Transmission Fluid


To reduce wear and friction inside a transmission, the most commonly used transmission fluids are mixed with friction modifiers.

  • If an automatic transmission fluid (ATF) without friction modifiers is used in a transmission designed for friction modified fluid, the service life of the transmission is not normally affected. However, firmer shifting will result and the driver might not welcome this change in shifting quality.
  • Transmission durability is affected by using friction modified fluid in a transmission designed for nonmodified fluids. This incorrect use of fluid will cause slippage, primarily when the vehicle is working under a load. Any amount of slippage can cause the clutches and bands to wear prematurely. Also, because of the high heat generated by the slippage, the fluid may overheat and lose some of its lubrication and cooling qualities, which could cause the entire transmission to fail.

The formulation of an ATF must also be concerned with the viscosity of the fluid. Although the fluids are not selected according to viscosity numbers, proper flow characteristics of the fluid are important in the operation of a transmission.

  • If the viscosity is too low, the chances of internal and external leaks increase, parts can prematurely wear due to a lack of adequate lubrication, system pressure will be reduced, and overall control of the hydraulics will be less effective.
  • If the viscosity is too high, internal friction will increase resulting in a greater chance of building up sludge, hydraulic operation will be sluggish, and the transmission will require more engine power for operation.

To check the fluid, the vehicle should be level and running and the transmission should be at operating temperature. Check the condition of the fluid.

  • The normal color of ATF is pink or red.
  • If the fluid has a dark brownish or blackish color and/or a burned odor, the fluid has been overheated.
  • A milky color indicates that engine coolant has been leaking into the transmission's cooler in the radiator.

If there is any question about the condition of the fluid, drain out a sample for closer inspection.

The correct fluid specifications and checking procedures are stamped on the dipstick.
  • Some fluids have a burnt smell when they are in good condition, so be sure this is not the normal condition.
    • Burned fluid is usually caused by failed friction parts in the clutch packs or bands. Friction material might be evident on the dipstick. The transmission will probably require rebuilding soon.
  • Sometimes it is easier to tell if the fluid is contaminated by feeling it, rather than by seeing it. Place a few drops of fluid between two fingers and rub them together. If the fluid feels dirty or gritty, it is contaminated with burned frictional material.

After checking the ATF level and color, wipe the dipstick on absorbent white paper and look at the stain left by the fluid. Dark particles are normally band and/or clutch material, while silvery metal particles are normally caused by the wearing of the transmission's metal parts.

If the dipstick cannot be wiped clean, it is probably covered with varnish, which results from fluid oxidation. Varnish will cause the spool valves to stick, causing improper shifting speeds. Varnish or other heavy deposits indicate the need to change the transmission's fluid and filter.

Abusive driving can overheat a transmission and cause fluid oxidization and breakdown. Stay within the recommended towing load for the vehicle. Avoid excessive rocking back and forth when stuck in snow or mud.

Low fluid levels can cause a variety of problems. Air can be drawn into the oil pump's inlet circuit and mix with the fluid. This will result in aerated fluid that causes slow pressure buildup and low pressures that will cause slippage between shifts.

Excessively high fluid levels can also cause aeration. As the planetary gears rotate in high fluid levels, air can be forced into the fluid. Aerated fluid can foam, overheat, and oxidize. All of these problems can interfere with normal valve, clutch, and servo operation. Foaming may be evident by fluid leakage from the transmission's vent.

If the transmission fluid was low or there was no fluid, raise the vehicle and carefully inspect the transmission for signs of leakage. Leaks are often caused by defective gaskets or seals. Common sources of leaks are:

  • The oil pan seal,
  • Rear cover and final drive cover (on transaxles),
  • Extension housing,
  • Speedometer drive gear assembly, and
  • Electrical switches mounted into the housing.

The housing itself may have a porosity problem, allowing fluid to seep through the metal. Case porosity may be repaired using an epoxy-type sealer.