How heat affects your automatic transmission fluid:

Once a vehicle is in motion, it doesn’t take long for the automatic transmission fluid (ATF) to heat up. Even under normal driving conditions the ATF temperature will rise to 80 degrees C (175 F), which is the usual temperature range at which most fluids are designed to operate. In fact, if the temperature could be kept at 80 degrees C, the ATF would last a significant amount of time (and distance). But once the fluid temperature increases beyond this, the life of the fluid begins to decrease. Every 20 degree increase in operating temperature above 80 degrees C decreases the life of the ATF by half.

Even normal driving conditions can cause the ATF temperature to increase beyond safe limits. But other factors can cause an increase in temperature as well, such as towing a trailer, mountain driving, driving at sustained high speeds during hot weather and stop and go driving in traffic. If there is a problem in the cooling system itself such as a low coolant level, defective cooling fan, thermostat or water pump or obstructed radiator etc, then this will also diminish ATF cooling efficiency.

At elevated operating temperatures the ATF oxidizes and turns brown, taking on a smell like burnt toast. As heat destroys the fluid’s lubricating qualities and friction material, varnish begins to form on internal parts (such as the valve body). This directly interferes with the correct operation of the transmission. If the temperature increases above 120 degrees C (250 F), the rubber seals begin to harden which in turn leads to fluid leaks and pressure losses. At higher temperatures the transmission begins to slip which aggravates the overheating even more. Eventually the clutches burn out and the transmission fails.

How to care for your automatic transmission:

Many vehicle manufacturers today tell us that the automatic transmissions in our cars are “filled for life” on the assembly line and will never need servicing. But to take that advice on face value is to court disaster. The fluid in an automatic transmission is its lifeblood. It depends on it to operate efficiently over a long period of time and to ignore it is to risk an expensive meltdown.

Even though some vehicle manufacturers tell you automatic transmissions don’t need servicing, as a transmission specialist, we highly recommend regular maintenance to keep your automatic transmission working the way it should. It doesn’t matter what you’re told, oil breaks down over time & with high temperature its efficiency is reduced.

The service routine we recommend for cars in normal use is to change the automatic transmission fluid (ATF) and filter every 12 months or 20,000 km and to carry out a full flush of the transmission every 2 years. For vehicles working in more punishing conditions, such as hilly terrain, on sandy tracks, using 4-wheel drive or towing regularly, those intervals should be reduced.

It’s also important to ensure the fluid doesn’t overheat, which usually happens because the transmission is worked too hard. If you plan to tow with your vehicle on a regular basis, consider having an external transmission oil cooler fitted.