What Causes Transmission Fluid to Turn Yellow?
2 Answers
Transmission fluid, also known as gear oil, may turn yellow after some time of use due to the presence of worn metal particles and high temperatures. This is a normal phenomenon. The functions of transmission fluid are as follows: Reduce friction and wear, control clutch friction and synchronizer performance. Bear loads (low speed with high torque, vibration loads), dissipate heat, reduce vibration and noise, and remove contaminants from gear meshing areas (cleaning function). Prevent corrosion, pitting, and surface metal tearing, isolate metal surfaces to prevent direct contact between metal parts. The methods for changing transmission fluid are as follows: Drain the fluid by loosening the drain plug at the bottom of the transmission and then refill it. Remove the transmission oil pan directly to drain and refill the fluid: This method is more thorough than the previous one, as it removes impurities from the bottom of the oil pan. Use a circulation machine to change automatic transmission fluid: Dynamic fluid exchange with an intelligent circulation machine can filter and clean sludge and impurities inside the automatic transmission.
There are quite a few reasons why transmission fluid turns yellow. Having run an auto repair shop for many years, the most common cause I've seen is fluid oxidation—meaning it gradually deteriorates upon exposure to air, changing color from red to yellowish-brown. Another issue is contamination, such as when dust or moisture mixes in, making the fluid appear cloudy and yellow. This tends to happen more frequently when driving often on dusty roads. High temperatures also play a role; if the transmission operates at excessive heat, the fluid breaks down, altering its color. Additionally, worn metal particles may mix in, accelerating the discoloration. Delayed fluid changes—waiting several years between replacements—can lead to natural aging and yellowing. It's crucial to check the fluid level promptly, as abnormal color may indicate internal transmission damage, often accompanied by rough shifting. Replacing the fluid early can prevent major repairs, whereas prolonged neglect will cost more to fix later. I recommend inspecting the fluid color every 40,000–60,000 km—keeping it a vibrant red is the safest practice.
