- Automatic transmission fluid
Automatic transmission fluid (ATF) is the fluid used in vehicles with self shifting or automatic transmissions. It is typically colored red or green to distinguish it from motor oil and other fluids in the vehicle. On most vehicles its level is checked with a dipstick while the engine is running.
The fluid is a highly specialized oil optimized for the special requirements of a transmission, such as valve operation, brake band friction and the torque converter as well as gear lubrication.
Modern ATF typically contains a wide variety of chemical compounds intended to provide the required properties of a particular ATF specification. Most ATFs contain some combination of rust preventatives, anti-foam additives, detergents, dispersants, anti-wear additives, anti-oxidation compounds, surfactants, cold-flow improvers, high-temperature thickeners, gasket conditioners, and petroleum dye.
There are many specifications for ATF, such as the DEXRON and MERCON series, and the vehicle manufacturer will identify the ATF specification appropriate for each vehicle. The vehicle's owner's manual will typically list the ATF specification(s) that are recommended by the manufacturer.
Automatic transmission fluids have many performance-enhancing chemicals added to the fluid to meet the demands of each transmission. Some ATF specifications are open to competing brands, such as the common DEXRON specification, where different manufacturers use different chemicals to meet the same performance specification. These products are sold under license from the OEM responsible for establishing the specification. Some vehicle manufacturers will require "genuine" or Original Equipment Manufacturer (OEM) ATF. Most ATF formulations are open 3rd party licensing, and certification by the automobile manufacturer.
Current OEM formulations are made from synthetic base stocks. Each manufacturer has specific ATF requirements. Incorrect transmission fluid may result in transmission malfunction or severe damage.
- ATF+4 - Most Dodge, Jeep, and Chrysler 
- Mercon V - Most Ford, Mercury, Lincoln
- Mercon LV - Some Ford(DuratecHE),MAZDA in Europe or Asia
- Dexron VI - Most GM products, some Ford applications
- ATF DW-1 - All Honda and Acura (except CVT)
- SP-III - All Hyundai, Mitsubishi and Kia (except CVT and dual clutch)
Synthetic ATF is available on aftermarket brands, offering better performance and service life for certain applications (such as frequent trailer towing).
The use of a lint free white rag to wipe the dipstick on automatic transmissions is advised so that the color of the fluid can be checked. Dark brown or black ATF can be an indicator of a transmission problem, vehicle abuse, or fluid that has far exceeded its useful life. Overused ATF often has reduced lubrication properties and abrasive friction materials (from clutches and brake bands) suspended in it; failure to replace such fluid will accelerate transmission wear and could eventually ruin an otherwise healthy transmission. However color alone is not a completely reliable indication of the service life of an ATF as most ATF products will darken with use. The manufacturer's recommended service interval is a more reliable measure of ATF life. In the absence of service or repair records, fluid color is a common means of gauging ATF service life.
In the 1950s, 1960s, and 1970s, ATF contained whale oil as a friction modifier. But since whale oil would break down at higher temperatures, cars produced in the 1970s and later would not be able to use whale oil because of the higher engine coolant temperatures employed to reduce emissions and save fuel. A moratorium on whale oil at that time prevented the continued production of older ATFs such as the original DEXRON formulation (Type B), and the Type A which preceded it. General Motors began marketing Dexron II Type C and later Dexron II Type D to replace the whale oil-derived fluids.
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