Monday, July 15, 2024 Detailed Auto Topics
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An automatic transmission is one of the most expensive components on a vehicle so protecting it where possible only makes sense.  While this should be simple misinformation is everywhere and knowing at which mileage to service an automatic transmission can be difficult.

The recommended service interval

The published maintenance schedule is often used by companies that rate vehicle.  The lower the maintenance listed the higher they may rate. Manufacturers know this and intentionally extend their service recommendations. While this may make costs appear lower it may not in the best interest of people that keep vehicles a long time. For instance recommending an automatic transmission service at 100,000 miles appears to cost half as much as recommending 50,000 miles. The problem is the odds of a failure can increase drastically at higher mileage.  When we fail to service our automatic transmission in a timely fashion the odds of failure are much higher.

For overall lowest cost we find it best to replace the fluid and if possible the filter before they are completely depleted. A proper automatic transmission service at 50,000 miles may prevent the need to rebuild the automatic transmission at 120,000 miles. Under severe condition, such as towing, a 30,000 mile interval is better. The same is true with automatic transmissions that have non replaceable filters.  On these units the fluid is replaced to prevent problems.

Automatic Transmission Operation

In normal operation automatic transmissions generate heat and some debris. Most of the debris comes from wearing of gears and bushings. This material circulates with the automatic transmission fluid until the automatic transmission filter removes it. In use the debris may block the pores in the filter. When this occurs the flow of automatic transmission fluid is restricted and damage can occur in the automatic transmission. A proper automatic transmission service will greatly reduce the chance of such a problem.

Heat is also a factor and causes the automatic transmission fluid additives to fail. Most heat is generated in the torque converter. The torque converter supplies the mechanical link between the engine and the automatic transmission. It is also designed to slip like a clutch, allowing the vehicle to stand still while the engine continues to run.

The basis of how an automatic transmission torque converter works

Torque converter lockup

To lessen heat buildup and help prevent energy loss torque converters have a lock-up feature.  This means once the vehicle is in motion a clutch, activated by the computer and "locks" the converter to stop the slip.  Automatic transmission makers accomplish this using fluid pressure and a clutch built into the torque converter.  When the driver attempts to stop the vehicle, the computer releases the torque converter clutch.  Releasing the clutch allows the torque converter to slip when the vehicle stops. 

Modern automatic transmission fluid has special additives that allow smooth engagement and release of the torque converter lockup clutch.  As these additives age, they lose the ability to perform their function.  This may cause a shudder when the  torque converter goes into the lockup. A torque converter shudder feels like a brief vibration. This often occurs at 35 to 50 MPH and is similar to running over a rough spot in the road. The shudder only last a few seconds, but if left untreated may damage the torque converter.   Often a proper automatic transmission service will help this condition by replacing depleted additives in the automatic transmission fluid.

Automatic transmission fluid color 

Heat causes automatic transmission fluid to darken in color.  Darkening of automatic transmission fluid is normal and this alone does not mean the fluid is bad. With more heat the automatic transmission may be damaged. This is why manufacturers equip most vehicles with an automatic transmission cooler.  The cooler helps eliminate excess heat and reduces automatic transmission wear.  A normal automatic transmission temperature is around 175 degrees Fahrenheit.  When a vehicle is used to tow the automatic transmission temperature will rise.  This causes the automatic transmission fluid to deteriorate faster.  Heat and additional wear are why vehicles that tow require more frequent service.

Another problem that develops over time is debris in the automatic transmission pan and filter.  Most debris generated in an automatic transmission is from the gears meshing together and "normal" wear from bushings and thrust washers.  With today's technology, clutches rarely wear out or generate much debris, unless caused by another problem.  A small amount of debris in the automatic transmission pan is normal, but metal particles and small pieces of metal show a problem.

Typical debris in a high-mileage automatic transmission

More metal than there should be, but not a big problem yet

Finding no metal particles in the pan is best.  In the photo above, we see some metal sticking to the magnet in this pan.  This amount of metal suggests a problem is developing, but very likely this transmission will continue to operate for quite a while. In this condition, a proper service will help this automatic transmission.  The amount of metal that is acceptable varies among automatic transmission types and models.  A trained technician can advise on how much metal in the pan is too much.


The small yellow stopper, with the O-ring, is normal in many Ford automatic transmissions. When they assemble the automatic transmission, they use the stopper to fill the dipstick hole. This keeps foreign material out of the automatic transmission when they ship it. When they insert the dipstick, the plug falls harmlessly into the pan. The service technician normally removes and discards this plug at the first automatic transmission service.

