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Automatic Transmission Fluid Properties and Functions

Automatic Transmission fluid performs many functions during service: It operates the hydraulics, lubricates the moving parts, dissipates heat through the transmission cooler, and acts as the fluid medium for the torque converter. 40 years ago only two types of automatic transmission fluid were commonly used, today there are over a dozen. Vehicle manufacturers now produce and promote their own brands of fluid for transmission service. Some vehicle manufacturers do not produce their own transmissions, but instead, buy them from worldwide transmission manufacturers such as ZF and Aisin Seiki. These transmission manufacturers can design one type of transmission and sell it to different vehicle manufacturers to be used in different car and truck models, providing economies of scale for both parties. The same type of transmission may require different types of automatic transmission fluid, as determined by the vehicle manufacturer. Here is more information about Automatic Transmission Fluid.

Though not often noticed, automatic transmission fluid serves four functions. First and foremost, it is used as a hydraulic fluid to operate the valves that control pressure, regulate shifting, and actuate the Pistons and servos that engage the bands and clutches. During service, all types of transmission fluids perform this job equally well in any type of automatic transmission, even those for whose use was not intended. Because of its relatively low viscosity, only very cold temperatures affect hydraulic function, and even then only for the first couple minutes of vehicle operation. Synthetic automatic transmission fluids have better cold flow properties, so can be used to minimize any adverse effects that very cold temperatures may have on your vehicle, and do not require transmission service as frequently.

Lubrication for the moving components is also provided by the automatic transmission fluid. Due to its low viscosity, its lubricating qualities are not nearly as effective as that of heavier petroleum lubricants, such as motor oil. Transmissions are designed with this in mind, so most do not have lubrication issues. There are some manufacturers, however, that allowed inherent design flaws to persist for years; flaws that were easily correctable. This was very apparent in the Ford Taurus, whose 1986 to 1994 models had what is arguably the worst transmission at that time. Companies such as Transgo, Transtec, and Sonnax have created products which correct these design flaws which Suburban uses in all its overhauls.

Transmission Cooler Installation Reduces Heat

Heat builds up within the transmission during normal service, so automatic transmission fluid is used to dissipate it through the fluid cooler. Most transmission fluid coolers are located within the engine radiator and the heat from the transmission fluid is dissipated into the engine’s coolant through a heat exchanger. On some vehicles, an additional cooler is located in front of the radiator or the air conditioning condenser coils. It resembles a very small radiator and uses air to cool the automatic transmission fluid. There is a lot of misinformation put out primarily by those who profit from the sale of automatic transmission fluid coolers, for accurate information please read about temperatures and external transmission fluid coolers.

Though rarely discussed, the torque converter is filled with automatic transmission fluid, a vital component for its operation. The torque converter is analogous to a clutch for a manual transmission. The torque converter allows you to put the vehicle into gear while stopped, without stalling the engine. You can visit this link to read more about torque converter theory.

Transmission Flush Service and Power Purge

Automatic transmission flushes and power purges are often a waste of money and can even be detrimental to the health of the transmission. For many years, this type of transmission fluid service has been shamelessly marketed to car and truck owners as a fix for automatic transmission problems. For the record, a transmission service will never, ever, repair or undo the wear or damage within an automatic transmission that is causing it to malfunction. Furthermore, it cannot eliminate the effects that the wear or damage has on the operation of the transmission. Replacing the old fluid with new can have benefit, as it will occasionally reduce the adverse effect that the underlying transmission problem has on the operation of the transmission, but this only helps for very specific types of problems, and then usually only for a few months. Keep in mind that any type of transmission service replaces some, but not all, of the transmission fluid, because the torque converter is completely filled and cannot be fully drained in its installed position. If it has been determined that an automatic transmission fluid change will buy you some extra time, you should consider it, but steer clear of the power purge service.

Flush and Power Purge Not Recommended

We strongly recommend that you avoid anything marketed as a transmission flush or power purge, and choose to service the automatic transmission fluid using the drain and fill method, by dropping the transmission pan if necessary. At its best, a transmission flush or power purge does nothing more than changing out some, but never all, of the transmission fluid. In many cases, however, it can do much worse. Many of these power purge flush machines use a diaphragm to force the new transmission fluid in as the old comes out. These diaphragms have seals which can leak over time, thereby allowing old fluid to reenter your transmission along with the new. Even worse, it may allow metal particles and other debris from many other customers’ vehicles to contaminate your vehicle’s transmission. Moreover, transmissions contain magnets to collect any metal that may have accumulated over time in the pan. With a flush or power purge, this metal will be dispersed throughout the transmission, possibly causing damage.

Fluid Service Interval

Automatic transmission fluid services and changes are specified by the vehicle’s manufacturer. Unlike motor oil, automatic transmission fluid can provide many tens of thousands of miles of service before a transmission service or a fluid change is required. Engine oil breaks down more quickly because the air that is constantly pumped through an engine oxidizes the motor oil at an accelerated rate. Gasoline as well as the byproducts of combustion also contaminate motor oil and reduce its effectiveness as a lubricant. Automatic transmission fluid does not experience these conditions, so lasts much longer. Changing the transmission fluid can create leaks and may lead to other problems if not done properly. It can also be expensive so should not be done needlessly or in cases where it does not produce a positive result. Upgrading to a fully synthetic transmission fluid is a viable option for use in harsh conditions but the specific type should be properly chosen.

Transmission Fluid Level Check

When is the last time you checked your transmission fluid? Before you answer, please read again. When is the last time YOU checked your transmission fluid? Most vehicles have an easily accessible dipstick and tube, and it only requires a couple minutes to check the transmission fluid. You should have it checked during an oil change. It is not an unreasonable request to ask to personally observe the process.

Low Fluid Level

Your transmission may be low of fluid for a number of reasons. Although you may not have a noticeable leak, you can still lose fluid over time. Quick lube centers are not required to check the transmission fluid level on vehicles manufactured without an accessible dipstick. Automotive service centers might not check the level unless you specifically request it. Transmission mythology has recently added fictional phrases such as “not serviceable” and “lifetime fill”. If your vehicle does not have an accessible dipstick making it difficult to check your transmission fluid level, make sure you have it checked at least once a year by an experienced transmission technician.

Maintenance of Transmission Fluid

When is the last time you checked your transmission fluid level? Before you answer, please read again. When is the last time YOU checked your transmission fluid level?

Your transmission may be low of fluid for a number of reasons:

  • Although you may not have a noticeable leak, you can still lose fluid over time.
  • Quick lube centers are not required to check the transmission fluid level during maintenance on vehicles manufactured without an accessible dipstick.
  • Automotive service centers might not check the level unless you specifically request it.
  • Transmission mythology has recently added fictional phrases such as “not serviceable” and “lifetime fill”.

Most vehicles have an easily accessible dipstick and tube, and it only requires a couple of minutes to check the transmission fluid. You should have it checked during an oil change. It is not an unreasonable request to ask to personally observe the process.

Some vehicles do not have accessible dipsticks and should be checked by a professional. If checking your transmission fluid cannot be easily done, make sure you at least have it checked at least once a year. Do the maintenance for the week of your birthday or anniversary. We know this does not sound very scientific, but you are more likely to remember if you set it as an important date.

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