If you have an automatic transmission in just about any car or truck that has been made, it has a trans cooler built into the radiator. These do just fine if all you do is drive around normal. But if your going to tow anything, offroad or drive it like you stole it then you need an aux. trans cooler that mounts in front of the radiator. The best kind you can get are the "stacked" plate designs.

 

If you are referrring to the tube that goes in front of the radiator, that is the power steering cooler...(ignore the drawn circle in the pic)

 

Here is a picture that I took a while back, showing the Trans lines going into the lower portion of the radiator.

Also never noticed how rusty my oil pan is. Guess I better check that thing out soon.

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