Automatic transmission care

Updated: December 12, 2021
An automatic transmission is one of the most expensive to repair components of your vehicle. It has to do a lot of work and doesn't require as many regular services as your engine.
Automatic transaxle Mazda automatic transaxle (transmission).
We all know that the engine oil must be changed regularly for the engine to last, but what about the transmission? What care does it need? What are the common causes of transmission failures and how to prevent them?

An automatic transmission depends on the transmission fluid to do all the work: the transmission fluid transfers the hydraulic pressure to shift gears, lubricates moving parts and cools down the transmission. The transmission fluid lasts longer than engine oil, but still deteriorates over time. For this reason, keeping your transmission fluid clean and at the proper level is vital. It's also important using only the proper transmission fluid type.

When to change the automatic transmission fluid

The first source of this information is the maintenance schedule for your car. However, for many modern cars, the maintenance schedule doesn't even mention the transmission fluid. Does it mean that the fluid is 'filled for life' and can last for the lifetime of the vehicle? Most mechanics will tell you that this is not the case. Any mechanical component that has moving parts wears out and fluid degrades over time as a result of load and high temperatures.
Transmission fluid condition Transmission fluid degrades over time and becomes darker.
Should you change the transmission fluid if it's not mentioned in the maintenance schedule? We'd recommend asking your dealer. They know their cars and if they know it's safe to do, they can change your transmission fluid using the factory-approved fluid type and procedure. If your car does have a transmission fluid dipstick, a mechanic can check the fluid level and condition and advise you based on the findings. There are two methods to change transmission fluid: 'flush' using the machine and 'drain and refill'. Read more about the difference below.

What can damage your automatic transmission?

Most transmission problems start from overheating. Under heavy load, such as when towing a trailer, freeing the vehicle from the snow, driving in continuous stop and go traffic in hot weather, racing, etc. the transmission fluid can overheat. At higher temperatures the fluid oxidizes, losing its lubricating qualities and leaving deposits inside the transmission.
Deposits inside a failed automatic transmission Deposits inside a failed automatic transmission.
If you'd check the automatic transmission fluid in an overheated transmission, it would be dark and dirty, with a strong burnt smell.

Exposed to heat, the rubber seals, O-rings and gaskets inside the transmission become hard and brittle. This causes drop in the fluid pressure. The metal parts warp causing valves to stick. All this, sooner or later, results in transmission failure. For example, one of our friends has fried his automatic transmission when he was spinning the wheels too hard trying to free his shiny Audi from the snow; it was on the next day after he bought it!

Another common cause of transmission failures is lack of transmission fluid caused by leaks.
Leaking transmission seal Leaking transmission seal.
Common sources of transmission fluid leaks are the axle seals and transmission pan. Often a transmission fluid leak can go unnoticed for some time. When the fluid level becomes too low, the transmission starts slipping, overheats and fails.

Other causes of automatic transmission problems include design flaws, faulty parts or using the wrong transmission fluid type. One person we know added gear oil into the automatic transmission of his Toyota by mistake. What happened? The transmission failed after 30 minutes of driving!

How to prevent automatic transmission problems

If your car does have an automatic transmission fluid dipstick, check the transmission fluid level and condition once in a while. Change the fluid if it looks dirty or as often as recommended in the maintenance schedule.
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Use only the specified type of transmission fluid. For example, the Owner's Manual for the 2008 Honda Civic says "Using transmission fluid other than Honda Genuine ATF-Z1 may cause deterioration in transmission operation and durability, and could result in damage to the transmission".

The transmission in your car is cooled by the same coolant as your engine. If the engine coolant level is low or there is some other problem that causes your engine to overheat, your transmission can overheat too.

Never shift to Reverse or Park until the car comes to a complete stop. Don't shift the transmission from 'Park' to other modes at high engine RPMs. Avoid full-throttle acceleration when possible.

In many cars, the automatic transmission can be damaged if towing with the drive wheels on the road. If you need to have your vehicle towed, check your owner's manual for the proper way of doing it.
CVT Audi Variable Automatic Gearbox (CVT).

If you planning on towing a heavy trailer with your vehicle, consider installing an additional transmission fluid cooler. It's also a good idea to change your transmission fluid more often when towing a trailer.

Upgrading or Installing an additional transmission fluid cooler is one of the popular option for vehicles with a CVT transmissions. Watch these videos.

Transmission flush vs drain and refill

There are two ways to replace transmission fluid. The drain and refill method involves draining transmission fluid from the drain plug, or if there is no drain plug, from the transmission pan and refilling the transmission with new fluid. In some cars, there is a transmission fluid filter that also should be replaced when the fluid is changed.

The transmission flush is done with a special transmission flush machine that is connected in series to one of the transmission fluid cooler lines.
Transmission flush machine Transmission flush machine.
With the engine running the transmission flush machine gradually pumps out old transmission fluid replacing it with new fluid at the same time. What is the difference? With the 'drain and refill' method only about 40%-45% of transmission fluid can be changed at once, because a large portion of fluid remaining in the torque converter, valve body and other parts of the transmission cannot be drained. Using the transmission flush machine allows replacing around 80-95% of transmission fluid at once.

Which method is better? Opinions are divided. Using a transmission flush machine is a quicker and more efficient method and it is successfully used in many car dealerships. However, half of mechanics you ask, will tell you that it's better to repeat a 'drain and refill' method a few times, as it's safer. Some car manufacturers even don't recommend using transmission flush machines on their cars. For example, Honda mentioned in one of the newsletters for Honda technicians that "Transmission flush systems are not approved or recommended for Honda A/Ts".

In general, the transmission flush method, if approved by a car manufacturer, might work well for low-mileage cars, while the 'drain and refill' method is safer for high-mileage vehicles. With either method, only the recommended transmission fluid type should be used.

When it's time to visit a transmission shop

If you experience any problems with your transmission such as leaks, noises, delayed engagement, problems with shifting, etc., don't wait until the problem gets worse.
Dual-clutch automated manual transmission Volkswagen DSG (Dual-clutch) Automatic Transmission.
Visit your trusted local transmission shop. The automatic transmission problems never disappear by themselves. Also, when going for the repair, try to explain to a service advisor in greater detail: when does the problem happen, when the engine is cold or after some driving? Does it slip/jerk only when shifting between certain gears? At what speed? This will allow a mechanic to better diagnose the problem. Inquire about the warranty; the longer the warranty they provide, the better the repair.

What is Overdrive?

Some older cars have an Overdrive or O/D button. The overdrive is the highest gear in the transmission. On many older cars, the automatic transmission had three or four gears and an Overdrive (forth or fifth gear). When you switch O/D on, you allow the transmission to shift into the highest gear. When it's off, the transmission will not shift into the overdrive gear. In normal driving condition the overdrive should always be on.

The transmission automatically shifts from O/D to lower gear. Sometimes, under certain conditions, e.g. when driving uphill or towing a trailer, the transmission may repeatedly hunt for a gear and shift from O/D to lower gear and back to O/D. This is a good time to switch the overdrive off.
You also may need to switch it off when driving downhill to use engine braking. For more details how to use an overdrive, check your owner's manual.