AT Overfill causes higher pressure?

[Klutch9]

Today while doing another drain/fill on the Sedona, setting the final fluid level, getting fluid burped in my face, etc... I recall reading consistently on the internet that overfilling an automatic transmission can, amongst other things, cause higher pressure within the transmission. Pardon me if I'm missing something, but isn't this impossible? Unit pressure is determined by the pump and seals, and could certainly DROP if underfilled, but in no way INCREASE if overfilled. It's not like the transmission operates on gravity feed pressure or anything. Pressure doesn't increase simply because the reservoir the pump is drawing from increases in volume. Someone tell me I'm sane

 

[MolaKule]

You're forgetting that overfilling causes air infiltration of the fluid which whips air into the fluid.

Only refill with the same volume that you removed.

 

[DWC28]

Also the volume may not be directly related to the pressure. Pressure is resistance to flow, so a small volume with high resistance will have higher pressure.

 

[Klutch9]

Originally Posted By: MolaKule
You're forgetting that overfilling causes air infiltration of the fluid which whips air into the fluid.

Only refill with the same volume that you removed.


This is true. But even if air was injested, wouldn't the effective pressure drop, since air is compressible and fluid is not? And FWIW my fluid level is good, it just got me to thinking.

 

[Klutch9]

Originally Posted By: DWC28
Also the volume may not be directly related to the pressure. Pressure is resistance to flow, so a small volume with high resistance will have higher pressure.


But if there is excess fluid in the sump, the transmission will not use more fluid for a given operation or shift. There will just be more fluid in reserve in the sump, which is not under pressure.

 

[ammolab]

Maybe not higher "pump" pressure but just higher pressure in the case of the transmission due to less free case volume above the fluid level.

 

[Klutch9]

Originally Posted By: ammolab
Maybe not higher "pump" pressure but just higher pressure in the case of the transmission due to less free case volume above the fluid level.


But the reservoir where the excess fluid sits isn't under any pressure. If it was, air pressure would blow out of the fill tube when you remove the dip stick, since it goes directly into the sump. Or it would come out on its own when the motor was running. My dipstick in particular has a pinhole on the cap, allowing air movement between the sump and the outside.

 

[SOHCman]

Perhaps the problem with over fill is not higher pressure at all, but simply air infiltration like MolaKule insinuated?

Air in the fluid can cause pump damage through cavitation if I am not mistaken?

S

 

[zeng]

I'm with you OP, and I believe you got all the above concepts right.

 

[Klutch9]

Originally Posted By: SOHCman
Perhaps the problem with over fill is not higher pressure at all, but simply air infiltration like MolaKule insinuated?

Air in the fluid can cause pump damage through cavitation if I am not mistaken?

S


This is what I'd imagine to be true. Even still, how would the fluid foam or cavitate? There's no crankshaft whipping around, and the gear set is well above the sump traditionally.

 

[Trav]

Originally Posted By: SOHCman
Perhaps the problem with over fill is not higher pressure at all, but simply air infiltration like MolaKule insinuated?

Air in the fluid can cause pump damage through cavitation if I am not mistaken?

S


+1