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I recently got a new to me 2013 Tundra with just under 17,000 miles on it. I’ve been replacing old fluids with new. The big question; should I have the transmission flushed or drop the pan change the fluid and pump out the old?
 

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A 2013 with 17K miles on it? Nice find! Personally, I would leave the tranny fluid alone until around 50K THEN change it. My 2010 has just over 40K on it and I will change out the tranny fluid at 50K. Yours is most likely a sealed unit like mine. Your fluid is probably still very clean like mine is. When the time does come, I will have the pan dropped.
 

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I dropped my pan and replaced the filter. The pan had a lot of deposits in the tray, cant be seen otherwise. Shifts a lot smoother after the change.
 

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The Tundra is a sealed transmission system. Air is what causes the transmission fluid to go bad. Toyota says if you don't tow with it, you never (never) have to change the fluid. If you read up on what a sealed transmission system does for transmission fluid life, it makes sense. I do tow a little bit so I changed my trans fluid in my current 15 CM 5.7 at 110K. But in all my other Tundra's, I never changed the trans fluid. I ran them all to a little over 100K each without issues. Your not going to hurt anything by changing the fluid, but be aware that all they do at the dealership is pull the pan, change the filter, and add whatever fluid is lost in the pan (which is only a couple quarts) so it is not much of a fluid change.
 

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A . Yes we know.
B. Please explain . I would but i'm still chuckling .
It seems like you don't know what a sealed transmission system is. There is no dip stick It is a completely sealed system. No air enters or exits the transmission. This is unlike other manufacturers that have a dip stick because those transmissions allow air to breath in and out of the transmission. But a Tundra transmission is not like this. It is a completely sealed system so no air enters or exits that transmission. They did this because exposure to air is what causes the breakdown of transmission fluid over time. If the system is sealed, and no air enters or exits the transmission, the transmission fluid does not break down, and therefore the fluid never needs to be changed.

Toyota recommends that if you tow with your truck that you change the transmission fluid. But if you do not tow, they recommend you never change it. And even if you do tow with your truck, the service they do is just to drop the pan, change the filter, and replace whatever fluid was in the pan, which is not much. So, you really are not changing the fluid.

I am guessing that you could take it to a quick lube store and they could maybe use one of the transmission fluid exchange machines they have to change all the fluid in the transmission, but I am not sure that machine works with the Tundra.

As I said above, you won't hurt anything by having the fluid changed, but the transmission is designed so that the transmission fluid never needs to be changed for the life of the truck with normal use. Never is a long time. And a lot of Tundra owners struggle with never. But, the sealed transmission is designed to never require a fluid change.
 

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It seems like you don't know what a sealed transmission system is. There is no dip stick It is a completely sealed system. No air enters or exits the transmission. This is unlike other manufacturers that have a dip stick because those transmissions allow air to breath in and out of the transmission. But a Tundra transmission is not like this. It is a completely sealed system so no air enters or exits that transmission. They did this because exposure to air is what causes the breakdown of transmission fluid over time. If the system is sealed, and no air enters or exits the transmission, the transmission fluid does not break down, and therefore the fluid never needs to be changed.

Toyota recommends that if you tow with your truck that you change the transmission fluid. But if you do not tow, they recommend you never change it. And even if you do tow with your truck, the service they do is just to drop the pan, change the filter, and replace whatever fluid was in the pan, which is not much. So, you really are not changing the fluid.

I am guessing that you could take it to a quick lube store and they could maybe use one of the transmission fluid exchange machines they have to change all the fluid in the transmission, but I am not sure that machine works with the Tundra.

As I said above, you won't hurt anything by having the fluid changed, but the transmission is designed so that the transmission fluid never needs to be changed for the life of the truck with normal use. Never is a long time. And a lot of Tundra owners struggle with never. But, the sealed transmission is designed to never require a fluid change.
Thanks but , It is easy to grasp what a sealed system means . Pretty sure i got that part down.
The part that makes no sense is " AIR is what breakdowns the ATF " .
ATF is oil , synthetic oil , with many additives .
What will breakdown any oil or makes it in effective Is : dirt , heat , combustion by products blown past the piston rings , and metal particles . Of coarse one of those will not apply to ATF oil.
I will agree a 100% sealed system will be cleaner , but the notion of AIR breaking down ATF is hilarious .
 

