Like a radiator, the transmission oil cooler may block or create a leak. Blockage or leakage can reduce the efficiency of the oil cooler when cooling the transmission fluid. As you might expect, the biggest problem facing the oil cooler line is the leak. The piping may leak oil, reducing the overall function of the oil cooling system. This puts pressure on the transmission. Shifting may slip during a shift or may suddenly change gears. Transfers can happen at odd times, too early or too late. If the transmission fluid is changed, an abnormal amount of sludge or debris may be found in the liquid.
If the oil leaks onto the hot parts under the hood, you may see black smoke under the hood. Leaked oil can also enter the engine, causing it to run poorly and expel black smoke from the exhaust. The best way to identify a leak is to visually inspect the cooler.