Excess metal, automatic transmission problems likely

Too much metal in the transmission pan

The fluid we see in this pan is full of metallic debris. Small metal flakes cover the pan bottom and shifting concerns are very likely. An automatic transmission with this amount of metal will likely fail in the near future. Metallic debris will stick to the magnetic solenoids and cause them to bind. This creates shifting problems and may set a check engine light. Servicing this automatic transmission is not going to cure the problem, as the source of the metal will continue to generate more.

Excess metal, major problem occurring

Failure has occurred in this transmission

The amount of metal on the magnet and in this pan shows the automatic transmission already has major problems.  If the filter is not plugged, it will be very soon.  Service will not help this automatic transmission as the damage has already occurred. Rebuilding, repairing or replacing the automatic transmission are the only long-term solutions.

Magnetic shift solenoids

Beginning in the late 1980's, vehicle manufacturers began using computers and [electromagnetic] solenoids to control when and how the automatic transmission shifts.  They mounted the computer on the outside of the automatic transmission, where it is protected from heat and debris.  The solenoids, mounted inside of the automatic transmission were vulnerable to the metallic particles being generated. 

Solenoid controlled transmission


Early automatic transmissions also use as little as one solenoid for torque converter lock up and two for gear changes.  Today automatic transmissions may have twelve or more solenoids.  Worse, the computer itself may be inside of the automatic transmission.  This exposes the computer to the heat and debris circulating in the automatic transmission.  Since some of these solenoids and computers come only as an assembly they are VERY expensive.  To protect these expensive components, the manufacturer uses one or more automatic transmission filters.  The automatic transmission filter is the primary protection for the transmission.

automatic transmission filters

Cutaway view of a typical transmission filter

Automatic transmission filters are designed somewhat like a sock. That is, fluid enters the neck of the "sock" and flows out through the fabric. The pores in the fabric are small enough to trap debris yet large enough for fluid to pass. Once inside the filter the debris is trapped and will not escape.

As more debris accumulates, more filter pores are clogged. In time the filter will become restricted and lessen the flow of fluid. This is critical as automatic transmission fluid acts as the lubricant for the automatic transmission. The automatic transmission fluid also acts as a hydraulic oil to activate clutches as well as a coolant, a cleaner and a liquid coupler in the torque converter.

When fluid flow becomes restricted, the automatic transmission can block fluid from lubricating the gears.  It does this to insure enough fluid is available to activate the clutches.  This keeps the vehicle moving but leaves the gears without lubrication. The problem gets much worse very quickly. This is why a proper automatic transmission service can help extend the life of a automatic transmission.

By design, an automatic transmission filter traps debris. If we could remove the debris, the filter is ineffective. We cannot clean an automatic transmission filter. When restricted we must replace an automatic transmission filter that is replaceable.

Toyota automatic transmission screens

Toyota refers to some of their automatic transmission filters as a screen. This is a matter of semantics and does NOT change the need for replacement. Well-meaning salespersons have told clients the screen is permanent or that it does not require replacement. This is not correct. Automatic transmission filters are sacrificial by design. That is, removing the debris that would otherwise damage the automatic transmission restricts them.

Proper Automatic Transmission Service

Before we perform any service on an automatic transmission, we should drive the vehicle thoroughly, paying attention to any symptoms. It is very helpful to have a trained technician drive the vehicle. The test drive may suggest problems, or bring attention to needed adjustments that we can take care of during the automatic transmission service.  With a complete automatic transmission service, the unit is then inspected for leaks or any external signs of problems, before draining or any disassembly.

Some of the steps in a proper transmission service

We drain the old fluid and inspect it.  Excessive metal content, a very dark color or burnt smell may show a problem.  On transmissions with removable pans, we should always remove the pan. This allows inspection of internal transmission components. A careful inspection can reveal early warning signs of trouble. Many times tightening of a bolt, or an adjustment at this point will prevent a major failure in the future.  This is also an excellent time to address small leaks.  We can easily repair a shifter-shaft or fill tube seal at this time, preventing expensive re-work.

On automatic transmissions with replaceable filters, we also remove and replace the automatic transmission filter. The best technicians often cut the old filter open and examine its contents. By the nature of the debris in the automatic transmission filter much can be learned about the condition of the automatic transmission. Some automatic transmissions do not have a removeable pan, and the filter may not be able to be replaced. On this type of automatic transmission, the fluid is drained and inspected. The automatic transmission is refilled, driven and the fluid is immediately drained again. This removes a majority of the debris and replenishes depleted additives in the fluid. This is not as helpful as a full service, but if done before we have fully depleted the fluid it is effective.  Automatic transmissions with a filter that cannot be replaced should be serviced more frequently to prevent problems.