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Thanks but , It is easy to grasp what a sealed system means . Pretty sure i got that part down.
The part that makes no sense is " AIR is what breakdowns the ATF " .
ATF is oil , synthetic oil , with many additives .
What will breakdown any oil or makes it in effective Is : dirt , heat , combustion by products blown past the piston rings , and metal particles . Of coarse one of those will not apply to ATF oil.
I will agree a 100% sealed system will be cleaner , but the notion of AIR breaking down ATF is hilarious .
There are primarily three things that cause transmission fluid to deteriorate. Oxygen chemically reacting with the oil, heat/stress, and dirt in the fluid.

Transmission fluid is both a lubricant and has specific viscosity properties that create friction when the plates in the torque converter spin in close proximity to each other transferring power from one plate to the next. Oxygen and heat/stress both cause the fluid to breakdown over time and affect the viscosity of the fluid which affects its ability to transfer power. As I understand it, the breakdown of the fluid is primarily that the viscosity properties go out of spec, but I don't believe it causes a significant change in the lubricity of the fluid (but I could be wrong about that, it may be both).

It has been known for a long time that transmission oil breaks down by the chemical reaction of oxygen in the air with the fluid. I believe this was first discovered by Chrysler in the 1930's where they found if they kept transmission fluid in a hermetically sealed system, the fluid did not deteriorate over time.

A sealed transmission has some air in it when it is first assembled. This air (specifically the oxygen) interacts with the transmission oil and is absorbed into the oil, and what is left is an inert atmosphere of nitrogen in the transmission. By keeping any additional air from entering the system, the chemical reaction of the oxygen with the transmission fluid stops. Any time any part of the transmission is opened, air enters the system, so the dip stick has been removed completely and there are no vents on the transmission. It is a totally sealed system.

If the transmission oil is then not excessively stressed or heated, then the fluid retains its viscosity and lubricating properties and lasts a "lifetime" never needing to be changed.

If you tow with the transmission, then the transmission fluid is experiencing much higher levels of loading and heat and that will cause the fluid to lose it's viscosity properties and will require it to be changed. The thing that does the most damage to transmission fluid is the torque converter. When it is locked up, there is no stress being applied to the oil. But when it is unlocked and the power of the engine is being applied to the transmission through the fluid in the torque converter, that causes extremely high heat and stress on the fluid and breaks down the viscosity properties of the fluid.

The bands (clutches) in an automatic transmission do wear and the debris that comes off the bands are filtered out by the filter. Again, towing puts a lot more stress on the bands, especially when shifting from gear to gear, and the higher the load on the engine when it shifts, the more wear and the more debris in the filter. This also contributes to the need to change the filter when towing.

The fallacy of all this is that when you do take your truck in for a transmission fluid change, they only drop the pan, change the filter, and replace the oil that was lost in the pan. That is not a fluid change. It is really just a filter change. They maybe replace 2-4 qts of fluid so you still basically have the same fluid in the transmission that you started with.

I have had 4 Tundra's. The first 3 I ran to 99K, 86K, and 132K and I never serviced the transmission and I never had any problems. I occasionally towed a large TT but not for long distances. My current Tundra is a 15 CM and I have towed trailers for long distances with it and so I had the transmission serviced at about 120K (filter change), and I can't tell any difference. If I had to do it over, I think I would not go back and do the transmission service. It is kind of expensive and I don't think it really accomplished anything. I did it because I thought they were going to change all the fluid, but I found out after the fact that they only replaced what was in the pan.
 

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The service on the transmission is not as you say. Its in the service manual, all the fluid is changed.
If you have just the fluid replaced that is lost when the pan is dropped, that's a real half assed service. Maybe you need a better place to do you're transmission service.
I think you are assuming a lot about what a sealed transmission is. If it was sealed as you say, where does the pressure go when the fluid expands?
The sediment in the pan is well worth cleaning out, my transmission shifted much smoother after a service.
 

