Consumer Guide: Automatic Transmission Care & Fluid Flush Service

The ubiquitous automatic transmission is something that most of us take for granted in our daily routine, often leaving it negelected and misunderstood. Being complicated with many internal parts, it is one of the most expensive components to repair in our cars and trucks. This guide is for the daily user to get the most out of their automatic. It is not a “how to” tutorial on servicing and rebuilding transmissions nor does give a detailed explanation how an automatic transmission works. My goal is to dispel the many myths and otherwise bad information out there to help you, the consumer, make informed decisions when having your transmission serviced. And of course, this information should be useful anyone that services their own vehicle.

Shift Into Reverse

The first mass produced automatic transmission was GM’s Hydra-Matic introduced in 1939. With humble beginnings, automatic trasmissions have progessively gotten more and more sophistcated over the years. Throughout the 1950s, 2 speeds were the norm like in the Chevy Power-Glide. By then end of the 1960s, 3 speeds became the standard. A fourth, overdrive gear followed suit with electronic controls coming online in the 1980s. Now 5 and 6 speeds are common and there’s even an 8 speed available from Lexus! An automatic trasmission has hundreds of parts with some of these newer units having over 1,000 pieces which translate into more expense if they require replacement.

What’s going on in there?

Automatic transmissions basically work with various internal components that are “held” and/or “applied” by friction devices known as clutches and bands. With the given combination of these applications, power is transmitted through the transmission at the necessary forward ratios or reverse.

The torque converter itself is a hydraulic coupling device that basically serves as a clutch between the engine and transmission. It is what allows your car to sit still at a redlight while in gear. Add engine speed and the converter hydraulically engages moving the car.

Automatic Transmission Fluid (ATF) is a hydraulic fluid that serves to clean, cool and lubricate the internal components of the transmission. It is the same fluid that creates the “coupling” in the torque converter. ATF is also what hydraulically applies the components mentioned above. The fluid distrubition is through the valve body, a device with a fine maze of small passages and valves. There are numerous gaskets, rubber o-rings and seals that keep the fluid where it belongs.


Knowing how this network of parts work is not as important as knowing these factors:

  • There is pressure and friction going on inside of your transmission which means HEAT.
  • There are small moving parts with close tolerances. They must remain CLEAN and move freely.
  • Heat and contamination can damage and/or impede the operation of these components.
  • The failure of a single component will cause a malfunction or complete failure of an automatic transmission.
  • There is no such thing as a mechanic in a can.


Once something fails internally, it’s a diminished return; however there are some components that can be serviced without transmission removal or complete overhaul if they are dealt with promptly. Many times though, one failure will lead to another like dominoes.

Example: A small rubber seal loses its integrity because of excessive heat. This results in a loss of fluid pressure that would apply a component such as a clutch pack. That clutch pack begins to slip a little. The slippage generates even more heat damaging further components which causes even more heat. Friction media begins to shed itself into the “bloodstream” of the transmission. A particle of contamination impedes the function of another component and so on. It’s a vicious cycle of runaway temperatures until something finally burns itself out leaving you stranded.

More on the ATF…

Automatic Transmission Fluid is an often overworked and abused multi-tasker, as it cools, cleans, lubricates and is required to move parts under pressure. Over time, the heat and hard work will deteriorate the fluid and its ability to do the jobs we’ve asked it to do. It’s impotant for the fluid to be replaced at the manufacturer’s service intervals if not more frequently. This is especially important in heavy duty applications or severe operating conditions.

Something that is overlooked is the type of fluid that goes into your transmission. In the old days, it was pretty much Type F for Ford and Dexron for GM. Those days are gone and there are different types of fluid out there that are specifically engineered for the transmission they go in. Among other things, they each have different friction properties. They do not mix well and adding the wrong fluid type to your transmission can mean trouble down the road. Be wary of “universal” fluids. Your owners manual and sometimes dipstick will tell you what fluid goes into your transmission. I promise you that it will not say “Use only Universal Fluid from El Cheapo Auto Parts.” Some makes, especially German cars, specify a fluid that the chain parts houses do not carry. This means it will come from a dealer or import specialist and it’s expensive. It’s still better than replacing a $5,000 transmission though.

Use your dipstick…

The fluid level and condition should be checked once a week. If you don’t know how to check your ATF, it’s pretty simple to learn. A mechanic should be glad to show you, especially if you’re a lady. The transmission should be completely warmed to operating temperature. The car should be on a flat, level surface and in P(ark) or N(eutral). With the warm engine running, pull the dipstick and wipe it clean with a lint free cloth or paper towel. Replace the dipstick making sure that it is fully seated. Pull the stick again and note its level in relation to the marks near the bottom of the stick. There are generally 2 marks around an inch apart. The warm fluid level should be up to the upper mark but not past it. A level showing between the marks is usually acceptable. If it is short of the lower mark, then the proper fluid should be added. This will require a funnel that will fit into the dipstick tube as it is added there. Only add a small amount at the time, checking the level frequently as you add. Do not over-fill your transmission. Different manufacturers will mark their dipsticks differently and often have some type of directions stamped onto the dipstick itself. Your owner’s manual or workshop manual should give you the information specific to your car.

