If your radiator is running a constant temp, your feed is always into the top of the radiator, out the bottom, then into your extra cooler, and then into the transmission. Your fluid temp is one temp, once it hits the radiator, it will be the same temp as the water in the radiator when it leaves the radiator. Your external will cool it down even more so before the return to the transmission. So to even make it cooler, and you want that extra fluid cooler on your ride, mount it in front of the radiator and add a pusher fan to to the cooler. Then your fluid will be near close to the ambient temperature when returning to the transmission. The reason for the pusher fan, when in city driving, you are not moving fast enough to have the full effect of the external cooler. The fan is actuated by a solenoid sensor separate from the water temp control sensor. Good luck.