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There may be a service procedure that replaces all the fluid in the transmission. When they talk about a drain and fill, they are talking about only what is in the pan of the transmission. A flush and fill or comprehensive fluid change would be an entire fluid change. Good or bad, I am positive that what my dealership did was a drain and fill and replace the filter.

As for the sealed transmission, I am correct that it is sealed. The pressure would only rise to 6 psi for the transmission to run at 200F, or 14 psi at almost 500F, so there is no need for a vent for such small pressures.

Note, I am not trying to argue for or against changing your transmission fluid. You are not doing anything wrong by having it serviced. I am only trying to explain what a sealed transmission does and why Toyota says you never have to change the transmission fluid. My personal decision is that since I am no longer towing as much with my current Tundra, and I had the transmission serviced a few months ago, from this point forward I am not going to have the fluid changed. If it starts shifting bad, I'll get it serviced but if nothing seems to go bad, I am going to just run it as is and see what happens. I expect to be able to get 300K out of this truck without any issues, but we shall see.
 

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I don't know how you calculated any pressures in the transmission. That isn't really possible without knowing the volume of air and the coe of the fluid.
The transmission has a breather hose on it, so there isn't any pressure.
I'm not making assumptions about servicing the fluid, look in the service manual. There IS a procedure to properly service the transmission. I have no idea who talks about a drain and fill , but they have no idea what they are doing and shouldn't be working on the truck.
 

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Say if the trans holds 10qts of oil at 30C, you will have appx 10.49qts at 100C.

So yes its a .95% increase in volume.

These calcs are for engine oil . ATF would be simm since its 10w hydro oil.

Air expands a great deal more so ,, IF the trans indeed has a breather of some sort , ITS NOT A SEALED UNIT . Aka , moist air in .......... hot air out . Repeat .
 

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The pressure is calculated by PV=nRT. P is absolute pressure, V is volume, n is the number of moles of air, R is the gas constant, and T is absolute temperature. If you divide both sides by (nRT), then the equation becomes PV/nRT = 1. Therefore you can write the equation P1*V1/n1*R1*T1 = P2*V2/n2*R2*T2. Since the volume of the transmission does not change, n is the same, and R is the same, we can reduce the equation to P1/T1 = P2/T2, or P2 = P1*T2/T1. Note, these values are in absolute units, so the pressure is gauge pressure plus 14.7 psi and the temperature is Deg F + 460.

So, to calculate P2 at 500F, we can assume that the original pressure and temperature of the air when the transmission was assembled was 0 psig and 60F. Therefore, the equation is (0+14.7)(500F + 460)/(60+460) = 27.1 psia, or 12.43 psig. You can substitute other temperatures to calculate the pressure in the system, but the point is that it is not much.

The oil does expand a little bit with temperature and the transmission case also grows with a temperature increase, but I am assuming these effects are not significant. The majority of the pressure change will be due the air pressure inside the sealed case due to the temperature change. Also note, that when they fill this transmission, it is done with the transmission heated up so that they control for the expansion of the fluid. If I remember right, it is filled and checked with the transmission at about 130F.

Non-fluid areas of the transmission may be vented, but the fluid areas are completely sealed to prevent any air from breathing in or out of the fluid area. The whole idea of a sealed transmission is to prevent air from entering and leaving the system so that an inert atmosphere of nitrogen is formed around the transmission fluid so there is no chemical reaction between the oxygen and the transmission fluid, thus eliminating a significant source of degradation of the transmission fluid.

Honestly, this is a great design. I hated having to change my transmission fluid in my Fords and Chevy's. It was a $300 bill every 30-60,000 miles. I think having a system that never needs this under normal operating conditions is an awesome engineering feat.
 

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The pressure is calculated by PV=nRT. P is absolute pressure, V is volume, n is the number of moles of air, R is the gas constant, and T is absolute temperature. If you divide both sides by (nRT), then the equation becomes PV/nRT = 1. Therefore you can write the equation P1*V1/n1*R1*T1 = P2*V2/n2*R2*T2. Since the volume of the transmission does not change, n is the same, and R is the same, we can reduce the equation to P1/T1 = P2/T2, or P2 = P1*T2/T1. Note, these values are in absolute units, so the pressure is gauge pressure plus 14.7 psi and the temperature is Deg F + 460.

