Contaminated Transmission Fluid
Inspect the automatic transmission fluid (ATF) for level, color, and condition. If it's prematurely dark brown or black, it's due to excessive heat. Heat causes discoloring and oxidation. Oxidation causes a thick brown film 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 cooling system to function correctly. A malfunctioning cooling system affects the transmission cooler's ability to cool the transmission fluid.
Cross-Contaminated Transmission Fluid
The transmission cooler is inside the radiator; a leak will result in transmission fluid swirling around in the radiator's neck. It also results in pink, milky transmission fluid. The cross-contaminated fluid damages transmission seals and friction material.
The transmission may 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 helps show its actual color and any debris mixed into the fluid.