There has been a variety of conversations by owners and mechanics about the servicing of automatic transmissions that have seen high mileage. Some of these thoughts on service are really not good practice although they may seem to be safeguarding the owner they can lead to problems and should be addressed.
When a vehicle has not seen perfect maintenance over a long period or when a new owner assumes a used vehicle there is a time where all maintenance should be verified and performed. It is pretty much expected that when you purchase a used vehicle that one of the very first things you will do is change all of the fluids, filters and parts that wear out. This includes everything from changing the motor oil to installing new tires, brakes and so on.
However there has become a practice in the industry to tell people that own vehicles with automatic transmissions that unless your maintenance has been perfect that you should not consider changing the fluid in your system.
In my opinion this is really bad advice but there is a reason behind it. During the 2000’s decade there were a number of manufacturers that had bad experiences with their transmissions. Honda is one of these and owners would go into a quick lube place and have their oil changed and the tech would recommend a transmission flush which normally meant draining what they could and replacing the fluid with generic crap fluid. Unfortunately Honda transmissions of this era require only official Honda transmission fluid. So while you thought you were doing the right thing and saving a few dollars by getting it done outside of the dealership it resulted in meltdowns of many transmissions. Other manufacturers have had the same experience and what became the fable was that if you change your transmission fluid on a high mile vehicle you have the chance of causing it to slip. The premise was that the metallic fragments and brake lining type materials on the bands and disks that were suspended in the fluid kept the transmission functioning and that removing this material by changing it with new fluid took away that friction ability.
Consider this, you bring your car in for inspection and the tech tells you an entire section of your frame is solid rust. Then they suggest that you do nothing about it because it is the rust that is holding your car together. Yes, This is pretty much the insanity behind the idea that you shouldn’t change your fluid in your automatic transmission if it has high mileage.
First to understand this you have to realize what transmission fluid is for. It is a hydraulic fluid just like in your brakes that is used because it is not easily compressed under high force and temperature. It is used to transfer the energy of your engine through the transmission and then finally out to your drive wheels and tires. If the fluid is compromised then it needs to be replaced because it will not be performing at its best. Compromise can be from materials that are in it or maybe it has moisture or it is burnt. Whatever the reason the hydraulic properties of the fluid have been degraded and it needs to be replaced.
Now techs will say I have had this happen before I change the fluid and the transmission starts slipping. Well that is not the problem of the fluid it is because the transmission is worn. If the same fluid is allowed to stay in the transmission it will continue to cause wear on the internal parts and it could also clog tiny passages that route the fluid while different gears are selected.
It is just a really bad concept to say rust is holding your car together so you should just let it be and be ignorantly happy. If the parts in your transmission are compromised and changing the fluid exposes this then there may be an option to adjust the bands like you would a brake shoe or you will need to service the transmission.
It is like using engine stop leak for anything but an emergency. Engine or Radiator Stop Leak is a material that can stop small leaks in your coolant system but if it can stop a hole in your radiator it is also going to gum up the entire inside of your engine and radiator. Sure it might get you 15 miles down the road in an emergency but it does so at a cost. Keeping bad transmission fluid in your transmission because its filling a void between surfaces is the same thing. It is not proper and anyone that tells you it is the right thing to do is going to cause you more trouble in the future.
Now this is not to say its always wrong I have to admit that. If you are preparing for a trip you probably should not do this because you could have a problem while you are on the road and out of your normal area. The same might be true if you expect to trade in the vehicle in the next few months. It doesn’t make a lot of sense to do it then if your transmission is working. But if you expect to keep your vehicle for some time or you just purchased it then it is a good decision to keep up with all of your maintenance tasks.
Performing Transmission Fluid Changes
After all the talk about whether you should or should not perform a fluid change on your transmission we should consider the options available to do the work.
Every time you perform a service you should realize that there is a lot of fluid that you can not just drain from the system. A large amount of fluid will remain in the torque converter, pump, valve body and the rest of the system. For this reason if you are not using a positive pressure fluid pump to replace all of your fluid it is suggested that you perform a number of changes to your fluid within a reasonably short period that way you dilute and replace the old fluid with new fluid.
Older style transmissions that have both the ability to drain and replace the filter should have the filter replaced at each change. Normally this leaves fluid in the converter but by changing the filter every time you will remove material that is still held in the system. Cleaning the pan and if there is a magnet in the pan is important.
Newer systems that only have a screen and do not have a replaceable filter should have the fluid changed in a series of steps. If your system holds 10 quarts and only 2.5 quarts can be removed by draining then you would perform 4 changes. Unfortunately these systems do not have pans with magnets so flushing by changing is your best option until the fluid appears to be of normal color.
Sealed Transmissions may have service recommendations that a positive pressure pump be attached to the fluid lines that go from the transmission to the radiator. This is a service that can not be performed by a owner and needs to be performed in a shop.
Other options include removing the fluid line at the radiator and allowing the pump in the transmission to expel the old fluid while a container of fresh fluid is sucked into the system. This may be an option for all three types of transmissions however it is not something that a factory service manual will suggest for most vehicles.
Adjusting your bands is another service option and you should check your service manual for instructions on how to set your bands if this step is necessary. Normally you only do this if you are having problems and it is not a step that is required at every fluid change.
New Vehicles With No Service Transmissions
In the past few years some manufacturers have been selling vehicles where the transmission is not suppose to be serviced. I have talked with a number of my friends that are very experienced mechanics and many of them working at dealerships that sell these vehicles and they are worried about this situation.
Forty years ago it was very common to change your transmission fluid and filter every couple years. About twenty years ago filters were removed from many transmissions and a screen replaced them. The screen was unserviceable if you were doing a fluid change. Today no maintenance transmissions are starting to appear and the common thought is that this will lead to very expensive transmission replacements. Average mechanics can no longer service these new transmissions because you need special tools to perform the work. Some mechanics will go the cost and purchase the tools but it is not the same as it ever has been in the past.
With vehicles losing their value much quicker you can find used vehicles less than 10 years old that have basically no trade in value these cars are becoming disposable. Many of the systems are getting too complex to maintain and materials like ultra high strength steel and aluminum used in the bodies means general repair can no longer be done and cars are getting totaled out by insurance companies because they got in a small fender bender.
This is the way of the future and as both a technician that has worked for a factory dealer and as a consumer I find it to be pretty unacceptable. An average person with a good set of tools and the knowledge needed should be able to service and repair their vehicle for many years after the warranty runs out or for even decades if they desire. Not only that they should be able to easily modify the vehicle to fit their own needs. This is not the case anymore and we are all paying for it with cars we throw away instead of handing down to children or keeping for an extended period. Manufacturer’s expect that we purchase new every 3 to 7 years but for many of us that expense is not reasonable.
Final Note
So this has been an overview of whether you should service your vehicle or sit back and count the minutes until it fails on you. Servicing is your responsibility to keep your vehicle in its best condition. Some of those things are making sure its washed and cleaned and waxed so it won’t rust but other things are easy to ignore. Sure we might change the engine oil or the brakes when they start to make noise but things like transmission fluid never really come to mind as long as the car is shifting through its gears and getting us where we need to be.
If maintenance is delayed for any reason that is not a reason to think it can be skipped entirely. That is what some technicians would have you to believe but it is really not in your best interest to neglect maintenance for any reason.