So about a month ago my transmission started having shifting issues and started running in the 170 degree range. It's always run about 140, it's running through the radiator and a big external cooler. I pulled the trans and rebuild it and it's still running 170. Outside temp has been 90 lately. Last year pulling a trailer in this heat it never got this hot. I'm at a loss. Hoping mastertech will pipe in his knowledge. I'm working at a Transmission shop right now and my boss says it's normal. It's not normal for my truck from what I can remember though.

98 Durango. 318 44re transmission with aftermarket aluminum pan. Extra 2 quarts. 20000 btu cooler

 

Have you deleted the check valve going into the cooler for the radiator? If not that may be getting stuck. Otherwise hows the flow through the coolers?

 

Mine also ran thought the radiator and external cooler and still run 165-175 in heat and humidity. I even added the electric fan to help out a bit and still in the same range... honestly I would say its normal as well. I would not worry about it as long as your staying under 180.