Torque Converter Clutch
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
See TORQUE CONVERTER CLUTCH PULSE WIDTH MODULATED SOLENOID VALVE under TRANSMISSION under MISCELLANEOUS CONTROLS.