Automatic transmission fluid

Automatic transmission fluid (ATF) is the fluid used in vehicles with a self shifting or automatic transmission. It is typically colored red 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.

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The fluid is a highly refined mineral oil optimised for the special requirements of an automatic transmission, such as valve operation, brake band friction and the torque converter as well as gear lubrication. Synthetic ATF has also recently become available, which offers better performance and lifetime for high-stress applications such as trailer towing.

ATF is also used in some power assisted steering systems and in some 4WD transfer cases.

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 application. 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. Other vehicles will require "genuine" or Original Equipment Manufacturer (OEM) specification ATF, typically available from sources controlled by the car's manufacturer.

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 ATF can be an indicator of a transmission problem, abuse, or fluid that has far exceeded its useful life. Overused ATF has reduced lubrication properties and often has abrasive friction materials (from clutches and brake bands) suspended in it; failure to replace such fluid will accelerate transmission wear and will eventually ruin an otherwise healthy transmission.

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.