The TCC, or Torque Converter Clutch, is controlled by a solenoid which in turn is activated by the ECM under certain conditions, namely speeds in excess of 45 mph in third gear. The TCC engagement feels like a subtle shift to a "4th" gear in the tranny. Watch the tachometer as you accellerate to cruising speed on the highway, when you ease off the throttle to stabilize your speed you'll see the tach drop a few hundred RPM.
The most common failure mode of the TCC system is the solenoid sticking, keeping the clutch locked, and resulting in the engine stalling as you come to a complete stop. If the TCC does not engage your gas mileage will drop a bit due to the wasted energy used to slip the torque converter. 20mpg sounds about right for a fairly high mileage V6 auto, though.
JazzMan
