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What are the four key jobs of automatic transmission fluid?

Automatic transmission fluid, or ATF for short, is a type of transmission fluid designed specifically for use in vehicles that have entirely automatic or self-shifting-style transmissions. In order to distinguish it from conventional car oil and other essential vehicle fluids, it can sometimes be coloured green or red, although clear versions of this lubricant for transmissions are also available.

In the following passages, we’ll talk through the four important roles a reliable ATF will need to perform.

Acting as a hydraulic fluid

Automatic transmission fluid is effectively hydraulic fluid, as automatic transmissions employ pressurised fluid in order to change gears.

If your vehicle’s onboard computer system decides it must shift gear, it will send an electric signal directly to the required transmission solenoid. This solenoid will then direct fluid through a complicated selection of passages into the valve body, so it can engage the appropriate gear. The transmission fluid then squeezes a set of plates together within a clutch pack, so it can connect the vehicle engine to the transmission’s output shaft and then route power directly to the wheels.

If a transmission is functioning effectively, this complex process will occur seamlessly and instantly, and it will largely go unnoticed. However, if the transmission fluid’s viscosity is too high, making it overly thick, it will not flow swiftly. For this reason, you’ll find Automatic Transmission Fluid has a much lower viscosity than its manual counterpart.

An ATF that has started to foam can also lead it to fail its job as a hydraulic fluid. The accumulated foam bubbles will collapse when under pressure, leading to irregular and elongated shifts, along with gear wear. This is why an ATF must always contain dedicated foam inhibitors as part of its additive package.

Delivering the right friction levels

As mentioned earlier, a pressurised ATF works to squeeze the clutch packs, pressing them together to engage the necessary gears. The clutch packs are made up of plates that have been coated with specially selected friction material. Both disengagement and engagement must always occur smoothly and seamlessly to offer those behind the wheel the optimum driving experience.

An ATF’s frictional properties will often be the difference between this complex combination of moving metals and lubricant creating comfortable shifts and drivers opting to change their choice of transmission fluid as soon as possible.

Protecting gears from wear

The latest automatic transmissions in vehicles contain epicyclic gear systems that include parts referred to as ring, planet and sun gears, and these require effective lubrication to ensure they are protected against wear during operation with a protective film.

Fighting heat

ATFs must withstand heat as it can chemically break it down. When this happens, varnish and sludge occur, clogging oil passages and causing problems with vehicle’s shifting hard and jerking. Automatic transmissions usually run hotter than manual solutions, so ATFs must offer greater heat protection.

To conclude, ATFs are fully optimised to answer the requirements of transmissions, serving brake band friction, valve operation and the torque converter, along with lubricating gears.

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