To answer a PM here:
Yes I have seen a rusty dip stick above the oil level because the oil was staying too cold and the condensation not being evaporated from the ATF. And it was Mobil 1 ATF. Sure the real cool oil is good for parts.... but the water is NOT.
The thermostat splices into both of the lines. When the oil is cold the output oil from the trans is routed directly back into the trans bypassing all cooler function until the thermostat opens the path to the cooler. That makes the transmission warm up faster and maintains a minimum operating temperature just like an engine radiator thermostat does.
Just like an engine coolant radiator, the size of the oil cooler and the airflow across it determines the maximum trans oil temperature.
Contrary to popular belief, the best place to put a transmission cooler is behind the radiator..... between the radiator and the mechanical fan. = Lots of airflow in all conditions and does not increase radiator coolant temperature.
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