The fluid in an automatic transmission or transaxle is stored in a sump or reservoir at the bottom of the unit as shown. The fluid level in the reservoir must be checked and changed periodically according to manufacturer's specifications. Be careful to use the correct type of fluid in an automatic transmission or transaxle; the incorrect fluid can cause severe damage.

The automatic transmission or transaxle is a hydraulically operated device. This means it depends on fluids for its operation. The fluid must perform several functions. In an automatic transmission, it is used as the medium for transmitting the engine's power to the rear wheels. There is no mechanical connection between the engine and the powertrain. The connection is made by fluid flow in the torque converter.

Fluids are used to sense the vehicle's transmission requirements. Fluids under pressure are directed to components in the transmission that sense and select the correct gear ratio for the vehicle's operating requirements. The same fluid that transmits the engine's power and provides pressure to operate the transmission must also lubricate the transmission's components. The fluid is circulated in the transmission to provide lubrication for all the bearings, bushings, seals, and gears. The fluid circulates around the parts in an automatic transmission as a means of removing the heat from the transmission.

The automatic transmission fluid must serve to transmit pressure and power, and to cool and lubricate. It must do these tasks over a wide temperature range and under a wide variety of service conditions. The fluid must have a high viscosity index; this index determines how much a fluid's viscosity changes with changing temperature. A fluid that changes a good deal with temperature changes is said to have a low viscosity index. A fluid that changes relatively little with a given temperature change is said to have a high viscosity index. Fluids for automatic transmissions must have a high viscosity index because these transmissions must operate in the same way under both very hot and very cold conditions.

Automatic transmission fluid must also have the quality of lubricity. This refers to how well a fluid works to prevent wear between closely fitted working parts. Direct metal-to-metal contact is avoided by forcing the fluid between the parts. If the fluid has enough lubricity, it will stay in place and prevent wear.

Automatic transmission fluid must resist foaming. The rotating components in the transmission act as an eggbeater to constantly churn the fluid. If a foam were to form, it would interfere with the fluid's ability to lubricate and transmit pressure. The fluid must also resist oxidation. Oxidation occurs when a fluid combines chemically with oxygen. The three greatest factors that promote oxidation are air, heat, and contamination. All three are present in an automatic transmission. If an excessive amount of oxidation occurs, a sludge begins to form on components. This sludge deposit may interfere with the operation of the transmission. Water and oxygen can cause rusting of the steel and cast iron components inside an automatic transmission. Air inside the transmission can provide both oxygen and water. Rusting results in surface destruction and rust may be carried into the hydraulic system where it will contribute to the formation of sludge.

Finally, fluids for automatic transmissions must be compatible. This means they must not destroy the non-metallic types of friction materials used in the automatic transmission.

Several different types of transmission fluids have been developed over the years to meet all these requirements. As components and operating conditions have changed, new types of fluids have been developed. During the 1960s, the most common fluid was identified as ATF, Type A, Suffix A. Most automatic transmissions used this type of fluid. During this period the Ford Motor Company developed a new friction material for their transmissions. A new type of fluid--called ATF, Type F--was developed to work with this frictional material. Using Type A fluid in a Ford transmission could cause severe damage. In the 1960s General Motors developed specifications for a new type of fluid called ATF, DEXRON. This fluid has been updated several times. Currently most vehicles now specify ATF, DEXRON IIE.

Be careful to use only the type of fluid recommended for the car you are working on. The fluid is always specified in the owner's manual.

Powertrain Lubricants

Lubrication technicians must frequently check and refill the lubricant level in a manual transmission, transaxle, differential, or transfer case. Some of these units use automatic transmission fluid like Dexron IIE. Many manufacturers specify different types of gear lubricants for these units.

Like engine oil, gear lubricants are used in powertrain components to prevent wear due to friction. These lubricants are rated according to Society of Automotive Engineers (SAE) standards. Gear lubricants are made with very high viscosity ratings to prevent wear in powertrain components. This gear lubricant has a multiple viscosity rating. The 80W is the cold weather rating and protects like 80 viscosity when cold. The 90 indicates it has a rating of 90 viscosity when hot and protects like 90 viscosity when hot. Viscosity ratings up to 120 are typical.

Powertrain lubricants also have a service rating. A typical service rating is GL-5. The GL indicates that the fluid is a gear lubricant. The 5 is a service rating system for lubricants; the higher the number, the higher the wear protection.

Like automatic transmission fluids, gear lubricants have to be compatible with the nonmetallic materials inside a powertrain component. For example, many limited slip differentials have clutch plates faced with friction materials. The correct lubricant must be used inside these units or the friction material could be damaged. Always follow the manufacturer's recommendations for gear lubricants. The vehicle manufacturer provides a list of recommended lubricants. The lubricant may also be identified by a part number. This means they can be purchased at the parts counter in the car dealership.