Automatic transmission fluid

Automatic transmission fluid (ATF) is the fluid used in vehicles with a 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 refined mineral 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. Synthetic ATF has also recently become available, offering better performance and service life for certain applications (such as frequent trailer towing).

ATF is also used as a hydraulic medium in some power assisted steering systems and as a lubricant in some 4WD transfer cases.

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. Some manufacturers will recommend different ATF products for different kinds of vehicle use, such as frequent trailer towing or vehicles that are used for longer periods between routine maintenance. 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-III 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. These products are proprietary, typically not open to 3rd party licensing, and typically available only from sources controlled by the car's manufacturer such as a car dealership.

The use of a 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.

ATF is sometimes added to the fuel tank of diesel engines to lubricate and clean internal fuel system components like injection pumps and injector nozzles. However the legality of this (both from an emissions and a fuel tax standpoint) is questionable, as are the benefits to the fuel system. There are a variety of tested and approved diesel fuel treatment products on the retail market to perform this cleaning process.