Technician A says that in any type of fluid coupling not equipped with a mechanical lock-up some slippage exists resulting in a difference between input and output speeds. Technician B says that in torque converter coupling phase impeller and turbine rpms match. Who is correct

Both technician A and B are correct

Explanation:

What is a torque converter?

A torque converter is a type of fluid coupling which transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the power source to the load. It is usually located between the engine's flexplate and the transmission. The equivalent location in a manual transmission would be the mechanical clutch.

The main characteristic of a torque converter is its ability to multiply torque when the output rotational speed is so low that it allows the fluid coming off the curved vanes of the turbine to be deflected off the stator while it is locked against its one-way clutch, thus providing the equivalent of a reduction gear. This is a feature beyond that of the simple fluid coupling, which can match rotational speed but does not multiply torque, thus reduces power.

Some of these devices are also equipped with a "lockup" mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.