So, to calculate P2 at 500F, we can assume that the original pressure and temperature of the air when the transmission was assembled was 0 psig and 60F. Therefore, the equation is (0+14.7)(500F + 460)/(60+460) = 27.1 psia, or 12.43 psig. You can substitute other temperatures to calculate the pressure in the system, but the point is that it is not much.

The oil does expand a little bit with temperature and the transmission case also grows with a temperature increase, but I am assuming these effects are not significant. The majority of the pressure change will be due the air pressure inside the sealed case due to the temperature change. Also note, that when they fill this transmission, it is done with the transmission heated up so that they control for the expansion of the fluid. If I remember right, it is filled and checked with the transmission at about 130F.

Non-fluid areas of the transmission may be vented, but the fluid areas are completely sealed to prevent any air from breathing in or out of the fluid area. The whole idea of a sealed transmission is to prevent air from entering and leaving the system so that an inert atmosphere of nitrogen is formed around the transmission fluid so there is no chemical reaction between the oxygen and the transmission fluid, thus eliminating a significant source of degradation of the transmission fluid.

Honestly, this is a great design. I hated having to change my transmission fluid in my Fords and Chevy's. It was a $300 bill every 30-60,000 miles. I think having a system that never needs this under normal operating conditions is an awesome engineering feat.
How do you know the volume of air in the pan area? I calculate the fluid gains almost a quart of volume with a 100 degree temp increase.
 

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How do you know the volume of air in the pan area? I calculate the fluid gains almost a quart of volume with a 100 degree temp increase.
Good question. My calculations assume the air space volume in the transmission stays the same (I think I said that in my post). They fill the transmission at about 130F so that is really the starting point for the expansion calculations. The fluid will expand some with temperature and the transmission case will also grow with temperature as well which will offset some of the fluid expansion, It is not possible for me to know the exact start and ending volume.

But, we can calculate it based on a percent increase or decrease. If we assume the fluid expands by a quart, and the original air volume was a gallon, then the pressure would increase by the original volume divided by the new volume (1 gal/0.75gal). If the original air space was 2 quart, and the the oil expanded by 1 quart, then the pressure would go up by 2/1, or it would double.

If I had to guess, I would guess the air space in the pan is about a gallon, but I have not measure it.

If you double the pressure at 500F, it is only 24 psi. If you triple the pressure, it is 36 psi. The point is that we are not talking about hundreds or thousands of psi pressure in a sealed system. We are talking 10's of psi pressure, and that is not hard to seal against.

The only way for the transmission fluid to cause super high pressures is if there were no means for fluid expansion due to temperature a sealed, non-expanding system. If that were the case, since the oil is for the most part an incompressible fluid, the pressure would rise to extremely high values. But that can be controlled by the use of an expansion mechanism of some kind.

The air space in the pan is basically the expansion mechanism. If they had designed this transmission with no air space, then there would be something like a spring opposed piston that would allow for the expansion and contraction of the fluid.
 

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It's a moot point as there is a breather on the transmission. It can be found on the parts breakdown of the transmission.
 

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Here is a link to a 1991 Patent for the sealed transmission breather control valve and vent. It is an interesting read and goes into quite a bit of detail about the first patent on the breather that dates back to 1972, oxidation of the fluid, the purpose of a sealed transmission, how to restrict air breathing in and out of the transmission and problems that causes, as well as various dipstick designs and problems they have when used in a sealed transmission.

I don't know if Toyota is using this exact vent design or if they have something different, but I am sure that they are doing something similar to restrict the air flow in and out of the transmission.


One last note, Toyota is now recommending for 2021 Tundras that the transmission fluid be changed at 120,000 miles under normal conditions, and at 60,000 for severe service duty. I am not sure at what point they made that change. I know my 2015 never recommends the fluid be changed under normal conditions but does recommend that it be changed at 60,000 for severe service duty.
 
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