Automatic Transmission Fluid

Automatic transmission fluid is either a highly refined mineral oil or a newer synthetic blend. Manufacturers add detergents and other compounds to reduce oxidation, foaming and other general wear concerns. Transmission fluid lasts much longer than engine oil, because engine oil suffers from residue created in the engine’s combustion chamber. This does not happen in transmissions. The red colored automatic transmission fluid turns brown from heat and oxidation.

For older vehicles, transmission fluid should be changed every 30,000 miles. With many late model vehicles, a transmission fluid flush is recommended every 100,000 miles. The fluids life depends upon several factors:

Type of Fluid: Fluids offer different levels of protection: surfactants, anti-oxidation compounds, detergents, anti-foam additives, anti-wear additives and friction modifiers are added to help protect and add life to the transmission.

Type of Transmission: Older designs require fluids that don’t last as long as the newer designs that use the DEXRON III and MERCON fluids. Some vehicles use a screen-type filter designed to catch the larger debris while leaving smaller particles to circulate with the fluid. The good thing about a metal screen filter is that it can be cleaned and reused. The bad is the cloth screen does a better job of cleaning, picking up any smaller particles more efficiently. Ideally, transmission fluid should be flushed every 2 years or 24,000 miles.

Transmission Cooler

If the transmission fluid appears pink and milky, check for contamination. The transmission cooler is located in the radiator. If there's a leak, transmission fluid and coolant will mix. This will result in pink milky transmission fluid.

Temperature: Transmission fluids ability to withstand temperature depends on its design. Typically ATF is designed to operate at around 175° F. Transmission fluid breaks down quickly as it exceeds this temperature. A 20° F increase in temperature will decrease the fluid’s life by half. Transmission fluid breakdown is the result of heat generated by the torque converter, debris caused by friction surfaces on the bands and clutch packs, and just normal wear.

When fluid level is low, the transmission will slip and suffer delayed engagement. When fluid levels are high, the fluid becomes sudsy and aerated. Aerated transmission fluid causes noise and slippage. It’s vital to check and keep the transmission fluid at the correct level.