What makes automatic transmission fluid special?

The right automatic transmission fluid (ATF) is essential to the smooth running of an automatic vehicle, but why?

It’s actually one of the most complex fluids you’re likely to find in a modern automobile, because an automatic transmission often involves hundreds of moving parts, as well as O-rings, seals, and gaskets. All of these need to be lubricated and protected under very demanding conditions.

What does an ATF do?

In addition to lubricating the moving parts and preventing wear, much like a traditional lubricant, an ATF plays many other roles to ensure smooth running and shifting.

Heat transfer

The interaction of transmission gears typically causes automatic transmissions to build up heat, which then needs to be dissipated. This is especially challenging in modern vehicles, where contemporary aerodynamic designs limit the airflow, and the problem only gets worse under heavy loads. The ATF therefore plays an essential role in transferring heat and enabling it to be dissipated efficiently.

Hydraulic actuation

In most automatic vehicles, the traditional friction clutch is replaced by a fluid (hydraulic) coupling or torque converter to transmit rotating mechanical power, with a system of planetary gears being hydraulically locked and unlocked to accomplish gear changes. This means in addition to its other roles, an ATF needs to act as a hydraulic oil.

Foam prevention

Foam is rarely desirable in a lubricated system. It typically hinders the process by preventing the oil from forming an adequate protective film over surfaces, but it has other serious consequences inside an automatic transmission. Foam compresses, and this can hinder or even prevent hydraulic actuation, which as mentioned above, is essential to most automatic transmissions.

How do ATFs differ?

The challenge faced by ATFs are already substantial, but it is made even more complicated by the varying designs used by different manufacturers. If an ATF is being drained and replaced by something like Mobil ATF LT 71141, it needs to be compatible with the manufacturer’s specification.

First of all, the components of different ATFs may be made from different materials, so the ATF needs to be compatible with all the materials in a particular transmission, otherwise some components may become damaged, leading to failure.

Secondly, different transmissions involve different shifting characteristics, such as the number of speeds and load-bearing requirements, which is another reason why manufacturers design oils especially for their products.

While synthetic oil generally offers some desirable properties, semi-synthetics like Mobil ATF LT 71141 often deliver similar benefits. The main concern, however, is to always use a compatible oil.