This dipstick is from a 2002 Dodge Neon with about 150K miles and a fairly well maintained transmission. The fluid level is fine. The fluid condition is not bad but is dark enough to warrant a service in the near future. Notice that this stick has separate ranges for both “hot” and “cold.” The fluid still should be checked “hot” in which would be the upper 2 marks on this stick.

What’s that smell?

Along with the fluid level, it is important to pay attention to the condition of the fluid. A simple visual examination and smell test can tell you a lot about what is going on inside of your tranny.

Take a sniff of the fluid. It should smell like oil and nothing else. If it smells scorched, there is an internal problem. A friction component has gotten too hot and burned up. The scorched smell will coincide with very dark fluid color. Any particles on the stick, especially metallic, indicate an internal failure. If the car is still drivable with scorched fluid and/or a metal presentation, then it’s only matter of time before the transmission will die. Under those circumstances, servicing the transmission is a near moot point and may lead to a quicker break down.

Look at the color of the fluid after wiping some on a white lint-free cloth or paper towel. Fresh fluid will have a bright rosy color. It is normal for ATF to darken over its service life so don’t be alarmed if your fluid has a brownish tint to it. This is a good cue for a proper transmission service in the near future. If the fluid is really dark but not burned, the service needs to happen ASAP.

If the fluid appears to be milky, then water has gotten into the transmission. This usually means the factory transmission cooler, internal to the radiator, is leaking allowing engine coolant to contaminate the transmission fluid. This needs immediate repair to avoid transmission failure. The radiator should be replaced and the transmission properly flushed until all evidence of water contamination are gone. People have bypassed the cooler when this happens to avoid buying a new radiator. This is not a wise long term solution. At a minimum, an aftermarket cooler should be installed rather than running without a cooler at all.This still doesn’t deal with the fact that the radiator is failing and will eventually lead to an engine overheat related break down.

You gotta be cool…

Heat is the number one enemy of automatic transmissions. Letting high temps take charge is asking for failure. Auto manufacturers build a small transmission cooler within the radiator which means it is being cooled by hot engine coolant. It is better than nothing but still short of ideal, especially when towing or someone with a hot foot. You will find that part of a towing package is an auxillary transmission cooler.

Adding an aftermarket cooler to supplement the factory setup is relatively inexpensive and good added insurance for your transmission. There are absolutely no downside to this with proper installation. Most do-it-yourselfers can do this at home in an afternoon or a reputable shop can install one for you. These kits can be purchsed for well under $100 with some starting under $50.

Touching on fluid again: Low fluid levels will cause temps to elevate and if it’s low enough, things will slip starting that vicious cycle I mentioned before. Adding a cooler will give a slight increase in fluid capacity which will can handle the heat better.

The Great Debate.

There is much debate, confusion and misinformation online regarding the proper service of automatic transmissions. A lot of well intended but misguided information comes from auto enthusiast forums. Many times information is quoted as fact but is actually someone regurgitating something they’ve seen before. There are extremely knowledgeable people online that provide great information but there are far more people that just don’t know. Before banking information, you should consider if the source has had formal training and professional experience etc. Cousin It may be handy with home repair, oil changes and tune ups but it doesn’t make him a transmission professional. Who’s to say he didn’t learn a bad practice from the World Wide Web…?

A Clean Transmission Is A Happy Transmission.

Automatic transmissions need to be properly serviced at their required intervals if not before. The debate is over how they are getting serviced.

  •  Should the service be a drain and fill?
  •  Should it be dropping the transmission pan, cleaning it and replacing the filter?
  •  Should it be “flushed.”

These methods often get confused. There is actually more than one flushing method and a service doesn’t necessarily mean a flush was performed. The first is the “flush machine,” mostly found at the nation’s Fasty Lube chains. The other is a method used by dealerships, repair garages and the guys at home. Each have there merits and pitfalls.

Junk in the Sump

There is one underlying fact that you should consider before making a decision on where or how you will get your transmission serviced. At the bottom of your transmission is a pan. Think of it as resovoir of sorts for your fluid. It is removable, and in most cases, it is removed to replaced the transmission filter. Over time, fine particles from normal wear accumulate in the bottom of the pan. Most pans will have magnets to keep the metallic particles in place. When you remove a pan you will have an almost paste like “slurry” coating on the bottom of the pan which consists of friction material and fine metal particles blended with fuid. Higher mileage, negelct and abuse will present more matter in the pan. Removing the pan is the only way to clean this gunk out. The pan pictured here is a fairly mild case with normal use and service.

