Usually the red line is the hot fluid coming out of the transmission and blue is the return line to the sump on the transmission. If you have a single cooler with no flow markings it doesn't really matter, if it has flow markings then the red would go to the in side and blue to the out side. If you have a cooler in the radiator as well as a cooler outside the radiator then yo would normally route the fluid through the radiator cooler then to the external cooler.
Be sure that an external cooler is in the path of airflow, it won't do it's job if the air cannot get to it.
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Thursday, July 23rd, 2020 AT 3:06 AM