From wikipedia...
"A transmission control unit or TCU is a device that controls modern electronic automatic transmissions. A TCU generally uses sensors from the vehicle as well as data provided by the Engine Control Unit to calculate how and when to change gears in the vehicle for optimum performance, fuel economy and shift quality....Transmission solenoids are a key component to these control units."
A solenoid is a coil of wire that produces a strong magnetic field that causes an armature to move - converts electrical power to mechanical movement - changes gear for you.
Peccavi, July 2009
