ATF Doesn’t Stand for Alcohol, Tobacco, and Firearms

Automatic Transmission Fluid, The other oil change

After oil changes and cooling system flushes the fluid that is probably the next most important, if you have an automatic transmission, is the automatic transmission fluid. This is that magical red fluid that your transmission is completely incapable of functioning without. The engine will still run without oil and coolant, the steering system will still steer without power steering fluid, but the transmission will quit working altogether if it looses only a few quarts of ATF.

What is it

Automatic Transmission fluid is essentially a specialized hydraulic fluid. Like all other hydraulic fluids it’s primary task is to transmit force. The automatic transmission contains numerous hydraulic circuits that channel fluid force in order to activate hydraulic pistons and servos. These pistons and servos are responsible for actuating various clutches and other braking mechanisms in order to make the transmission function. ATF must also lubricate all of these mechanical parts that are constantly rubbing and touching and heating up against each other.

Clean ATF is usually a nice red color

This fluid is very heavily abused inside the transmission but it contains all sorts of additives to help it do its job and last a long time. Some of the additives include friction modifiers, detergents, antioxidants, dispersants, anti-foaming agents, and others. All of these things give ATF its unique properties. The red color that you usually see in ATF is actually a dye that is primarily added to help in differentiating between engine oil and ATF by sight. This fluid is so good at what it does that many manufacturers also use it for power steering fluid. If you ever get your hands extremely soiled with grease and grime, just scrub them lightly with some clean ATF and it will not only clean the tough grime out from under your finger nails but it will leave your skin feeling soft. The additives in ATF are meant to keep the inside of the transmission clean and those same additives work on your hands as well.

Check it

In the past every vehicle had a second dipstick, other that the engine oil dipstick, which was used for checking the level of the ATF. For vehicles that are still equipped with such, checking the ATF is very easy. Most cars require that the engine be running with the transmission in park. Some require that the transmission be in neutral. Honda trucks and cars with automatics require that the engine be off. If you are not sure what your vehicle requires, you can sometimes find directions on the dipstick itself. If that doesn’t work then consult the owner’s manual.

Most cars also require that the engine and transmission be warmed up in order to get the most accurate reading. The reason for this is that ATF expands quite a bit as it warms up. One might believe the fluid level to be low when in reality the fluid is just cold. Many manufacturers put separate marks on the dipstick that are used if the fluid is cold, but what if the fluid is somewhere between cold and warm? This is why it’s just best to check it with the fluid warmed up.

The goal when adding or checking fluid is to make sure that the fluid level is between the two marks that are found on the dipstick. If the level is below the lower mark then some fluid must be added, but if the level is between the marks, then that is satisfactory, and no more fluid is required. If fluid needs to be added then usually it must be poured down the dipstick tube. These dipstick tubes that double as a filler tube are usually wide enough to put the end of a funnel into them. If the dipstick tube is too narrow to fit a normal sized funnel into the end of it, then there is likely a filler plug somewhere else.

A tranny pan with a drain plug in it, the fill plug can be seen
in the housing above the pan.

Besides looking at the level the condition of the fluid can also be examined. If the fluid contains very tiny black particles that rub off on your oil rag or paper towel this is normal, but can it can indicate that the fluid needs to be serviced. These small black particles are bits of clutch pack material that are suspended in the fluid. This is a sign of normal wear and tear but if the particles become excessive, or if the particles are metallic looking; this could indicate some major problems. The last thing you can do that can help determine fluid condition is give it a sniff. Worn out fluid will have a definite burnt smell to it and fluid from a transmission that has completely failed smells downright disgusting.

Many new cars do not have dipsticks. On these vehicles the fluid must be checked by climbing underneath the car and removing some kind of plug from the side of the transmission in order to see the fluid level. Some of these newer cars will still have the dipstick tube but no dipstick in it. On top of the tube you will find a plug that says in order to check the fluid level you have to take to take the car to the dealership service department. Once there, the technicians can check it with a special tool that looks just like a dipstick. This seems silly and it probably is. The reason for no dipstick is that the car builders want you to believe that you don’t need to check or maintain the fluid. Many of them actually say that the fluid they use is good for the life of the vehicle. This is not exactly true but with modern synthetic fluids, the fluid is at least good for the warranty period and that’s good enough for them. Some cars have a sensor in the transmission that will monitor fluid, and the level can be checked via the information computer located in the instrument cluster.

