Transmission fluid (ATF / gear oil)
Definition
Transmission fluid lubricates gears and bearings, carries heat away, and in automatics also acts as the hydraulic medium for clutches, the valve body and the torque converter, so its friction characteristics are engineered precisely for each transmission family; using a fluid without the exact OEM approval (Dexron VI, Mercon LV, ZF Lifeguard 6/8, Toyota WS, VW G 052/G 055, Nissan NS-2/NS-3 etc.) causes shudder, slipping and solenoid damage even if it looks identical. Manufacturers often call the fill 'lifetime', but fluid oxidises and fills with clutch debris, and most transmission specialists recommend a drain and fill every 60,000-100,000 km, DSG wet clutches every 60,000 km, and CVTs even sooner. The level on many modern gearboxes has no dipstick and must be set at a specific fluid temperature (often 35-45 °C) through an overflow tube with the engine running, using a scan tool to read the temperature. Dark, burnt-smelling fluid, metal glitter on the drain plug magnet and a clogged filter indicate internal wear; a milky colour means coolant has entered through a failed transmission cooler. Codes: P0218 (transmission over-temperature), P0711-P0714 (fluid temperature sensor), P0730-P0736 (gear ratio, often from degraded fluid), P0741 (converter shudder), P0868 (low fluid pressure), P2757-P2763 (converter pressure control) and manufacturer fluid-age or service codes.
Codes often stored
- P0218Transmission Fluid Over Temperature Condition
- P0711Transmission Fluid Temperature Sensor 'A' Circuit Range/Performance
- P0712Transmission Fluid Temperature Sensor 'A' Circuit Low
- P0730Incorrect Gear Ratio
- P0741Torque Converter Clutch Circuit Performance or Stuck Off
- P0868Transmission Fluid Pressure Low