Converter slip, and loss of power through the transmission produces heat, which must be dissipated.
At stall, a lot of engine output is converted into heat, and this brings the oil operating temperature closer to its boiling point.
Excessive temperature rise can produce cavitation bubbles in the fluid, which produces noise and reduces converter efficiency.
Some converters use external fins on the case to circulate air through the housing, onto the converter.
Most automatic transmission vehicles use a heat exchanger in the lower tank of the radiator.
Fluid flows from the pump to the converter, then through the heat exchanger before returning to the transmission.
