[Mikecol]

So, the Tundra has a sealed transmission. That means that the fluid is not allowed to freely interact with air. The oxygen in air degrades the fluid over time and it has been known since the 1950's that if you seal transmission fluid from air, it is not degraded by the oxygen. The air that is in the transmission when assembled does interact with the fluid until the oxygen is used up and you are left with an inert atmosphere of nitrogen that protects the fluid.

In a sealed transmission, if you don't tow heavy, off road your truck, and drive in such a way that you minimize shifts and keep the torque converter locked up, your transmission fluid will last a lifetime, which probably means several hundred thousands of miles.

However, if you tow heavy, drive your truck so it is constantly shifting or the torque converter is running unlocked a lot (ie, around town driving, off-roading, etc.), the fluid is being stressed through the torque converter and will wear over time and need to be replaced. In these situations, you will need to replace your fluid.

It is normal for transmission fluid to turn maroon in color. But when it turns brown or black, that is bad. The brown/black is caused by the transmission fluid getting burnt from heat and stress and indicates that you definitely need to have the fluid changed and may have done damage to your transmission.

Personally, I only occasionally tow a light travel trailer and when I do, I keep the torque converter locked up and drive it easy. When I am not towing, I basically and driving long distance on highways with little to no stress on the transmission. In my case, I think a drop the pan, replace the filter and fluid in the pan, at about 100-150K interval is sufficient for my transmission fluid to last several hundred thousand miles (and that is what I have done so far and am at 180K and no issues).

But if you are in town driving, towing heavy and hard, off-roading, etc, you should change your fluid more often and consider a full fluid change instead of just replacing what is in the pan.

Last, you won't hurt anything by changing the fluid more often than needed. It just costs money. But you can damage a sealed transmission if you don't change it right and don't get the level right, so make sure you know what you are doing or that your mechanic knows what they are doing. It is not an easy process. I had mine done at the dealership and the filter/pan fluid change was under $200. I am sure a full fluid change is probably double that price.

 

[Mikecol]

Anyway, I am wondering how the transmission, being "sealed', handles the change in internal pressure due to expansion and contraction of the trans fluid. I could not find an answer in my research. I would think it would need some sort of expansion tank. Everything else filled with oil in the drivetrain has a breather fitting of some kind.

The air space in the transmission really regulates the pressure and allows for expansion and contraction of the fluid. And while the fluid does expand some, the transmission case also grows with temperature and offsets some of that expansion.

To calculate the air space pressure, you can use the equation pv=nRT to calculate the pressure created in the transmission from the change in temperature. V is volume and stays constant and n and R are fluid properties and so they stay constant as well. When you solve the equations, all the constants cancel out and you are left with p1(T2)/T1 = p2. T and p are in absolute units, and if we assume the final temperature is about 200F, you end up with 14.7(200+460)/(70+460) = p2 = 18.3 psia, or 3.6 psig.

If you assume the volume of air is reduced by 33% because of the fluid expansion, then you would multiply p2 by V1/V2 or 1.5/1, and the pressure would be 5.4 psig.

You can play around with the numbers and see what the pressure would be if the temperature was 300 instead of 200, or if the air space volume were reduced by 50% instead of 33%, but no matter what you do, the pressure is not very much and is easy for the transmission seals to handle.

 

[Mikecol]

I will prefer to do the change myself... However, I have some prepaid Oil Sample kits from Blackstone Labs, so as soon as I can I will suction out a sample thru the fill plug opening, (only a few ounces) and see what the report says first. I would intend to do a complete change of fluid, either myself or by the local dealer.

When you get the results back, please update this post and let us know what they say along with your driving style so we all have an idea of how much stress your trans fluid has experienced. The more data points we get from people who do the tests, the better.

I am curious how much a test costs. If it's not outrageous, I would love to have mine tested at the next change to see how it looks. My 15 CM 5.7L I would be at about 250k on my next service interval, which is probably 3 years from now, and all but what was in the pan would be original fluid at that point.

 

[Mikecol]

$200 is very inexpensive sounding for a fairly Labor intensive pan drop/filter/5 qt WS fluid service.
You sure they did the job???

Yep

 

[Mikecol]

Transmission has breathers. Heat is the enemy, not oxygen.

In a non-sealed transmission, heat, fluid stress, oxygen, and contamination all require the transmission fluid to be changed.

In a sealed transmission, heat, fluid stress, and contamination require the transmission fluid to be changed.

The breathers do not allow the transmission fluid to come in contact with outside air. If they did, it would not be called a sealed transmission.

 

[Mikecol]

Yeah, let's argue about the definition of a sealed transmission. This should be good. Get the popcorn folks.

 

[Mikecol]

OK. For those who want to learn, here we go.

Here is a link to the patent for a sealed transmission breather. This will explain why a sealed transmission has a breather and how it prevents or minimizes oxidation of the transmission fluid. US5129422A - Transmission breather control valve - Google Patents

Here is one from the society of automotive engineers (SAE) also about breathers on sealed transmissions, and although you have to pay to read this one, you can read the summary and get the idea of what they are talking about. Transmission Air Breathing Suppressor (TABS) Valve - A Device for Improving Automatic Transmission Fluid Life

Here are a series of 5 history of transmission fluid videos done by Prof John Kelly who is the most knowledgeable transmission nerd I know of. In part 4 of the history of Chrysler Trans Fluid he talks about the discovery of sealed transmission fluid longevity and explains why preventing people from checking the fluid and adding fluid and exposure to oxygen are all bad and led manufactures to eliminate anyone being able to check or change fluid easily. He talks about doing a lifetime fluid video, but it does not look like he every finished it. If anyone can find it, drop a link because I would love to watch that.

This is a good article that has a section that talks about lifetime fluids. It has a good history of transmission fluid and talks about the original discovery of hermetically sealing fluid in the transmission by Chrysler . https://everipedia.org/wiki/lang_en/Automatic_transmission_fluid

A couple more things:

I don't care one way or the other if you change your transmission fluid. I can only explain the engineering behind why manufactures are saying that under normal driving conditions you don't have to do that. You do what makes you feel good.

I am not going to explain the math. It's Jr high school level chemistry and algebra.

Last:

For those who want to argue against Toyota's recommendation to not change the trans fluid, I ask what is Toyota's motivation? Are they trying to boost their 15 year/300,000 miles transmission sales? Why would they want to sell less transmission fluid and why would the dealers want to do less maintenance work? And why would they want to purposefully shorten the life expectancy of their trucks over transmission fluid?

I think the answer is that they think their transmissions on average will last 3-400K miles without any service, and even when the transmissions does fail, it will probably not be because of the fluid. And that happens to be my definition of lifetime as well, so I don't plan to do anything more than a pan drop and filter change every 125-150K.

The best of the best to ya all!

 

[Mikecol]

Screen filter, not like Oil filter.
Would take a lot of clutch/metal grinds to make fail, but not a lifetime filter particularly, like the 'Lifetime Fluid'.
LOL

I really wish it was a filter.