A transmission cooling line is a solid metal line that connects a vehicle's transmission and transmission cooler. Each transmission cooler has two lines: the inlet that takes the coolant to the transmission and the outlet that takes the fluid back to the cooler.
While many vehicles have an external cooler, in some makes and models the transmission cooler is built directly into the radiator. In vehicles with an external cooler, it is normally mounted below the engine to allow air to pass through it as the vehicle moves. This helps to cool down the hot fluid coming back from the transmission. Most external transmission coolers closely resemble a smaller version of the vehicle's radiator.
If the cooler or either cooling line develops a leak, it can wreak havoc on the transmission because there is not enough coolant to prevent it from overheating. The lack of coolant can severely damage the transmission's moving parts, which can require expensive repairs. In fact, the transmission is usually one of the most expensive parts of a car to repair.
For external coolers, leaks can be diagnosed by fluid dripping from the cooler. However, for internal coolers, it is necessary to look for either radiator coolant in the transmission fluid or transmission fluid in the coolant.