Automatic Transmission Fluid Facts

It is easy to forget how important automatic transmission fluid is to the proper operation of the vehicle. This happens to be one of the more complex fluids in the automobile.

Although there are only a few different types it’s extremely important that the right one gets installed in the vehicle. The specifications section of your owner’s manual will provide you with exactly what kind of fluid belongs in your automatic transmission.

In the maintenance section will be guidelines for replacement and maintenance. Keep in mind that a great debate rages on about whether or not service is required at all. I believe in sticking to what the car-makers engineers recommend.

The reason I feel this way is because the hydraulic fluid circulating through the transmission and torque converter and over the internal parts of the transmission provides not only the lubrication but also the cooling. The hot fluid exiting the transmission moves to a transmission fluid cooler where the heat is removed.

In some cases this transmission cooler is located in the side tank of the radiator and uses engine coolant and airflow to remove the heat from the much hotter hydraulic fluid.The principles of heat transfer state that the direction of flow is from hot to cold.

On trucks, SUV’s and crossovers it’s becoming more commonplace nowadays is an external fluid cooler. Often these are mounted in front of the radiator and air conditioning condenser where it is exposed to large amounts of air circulation.

How Important are Transmission Coolers

The removal of heat from an automatic transmission is extremely important for the longevity as well as the proper operation of the unit. The reason this is so important is because excessive heat causes the fluid to break down.

If the automatic transmission fluid does break down it will no longer lubricate as per design intent and this can lead to internal damage to major components quickly. If an automatic transmission is operated a long period of time with overheated fluid a varnish can form inside.

This can plug small galleries that will in turn affect internal pressures in the individual fluid circuits that can cause transmission problems. Continuing to run broken down fluid in a transmission and ignoring signs of problems such as erratic shifting could lead to a complete overhaul.

Just about every transmission I’ve ever seen has some type of cooler setup. The most common would be transmission cooler lines that direct the hot fluid from the torque converter directly to a radiator cooler as described above.

After the heat is removed from the fluid it returns to the transmission through the other cooler line called a return line. Often this cool fluid is directed to the transmissions bushings, bearings and gears that are most affected by internal heat buildup.

The fluid is then directed in most cases back to the pan where it is filtered before it goes through the valve body. Amazing when you think about it! Transmission fluid is a coolant, lubricant and hydraulic oil that pushes on check balls and valves to control functions.

On heavy-duty vehicles that are often used for towing or carrying heavy loads an auxiliary automatic transmission fluid cooler may be installed by the factory to meet the needs of the vehicle.

The more the vehicle carries pulls or tows the more heat will be generated by the transmission. If you have a regular light duty vehicle and add an aftermarket trailer hitch for towing a boat or something similar you should seriously consider adding an aftermarket automatic transmission fluid cooler at the same time.

You can see your mechanic for his recommendations on this as well. For more information about how your transmission works I have provided some additional articles about the theory and operation. I have also included some stories about specific problems with automatic transmissions. Bookmark or favorite this automatic transmission fluid page. Share with a friend thinking about towing a boat without adding a transmission cooler.

If you would like more information about how to check your fluid as well as some recommended guidelines for maintenance I have an article on my blog that talks about automatic transmission fluid service.

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