found answer (sort of) on
Re: P0740 Torque Converter Clutch Circuit Malfunction Joel 5-10-07
If you get the P0740 code it means there is a problem with either the torque converter or the control system for the torque converter in your car. It might be such a slight problem that you dont notice anything, or it may be totally undriveable. It depends on what part of the system failed and how badly it failed. To fix it, it can be anything from a simple fluid and filter change (to get debris out of the fluid that might be clogging the solenoid up), or it could be an electrical problem (such as the solenoid, or wiring) or it could be the torque converter itself has failed (which would mean the entire transmission is likely ruined from the debris). It IS definetly a difficult problem to diagnose, and impossible to diagnose without taking anything apart (or at least test driving it with some diagnostic equipment). Starting with the basic (inexpensive) stuff like a fluid flush and filter change and working your way up from there is usually the best way to go, as long as the car is still driveable. Hope that helps (since everyone else only seems to know how to ask the same question over and over and nobody ever posts answers on here).