Automatic Transmission Fluid

Automatic Transmission Fluids (ATF’s) are among the most complex multifunctional fluids available because they serve many purposes within an automatic transmission. First, it lubricates the gears, bearings and clutch packs. Second, it acts like a fluid coupling inside the torque converter to transmit drive torque from the engine to the transmission input shaft. It also carries hydraulic pressure through the valve body to engage and disengage the clutch packs that change gears. Furthermore, it serves as a coolant for the entire transmission, carrying away heat generated by normal operation and friction. In summary ATF’s perform the following key duties:

  • Heat dissipation
  • Wear protection
  • Lubrication<
  • Foam inhibition
  • Shift quality

In addition, ATF’s must be compatible with all transmission components, operate at both low and high temperature extremes, and maintain constant performance for extended periods. In order to perform all of these functions for thousands of miles or kilometers and deliver satisfactory performance, a number of key additives are used to deal with the environment of close tolerances, high heat, and rotating components. These key additives are used to contribute to the following fluid challenges:

ATF is subject to a wide range of temperatures from -77F to 338ºF. When temperatures are low, viscosity increases and ATF does not flow well. As a result, shift timing may be delayed, slippage at bands and multiple disc holding devices may occur. On the other hand, if the temperature is too hot, the fluid thins out and the lubrication film may break down, causing metal-to-metal contact and wear. Therefore, viscosity is one of the most important factors affecting ATF’s ability to operate the torque converter, valve body components, and the holding device. ATF includes a viscosity index improving agent to maintain viscosity at high temperatures and pour point depressants to improve low temperature flow.

Thermal and Oxidative Stability
ATF temperatures reach around 212ºF at normal speeds and up to about 300ºF under severe operating conditions. The surface temperature of clutch discs my heat up to 660ºF or more. Therefore, ATF must have good thermal resistance. If it does not, deterioration due to heat causes a chemical reaction to occur, leading to greater oxidation of its oil molecules, which causes formation of varnish, sludge, and acids, which leads to internal damage. Oxidation inhibitors are used to combat heat related fluid breakdown.

Defoaming Characteristics
ATF is violently churned and sheared between the impeller and turbine in the torque converter. During periods of high vortex, the shearing of ATF creates a tremendous amount of heat. The churning and shearing of the fluid causes it to foam as air is mixed with the fluid. Foam reduces pressure and promotes slippage, wear, and oxidation of the fluid. An anti-foaming agent is added to ATF to prevent air bubbles and reduces the lifespan of bubbles that do form.

Corrosion Inhibitors
Water and oxygen cause rust formation or etching of metal components. Corrosion inhibitors are added to coat and adhere to metal components and prevent moisture from accumulating and causing damage.

To perform all of these functions well, the fluid must have the correct viscosity and friction characteristics for the transmission application, be clean (no contamination, dirt or debris), be in good condition (no oxidation) and be at the proper level. The wrong type of fluid can cause shifting and engagement problems. Dirty or contaminated fluid can accelerate wear and cause valves to stick. Worn out (oxidized) fluid can also accelerate wear and contribute to shifting and engagement problems. A low fluid level can delay engagement and cause the transmission to slip. A fluid level that’s above the full mark may allow the fluid to become aerated, which can have an adverse effect on shifting, cooling and lubrication.