I think every email I get about an automatic transmission problem starts with, "My transmission is having a problem and I think it's the torque converter." The truth is, it's not often the torque converter that's at fault. I think I hear this because the operation of the torque converter is rarely understood and it seems like a mechanic-ey thing to say. I mean, “torque converter”: It does sound kind of manly. So to combat the ignorance I've made this video on how a torque converter operates and what's inside it.
The torque converter is designed to take the place of a clutch assembly that a manual transmission uses. It also multiplies engine torque so you can take off from a dead stop easier. In addition to that, it also houses what is referred to as the TCC, or torque converter clutch. This is a clutch and works in the same way a manual transmission's clutch works. It's there to prevent slippage that normally occurs within the unit. At cruising speeds, the clutch locks up and allows full engine torque to be transferred to the transmission.
A problem with the TCC can cause a few things to happen. The first is poor fuel economy. If you notice a sudden drop in fuel economy, put the TCC on your list of suspects. It's difficult to check, but as your vehicle is shifting through the gears, count the shifts. If you have a four-speed transmission, count up through the gears as it shifts. After the transmission shifts into fourth, you might feel another shift after that. This is likely your torque converter locking up. When it's working properly it might feel like another shift at cruising speed. If you don't feel this shift, there could be a problem with the torque converter or its controls.
Another issue might be that the torque converter clutch does not release and stays applied. If this happens, it's just like leaving your vehicle in gear with your foot off the clutch when you come to a stop. It's likely your engine will stall. If you notice that your engine runs fine except when you come to a complete stop, you could have a problem with the TCC staying applied. Consult your service manual for testing procedures.