Automatic Transmission Fluid Types

A few of the many types of automatic transmission fluids

Because of the huge number of designs in common use, automatic transmissions often require a specific fluid. For instance, Honda requires one of two fluids, with different characteristics.  Honda fluid is also different than the fluid used in Toyota. In fact, even different model Toyota vehicles require different fluids. The same is true of Ford, Chrysler, Mitsubishi, late model GM and most others.

Often after design, a automatic transmission may exhibit a characteristic that is not desirable. When this occurs, design engineers may formulate a fluid to alleviate the undesired effect. The same is true for problems that occur in service. Sometimes automatic transmission fluid is revised and upgraded after releasing the automatic transmission. With so many fluids on the market it is imperative the correct fluid is installed in your vehicle.

Refilling to the proper level is also very important and far more difficult than it may seem. The vehicle has to be level and the automatic transmission must be at the proper temperature. Properly reading the dipstick is also imperative. Failure to observe these requirements results in an improper fluid level.

Many automatic transmissions today no longer have dipsticks. Such automatic transmissions require a specific procedure for checking the fluid. They often require a scan tool or other special tools to check fluid level. Precise fluid level is mandatory in modern automatic transmissions.

Why not flush an automatic transmission

In recent years, the practice of flushing a automatic transmissions has become very popular. Flushing is quick and easy, and very profitable to those that perform it. With this method the cooler lines are disconnected. With the engine running, a small portion of the fluid is pumped out of the automatic transmission while new fluid is added.  Contrary to claims, this does not replace most of the fluid.  Only a small portion of the fluid is circulated through the cooler lines.  The rest goes to the torque converter, clutches and is returned to the pan by the pressure regulator. 

With flushing, they catch the fluid flowing through the cooler lines.  Fresh fluid is added to the return line and flow to the automatic transmission pan.  This fresh fluid mixes with the old fluid in the pan and some is again pumped out of the cooler line.  A lot of fresh fluid is pumped out and a much of the old fluid remains in the unit.  In our opinion, this method does little good and is NOT a substitute for a proper service.

When flushing an automatic transmission, they do not remove the pan and do not replace the automatic transmission filter. It is not possible to properly inspect the inside of the automatic transmission. This method may also stir up debris in the automatic transmission and further restrict the filter.  After flushing, an automatic transmission may be worse than before.

Pan and filter from a transmission that was recently flushed

The filter in the above image is from an automatic transmission that was flushed about one week before. The filter is full of metal and clearly has not been cleaned by flushing the automatic transmission.   

Another problem with automatic transmission flushing is that often a single type of fluid is used to flush all automatic transmissions. It may not be practical to have a flushing machine for each type of fluid in use. Generic fluids are also considerably less expensive than the specialized fluid the engineer may specify.  For these and other reasons, GM and other manufacturers now state that their automatic transmissions are NOT to be flushed and we should only perform a proper service.

Other Possible automatic transmission Service Problems

The most common problem with automatic transmissions is fluid leaks. Leaks can occur when the automatic transmission pan is not properly sealed or the bolts improperly tightened. Leaks can also occur when cooler lines are removed for flushing. Bent lines sometimes leak or come disconnected when driving.  The loss of fluid from these leaks can destroy an automatic transmission.

Cooler lines and the transmission cooler should also be insprected

A proper automatic transmission service always includes an inspection for leaks.  We closely check the automatic transmission, all lines and hoses and the cooler and repaired if needed.  Even a small leak, will get worse in time and can ruin an automatic transmission.

Leaks result in the fluid level being lowered in the automatic transmission. As the automatic transmission fluid level drops the pump can not pick it up as it should. With a low automatic transmission fluid level, the pump draws air into the system, pressure may drop and the automatic transmission can shift to neutral or quit pulling. Extensive automatic transmission damage can be quickly occur from low fluid.

Other service problems

Other problems can also occur in an improper automatic transmission service. They sometimes install the incorrect filter or improperly install the proper filter. Some automatic transmissions use multiple filter depths. Each filter may appear very similar and may fit, but not function properly. For example, GM uses multiple filters on the 4L60E automatic transmission, depending on the depth of the automatic transmission pan. Each filter will fit, but only the correct filter for the application will work properly.

Many manufactures use different filters on a transmission

The correct automatic transmission filter for the application will fit near the bottom of the pan, leaving room to draw fluid in. If they use a filter designed for a shorter pan, it may draw air in, as the fluid sloshes in the pan. Filters designed for a deeper pan will bottom out. This restricts the intake of the automatic transmission filter. When they tighten the pan, they may also damage the seal at the top of the filter.  An expert technician will always make sure that the appropriate, quality filter is use and installed correctly.

A proper automatic transmission service can never harm an automatic transmission. A quality automatic transmission service can prevent many problems and extend the automatic transmission’s useful life. Improper service can be worse than no service at all. I feel it is very important to have only a trusted specialist service your automatic transmission.

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