission gears. Too much stress on the gears will lead to transmission failure.
Low Speed Acceleration Slip
If the one-way clutch fails and cannot hold its position, spinning freely in both directions, it has lost driving force. This causes the vehicle to slip or start off slowly in first gear, causing the engine to rev higher and putting it out of rotational synchronisation. In this case, the clutch is permanently damaged, which means you need a new torque converter.
Engine Shudders and Dies
Many torque converters have a lock-up device to improve fuel economy. If the lock-up device does not engage properly, there will be noticeable slippage in most driving ranges, not to be confused with low fluid levels. If the lock-up does not release properly or on time, the engine can die when coming to a stop or shudder when slowing down. A stuck lock-up valve or a grounded (shorted) solenoid sensor can be at fault here.
Stall Speed Test
A stall speed test must be performed with great care and safety in mind. Once the rear and front wheels are blocked with wheel chocks, apply your foot to the brake while the accelerator pedal is pushed slowly down to maximum RPM (this is the stall speed limitation). If a stall occurs before reaching the maximum RPM, this indicates that the torque converter clutch is slipping or that the transmission fluid level is dangerously low. Checking the fluid level first will rule out a low level condition.