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  1. #1
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    Why does Auto Trans Fluid level rise when Hot

    Im curious as to why automatic transmission oil rises on dip stick when hot..

    Engine oil, diff oil ect do not rise....Im sure its not the oil expanding when hot that makes it rise on the stick....

    Can anyone explain to me why this happens...I googled it and didnt find the answer..

    Thanks in advance
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  2. #2
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    I can't think of anything other than heat expansion. Same happens with coolant. And yes, the same does happen with engine oil.

  3. #3
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    Quote Originally Posted by rickf28 View Post
    Im curious as to why automatic transmission oil rises on dip stick when hot..

    Engine oil, diff oil ect do not rise....Im sure its not the oil expanding when hot that makes it rise on the stick....

    Can anyone explain to me why this happens...I googled it and didnt find the answer..

    Thanks in advance
    Rick
    Youre not serious right?


    Yes..heat expansion happens to all running gear vital fluids.

  4. #4
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    Ya im serious.....Im sure all fluids expand to some degree when heated.....but my engine oil doeasnt have a hot and cold check level, nore does the diff...and manual transmissions can be checked hot or cold, so im guessing there isnt much expansion going on there .......The coolant does have hot cold and so do most power steering.....(thinner fluids maybey) but the are" your not serious comment "answers nothing that I asked...But then again I have noticed that there are lots of people on lxforums lately that act this way....to bad I guess...this use to be a friendly helpful place to gain knowlage

    The reason I asked is because i was curious if the fluid expanded that much..(quite a bit of difference in trans hot and cold levels) or if it has to do with the way an auto trans works.....

    Ive asked several mechanics and they all have different reasons...some say bands and clutches swell when heated ect...I just thought there was more to it then heat...thats a lot of expansion...

    But if thats all it is then maybe it was bad question...Ive just never gotten a strait answer on this one....
    Last edited by rickf28; 11-13-2009 at 06:34 AM.

  5. #5
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    I think perhaps the thermal expansion issue is more evident in trannies because of the volume of fluid involved (roughly 3 gallons) and the fact that the dipstick enters the tranny at a place where only a small portion of the fluid pools (the pan as opposed to all the lines and especially the torque converter).

    I think you end up with something similar to the tidal effect, which is exaggerated in many parts of the world due to the structure, shape and length of the body of water open to the sea and other complex shorline and ocean bottom features (nothing to do with the gravitational pull of the moon and sun, which are cyclical mathematical constants).

    How's that for a guess?

  6. #6
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    Why it rises is irrelevant, it does and is documented.


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  7. #7
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    Why it rises is irrelevant, it does and is documented.
    Come on now, BigJim...it's an interesting puzzle.

    Personally, I think that if a transmission were 3" deep, 3' wide and 6' long, you wouldn't be able to detect thermal expansion of the fluid on a dipstick.

    So there.

  8. #8
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    Quote Originally Posted by lou1355 View Post
    Come on now, BigJim...it's an interesting puzzle.

    Personally, I think that if a transmission were 3" deep, 3' wide and 6' long, you wouldn't be able to detect thermal expansion of the fluid on a dipstick.

    So there.
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  9. #9
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    pretty much everything expands when heated (metals and fluids). the rate of expansion, and, as Lou pointed out, the shape of the reservoir, will dictate the amount of level rise. automatic transmissions are especially susceptible to level rise because the valve body takes up most of the room inside the pan. so, any any expansion will result in a noticeable increase in level. the reason fluid level is so critical in an automatic transmission is because if the fluid expands to the point where the rotating components can churn up the fluid and aerate it, pressures will become unstable.
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  10. #10
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    pressures will become unstable
    Well, we sure as hell don't want any of that!

    Unstable "pressures" you say? I think there is a lot of that going on in my particular hot rod.

    I'm going to have to look into that.

  11. #11
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    Quote Originally Posted by lou1355 View Post
    I think perhaps the thermal expansion issue is more evident in trannies because of the volume of fluid involved (roughly 3 gallons) and the fact that the dipstick enters the tranny at a place where only a small portion of the fluid pools (the pan as opposed to all the lines and especially the torque converter).
    thanks...this is more what i was looking for

  12. #12
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    Quote Originally Posted by bigjim View Post
    Why it rises is irrelevant, it does and is documented.
    we all know it rises...I just wanted to know why

  13. #13
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    Quote Originally Posted by fnkychkn View Post
    pretty much everything expands when heated (metals and fluids). the rate of expansion, and, as Lou pointed out, the shape of the reservoir, will dictate the amount of level rise. automatic transmissions are especially susceptible to level rise because the valve body takes up most of the room inside the pan. so, any any expansion will result in a noticeable increase in level. the reason fluid level is so critical in an automatic transmission is because if the fluid expands to the point where the rotating components can churn up the fluid and aerate it, pressures will become unstable.
    Thanks for the info

  14. #14
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    Very interesting read.

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  15. #15
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    Don't forget... It's not just the fluid that's expanding, but the solid materials inside the transmission that expand with heat too.

    When temperature increases, so does the length of the molecular bonds. As a result, solids typically expand in response to heating and contract on cooling; this dimensional response to temperature change is expressed by its coefficient of thermal expansion (CTE).


    Different coefficients of thermal expansion can be defined for a substance depending on whether the expansion is measured by:
    • linear thermal expansion (CLTE)
    • area thermal expansion
    • volumetric thermal expansion.
    These characteristics are closely related. The volumetric thermal expansion coefficient can be defined for both liquids and solids. The linear thermal expansion can only be defined for solids, and is common in engineering applications.
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