Simple by both design and function, torque converters are fairly easy to diagnose when they develop problems. Since these couplings exist only to transfer power from engine to transmission, any interruption in power transfer is a sure sign of converter malfunction.
This occurs generally under light acceleration from a stop, and feels like driving over a washboarded road. Shudder is caused by overheated tranny fluid, which must be changed to remedy the condition.
Slippage between engine and transmission is a result of converter malfuntion, and can usually be traced back to overheated fluid.
Failure of transmission/engine disengagement when coming to a stop is a sign of either a broken torque converter or one in which the lock-up mechanism is not disengaging.
Torque converters can suffer overheating if the vehicle is revved while in drive ("power-braking") or must ascend a steep hill while towing a heavy load.
An engine RPM that is unsteady under cruise is a sign of lock-up failure. This can be due to either damaged lock-up clutches in the torque converter or electrical problems associated with the system.
