"Transmission fluid, like any fluid such as oil or antifreeze, tends to break down with heat and time," says Ken Buehler, owner of Buehler Automotive & Transmission. "As the operating temperature rises, the fluid breaks down at a faster rate requiring replacement at shorter intervals. If the fluid lasts a certain amount of time at 200 degrees F, operating it at 230 degrees F may mean it will last 2/3 as long. And at 250 degrees F perhaps half as long."

Ken explains that it is very important to keep temperatures under control within the automatic transmission. He says that most vehicles use a heat exchanger (cooler) within in the radiator to cool the transmission. 

"Transmissions run a lot hotter than the engine, so there is no danger ( as some people believe) that the engine will overheat the transmission," says Ken. "The heat exchanger within the radiator helps the transmission initially by heating the fluid and helping to act as a thermostat. This keeps the transmission at a more consistent operating temperature. A transmission that is carrying a heavy load will greatly benefit by the installation of an auxiliary transmission cooler to help cool the oil."

Ken says that he has seen many recommendations for automatic transmission fluid change intervals, but he says that a "good rule of thumb is every 40000 km or two years." Ken adds that once the transmission fluid breaks down, it becomes dark in colour and takes on a foul odor. It also becomes more acidic and as a result hardens the rubber seals within the transmission. "When this occurs, the seals can leak internally and not do their job efficiently allowing the clutches to slip," Ken says. "This creates more heat which will eventually burn the clutch packs which in turn can result in shifting problems that may cause the vehicle to stop moving altogether. The only alternative then would be a complete overhaul which would include replacement of all clutches and seals."