How to Make Your Car Last Forever: Avoid Expensive Repairs, Improve Fuel Economy, Understand Your Warranty, Save Money

The automatic transmission achieves forward motion in a completely different
way. When a gear is selected, oil (transmission fluid) is routed under pressure
(created by its internal pump) through the valve body. The valve body is the
brain of the transmission, controlling up and down shifting through hydraulic
pressure and electronic commands from the vehicle’s performance system. The
valve body directs the flow of transmission oil to where it is needed in order for
the transmission to perform a function (such as the application of “passing gear,”
reverse, first, or second gear). Oil is then forced through the torque converter at
high pressure. This action creates a fluid coupling between the engine and the
transmission. The torque converter changes hydraulic pressure within an
automatic transmission to mechanical torque, which drives the driveshafts and,
ultimately, the wheels of your car.
In design, the torque converter is similar to a turbine engine. Fluid is forced
under pressure through small passages called fins. These passages vary in size
and flip flop in direction. As fluid is forced through the fins (which get smaller
in size), a strong, almost solid “fluid coupling” is created. This is what powers
the driveshafts and wheels. The fluid coupling effect is similar to what happens
when the clutch is applied on a standard transmission. The applied clutch
connects the engine to the transmission, and the torque converter does the same
thing through the fluid coupling.