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Why does Auto Trans Fluid level rise when Hot

22K views 27 replies 14 participants last post by  2005rtmag 
#1 ·
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
 
#3 ·
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 · (Edited)
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....
 
#5 ·
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?
 
#11 ·
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
 
#6 ·
Why it rises is irrelevant, it does and is documented.

 
#12 ·
#9 ·
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.
 
#13 ·
Thanks for the info
 
#15 ·
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.
 
#16 ·
Ok to continue with Daves thoughts, and with the thermal expansion the cavities the fluid occupies becomes smaller and forces more fluid into the pan to further add to the already rising depth.

Better Erik?
 
#24 ·
I learned that bit about water expansion/contraction between 0-4 C back in middle school, but it is not obvious to me why that characteristic is essential for life on the planet.

Educate me, please.

Edit: Or did I misread your statement? Is it life or the earth itself tht would not be possible?
Yes, why is this fact critical to life on earth?
BEATS TE HELL OUT OF ME.... Guess we're going to have to wait for IES99 to log back in and enlighten us.
 
#28 ·
Like fnkychkn says, everything expands with heat. When I replaced freeze plugs in engine blocks, I would put the plug in the freezer to shrink it. Then it slides in the hole in the block and expands rapidly. Usually have to knock it in a little when it sticks from expanding. It's an old trick to use with anything that is a press fit. You freeze one part to shrink it and heat the other possibly to make it easy to install. I worked in power plants for years. It's interesting to see how long runs of steam piping can grow longer when it receives steam. The pipe supports and connections have to be designed to accomodate this expansion from heat.
 
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