Generally the higher line on the radiator has fluid going INTO the transmission and the lower one goes back to the transmission, so the higher line has fluid coming out of it and the lower line sucks it back into the transmission. This is because the radiator works by taking in hot coolant from the top and as it goes down through the core it gets cooled by the air going over the tubes.
However, I would verify this by disconnecting the upper transmission cooling line at the radiator and putting it into a bucket. Then have some one start the engine and let it idle for 10-20 seconds, you should see the fluid being pumped in to the bucket.
linkw222 on October 03, 2010
thanks for the info on the durango transmission lines. i finally got around to installing my cooler and your info. was a big help. thanks again
Cadillacdave1 on August 08, 2021
Your response is poorly worded and suggests both lines go back to the transmission and you say it twice! If fluid comes OUT of the upper line, it is NOT a return line to the transmission! That means the fluid is being pumped into the radiator to be cooled. If you remove a line from the radiator and fluid can be pumped into a bucket from the line, that is the inlet line to the radiator (the hot trans fluid). Then it passes through the radiator for cooling and is then sent back to the transmission AFTER it has been cooled.
A lot of folks even transmission shops get this mixed up when installing external transmission coolers, in addition to the radiator. If installed backwards, as many do, it completely defeats the purpose of the cooler.
The proper installation is hot line from transmission to radiator in. Now fluid is cooled by radiator and then sent to external cooler for additional cooling and then returned to transmission.
