Around 80°C (176°F) is generally considered normal operating temperature for automatic transmission fluid. Anything over 90°C (194°F) is getting too hot. Transmission fluid, like engine oil, thins out with increasing heat, and wear protection will begin to decline. The change in viscosities can also cause functional problems with the transmission. Hauling your whole family, a large picnic basket while towing your boat up a mountain in Nevada on a hot summer day is probably a bad idea. An additional or larger transmission cooler helps keep the transmission cooler.