ATF Doesn’t Get Consumed

If you check your ATF regularly and find that you always have to add a little every few months then you have a problem. Transmission fluid does not get consumed as it is performing its job. Engines will naturally burn a bit of oil every time they are running which will cause the oil level to go down. Transmissions don’t burn fluid. If the fluid level drops it is most likely due to an external leak. Just because you don’t see puddle in the driveway doesn’t mean that there are no leaks. The only other place the ATF could go is into the coolant. The transmission fluid is pumped up to the radiator where it passes through a cooler that is located in the radiator, surrounded by coolant. If this cooler leaks it may allow transmission fluid to leak into the cooling system. This is very rare. Running the transmission low on fluid can be very damaging to the transmission. If the fluid is going away, find out where it is leaking and get it fixed.

Service Needed

Most of the time you can never go wrong following the manufacturer’s service recommendations, but when it come to ATF service there are a few things to consider. The thing that happens to just about every fluid in any automotive system is oxidation. When ATF becomes oxidized it is no longer able to perform its duties in an effective manner because it is no longer just pure ATF, it is some kind of oxide thereof. Think of the body panels on an old car. The rust that you see on an old car is oxidation of the metals that comprise that body panel. This oxidized metal is no longer good for maintaining the structural integrity of the car’s body panels. Oxidized ATF is the same way.

As mentioned previously, some cars supposedly come with ATF that is good for life. If this isn’t true then how can the ATF be serviced, and how often? The fluid can still be drained in the same ways that it is on older cars. There is always either a drain plug in the bottom of the transmission, or the pan that holds the fluid can be removed. Because these cars come with high quality synthetic fluid, this same fluid should be used when servicing such cars. The interval for this service will be fairly long, but it probably shouldn’t be ignored. If the supposed lifetime fluid were service every 80,000 to 100,000 miles, this would probably suffice.

Transmission fluid drain plug in the pan. Above that the spin-on type
fluid filter can be seen.

All of the regular cars out there that still have transmissions with a service interval usually need to be serviced about every 30,000 to 60,000 miles. Sometimes the service might just involve a drain and fill of the transmission, some require the replacement of a spin-on filter on the side of the transmission that looks like an engine oil filter, and some require that the pan on the bottom of the transmission be removed, and the filter screen behind it replaced. In reality, removing the pan and replacing the filter screen is really not that necessary. The screen should never get plugged by normal wear and tear, and if it does get plugged to the point where fluid flow is restricted, the transmission is probably on its way out anyway. 

The filter screen behind the transmission pan

In some instances a transmission flush may be appropriate. This involves hooking up the transmission fluid lines to a fancy machine that will remove all of the old fluid and replace it with new fluid. This is good for getting all of the old fluid out. When the transmission is drained through regular means you can usually only get about 4 to 6 quarts out, which isn’t that much considering the transmission actually holds upwards of 10 to 12 quarts. If the ATF is serviced as often as it should be, a flush isn’t really necessary, but if the fluid has long been neglected, or the transmission seems to shift a little harshly then a flush might be a good idea.

The last consideration when either adding fluid or servicing the fluid is what type of fluid to use. Like so many other things on the car, this has become complicated by the fact that every auto company seems to use some kind of proprietary fluid. Regular common ATF is usually referred to as Dexron/Mercon. The Dexron name came from GM way back in the day, and the Mercon name comes from Ford. Recently with the advent of synthetic ATFs these two names are not seen together anymore. Instead we have Dexron VI and Mercon V, which it’s safe to assume are not really the same thing. The biggest thing to remember is that if the transmission was designed for a nonsynthetic ATF, then Dex/Merc III is probably fine. If it was designed for synthetic, then use the Dexron VI or the Mercon V. Many cars that are neither GM products nor Ford products will use Dexron/Mercon. Honda automatics however, are not at all like regular automatics so they should use what is known as ATF Z-1. If this is too confusing then just go to the dealership parts department to get your fluid and then you know that it will be the right stuff. If you don’t use the proper fluid it probably won’t hurt your transmission, but the shifts might not feel as smooth as they should.