During a preliminary inspection, transmission fluid should be inspected for level, color and condition. The color of the fluid will serve as a guide to direct further inspection and diagnosis. If the fluid is prematurely dark brown or black, it’s due to excessive heat. Heat causes oxidation. Oxidation causes a brown tarnish that coats the dipstick, transmission case and other components inside the transmission. This tarnish also causes transmission valves to stick, resulting in delayed and unusual shifting. If the fluid is pink and milky, the transmission cooler is leaking.
The transmission cooler relies on the engine’s coolant system to function properly. It's important to make sure the engine’s coolant system is functioning properly. A malfunctioning cooling system will affect the transmission cooler’s ability to cool the transmission fluid.
Since the transmission cooler is located in the radiator, a leak will result in transmission fluid seen swirling around in the radiator neck. The leak results in pink, milky colored transmission fluid. The coolant mixed fluid will damage transmission seals and friction material. The transmission may even need a torque converter and a complete overhaul to replace these contaminated parts. Check the fluid with a piece of paper towel or white lint free cloth. It will show its true color and any debris mixed into the fluid.