The Traditional Service

The traditional automatic transmission service consisted of removing the pan, cleaning it and replacing the filter. Most pans do not have drain plugs so it can be messy. A drain plug is nice since you can drain the fluid then drop the pan. Some folks will tap the pan and add a drain plug for future service.

The downsides to this are: Not all of the fluid will be removed. The average car transmission will hold around 12 quarts of fluid, give or take. Trucks and SUVs will hold more. This method will only get 4-6 quarts however some cars have a drain for the torque coverter which may get another 4 or so quarts. Some consider this “invasive” and therefore risky.

The upsides are: You get that nasty gunk out of your pan. You get a new filter. The technician will get a good idea of the condition of the transmission based on what’s in the pan.

Some people elect to first clean the pan, change the filter then repeat changing the fluid until it has been mostly replaced. It’s easier with a drain plug of course.

Straight Flush

One way of flushing the transmission is by disconnecting a cooler line from the radiator and pumping the fluid out (engine running) while adding new fluid through the dipstick.

The advantages are: All of the fluid can get replaced with less fluid wasted than in the traditional method. It is simpler, usually less messy and less invasive than the traditional method.

The disadvantages are: There’s a risk of pumping too much fluid out before replacing it with new fluid thus causing damage. This may not be so much an issue for professionals that use this method as it could be for a shadetree guy. The pan does not get cleaned or the filter replaced with this method alone.

The pan and filter may be serviced first and then the remaining fluid be flushed. I prefer this and this is how I service my cars as well as customers cars.

Cooler Line Flush Machines

The 2nd flush method is via the hotly debated flush machine. These are mostly found at fasty lubes and muffler/brake/tire chains although a few shops and dealers are using them.

The advantages are: It’s fast and convenient. It replaces almost all of the fluid.

The disadvantages are: The obvious disadvantage is the pan and filter are not being serviced which in lies the problem of these machines. Some models of this machine use higher pressure to “power flush” the transmission. That alone may cause transmission damage. The bigger issue is that the abrasive sump slurry is being forced up into the filter and possibly into the delicate inner workings of the transmission.

Many people have had this procedure done and find themselves having their car towed into a transmission shop for a $2,000 repair bill. This doesn’t mean that using this machine will ruin your transmission but it has and may happen.

Defenders of these machines make these arguments:

  • Replacing all of the fluid is better than a partial replacement. That in itself is true.
  • Removing the pan is invasive and risky. False
  • The filter is a simple screen and can be left in place. False. It is a very fine mesh that may become clogged when that past like slurry gets washed up into it. They were designed to be serviced and sold everywhere for a reason.

They also argue that newer model machines don’t use high pressure blah blah blah. It doesn’t matter. These machines do not clean the pan sediment from the transmission nor do they replace the fliter however they may very well wash it up into the old filter and beyond.

The machines are like slot machines. They are there to make Fasty Lube money. You may win or you may lose. In most cases, oil change technicians are not formally trained and certfied mechanics nor have they had training in automatics transmissions other than running the flush machine. I’m not attacking the individuals; they have a job to do and they do provide a convenient means of getting your oil changed, which everyone should be doing. The hook is they will invariably try to sell you overpriced products or services that at times are not even needed. Women are often easy targets of these services.

The youtube videos and before and after pictures are not telling the whole story. They don’t show what was in the pan and how it may have been washed through the transmission. Of course a freshly replaced fluid sample will be brighter than the old fluid. It will remain bright until a clogged filter or contamination stir-fries the transmission.

Defenders also say transmissions were ruined because they had a pre-existing problem. That may be true at times however a transmission on its last leg will tell you by the condition of the fluid. It’s not a good idea for a car owner to run get a flush when the transmission starts misbehaving. The average consumer may not know that though. They should actually have a mechanic check it first. A lube place with good practices should take a quick look at the fluid and ask the consumer questions before pushing another quickie flush sale and getting them out of the door.

I don’t know of any transmission shops that approve of these machines unless the pan and filter have been serviced first. Most transmission professionals will tell you to avoid them like the plague.

Pump Inlet Flush Machines

Finally, there is a newer type of flush machine known as a pump inlet flush machine. In this case the technician removes the pan for cleaning and replaces the filter. New fluid goes directly into the inlet and old fluid is removed without being recirculated through the transmission.

This advantage of this method is that it covers all of the bases: The mechanic can inspect the contents of the pan and clean it properly. You get all new fluid and a new filter. Its only disadvantage is the service will cost more which is insignificant compared to transmission repair.


I know that this was a lot of information but I hope that it will help you have the advantage in getting the most out of your automatic transmission. A happy transmssion makes for a happy car. A happy car makes for a happy car owner.