What are the causes of automatic transmission lubricant deterioration?
4 Answers
Automatic transmission lubricant deteriorates due to the following reasons: 1. The automatic transmission oil specification does not meet the required standards; 2. Incomplete oil change, where only the oil in the oil pan is replaced, while the oil in the torque converter and radiator remains unchanged; 3. Engine coolant entering the automatic transmission cooling oil circuit; 4. Improper vehicle usage, frequently overloading or driving abnormally; 5. Blockage in the automatic transmission radiator or pipelines, or jamming of the radiator's pressure relief valve; 6. Excessive or insufficient clearance in the clutch or brake, or too small clearance in moving parts; 7. Insufficient main oil circuit pressure, causing slippage in the clutch or brake during engagement; 8. Malfunction in the torque converter.
I've been driving automatic transmission cars for over a decade, and transmission fluid deterioration is quite common. The main reasons are, first, prolonged high-temperature operation of the vehicle. The oil oxidizes under heat, darkens in color, turns black, and loses its lubricating properties—especially noticeable during traffic jams or mountain driving. Second is contamination, such as rainwater seeping in or car wash water mixing with the oil, forming a milky deteriorated substance; it could also be metal wear particles mixing in, causing corrosion. Not changing the oil for a long time is another pitfall, as additives lose effectiveness and age, making the oil too thin or thick. I personally experienced gear slippage once, and the diagnosis was old oil—replacing it fixed the issue. So now, I check every 40,000 kilometers, or advance it to 30,000 if city driving is frequent. Also, it's important to choose the right oil—don't skimp on this small expense, or you'll end up spending more on major transmission repairs. Deteriorated oil not only affects gear shifting but also shortens the transmission's lifespan. Regular maintenance is key.
As someone who frequently helps out at auto repair shops, I've seen plenty of transmission fluid degradation issues. The causes are straightforward: water contamination is the most common, like when aging seals leak and cause emulsification; high-temperature oxidation is also frequent, as prolonged high-temperature operation degrades and thickens the fluid. Metal wear particles mixing in are another culprit, accelerating fluid deterioration. When checking the fluid, a simple look at its color or smell can tell—if it's foul or sticky, change it immediately. For prevention, I recommend owners replace the fluid every 50,000-60,000 km, sooner for those often stuck in city traffic; during replacement, thoroughly clean the system to avoid old fluid residue. Degraded fluid causes overheating and even gear sticking—early action saves money and hassle. Other factors like dust intrusion or low-quality fluids also speed up degradation, so it's better to choose the manufacturer-recommended specifications.
Fundamentally, transmission fluid degradation results from chemical reactions. High-temperature oxidation causes oil molecules to break down, forming acidic substances that corrode components; moisture ingress leads to hydrolysis, diminishing lubricity; prolonged use depletes additives, rendering the oil ineffective. Contaminants like metal particles increase friction and accelerate aging. These factors cause discoloration and odor changes, impairing transmission performance. Owners should monitor driving habits, avoiding frequent short trips that allow moisture buildup. Regular fluid changes every 60,000 kilometers or so ensure safety.
