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.
A leak in the automatic transmission cooler allows coolant to seep into the cooler and contaminate the fluid. The transmission cooler is in the radiator and relies on the engine's cooling system to function correctly. A malfunctioning cooling system affects the transmission cooler's ability to cool the fluid.
Cross-Contaminated Transmission Fluid
Because the transmission cooler is inside the radiator, a leak will result in transmission fluid swirling around the radiator's neck. It also produces pink, milky transmission fluid in the case and on the dipstick. 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 transmission fluid with a paper towel or white lint-free cloth. It helps show its actual color and any debris mixed into the fluid.