Overfilling an automatic transmission...

[Externet]

Would be great to find a member that has professionally studied about these devices and can clear myths found everywhere including the web. Or that is not a myth.

 

Excessive fluid level in an automatic transmission is warned against. (Oil in engines too, but I understand why in them)

What parts become damaged ? (if damage is the concern)

Is there symptoms of overfilled ?

 

If you share the commonly found opinion that overfilling will foam the fluid; where in the transmission, how, why is foaming supposed to form ?
Wouldn't 'foaming' happen if there is too little fluid instead, and the pump is sucking some air from the pan ?
-That would be the opposite of overfilling-

 

Educate me please ?

 

 

 

[Phi for All]

I've always heard overfilling oil or transmission fluid risks blowing out your seals due to pressure. I've owned mostly manual transmissions (until my present vehicle), and I've never heard about the foaming.

 

[/non-professional hearsay]

[Externet]

Thanks.

Yes, 'heard' that one too. But as far as I know, the transmission pump has a pressure regulator built-in, that would not allow that to happen ¿?.

[Greg H.]

Overfilling your transmission can cause early failure of parts due to excess pressure. As Externet states most (I won't go with all, since I don't know if it's true) modern transmissions include a pressure valve that regulates the pressure after the pump, but excess pressure can cause premature wearing on that valve, which can lead to failure. This failure can cause pressure related failures in other components. So yes, maintaining too much fluid in the transmission can lead to premature failure of the system over time, and can potentially void any warranty you might have, if the cause of the failure is determined to be the presence of too much fluid.

 

All of that being said, I have only ever seen one transmission fail from having too much fluid, and that was in a 15 year old car that probably had other issues besides just too much fluid. So while it can, I expect it's not something that would happen quickly.

 

As for the foaming, this coiuld be caused by old oil, contamination, or air being sucked in from a failed seal somewhere. Regardless, foamed oil is one of those things my father would call "Very bad" for your vehicle.

[MigL]

All transmissions, manual or auto, have some means to relieve pressure.

the lubrication pump/system takes care of things in a normally filled engine/transmission. Foaming,in a transmission or an engine, occurs when rotating parts ( crankshaft weights in an engine, rotating gears in a tranny ) splash in the overfilled oil/fluid reservoir or crankcase.

Foamy oil or transmission fluid, I would think, is not as good a lubricant or heat transporter.

This splashing also leads to excessive fluid temps, the tranny fluid degrades very quickly, and the transmission ( auto ) fails.

[Externet]

... Foaming,in a transmission or an engine, occurs when rotating parts ( crankshaft weights in an engine, rotating gears in a tranny ) splash in the overfilled oil/fluid reservoir or crankcase...

Thanks.

Am condensing the text above to the pertinent as follows :

"...Foaming in a transmission occurs when rotating gears splash in the overfilled fluid reservoir..."

IF this condensing or rewording does not match your intended phrasing, ignore this post, as I do not want to distort your opinion.

 

----> There are no rotating gears splashing fluid in an overfilled fluid reservoir in a transmission. Gears are two 'levels' above the fluid and valve body, in another chamber.

[MigL]

The gear 'chamber' still needs to have fluid pumped into it for lubrication, and AFAIK drain holes carry this back to the pan.

Thickening of the fluid due to temp degradation, metal/friction particles from a failing tranny, or even just plain dirt can clog these drains and lead to foaming.

That being said, foaming doesn't seem to be much of an issue these days, hi-temp degradation is.

I've always owned GM, but my current transaxle on a V6 MDX doesn't even have a replaceable filter, and I've always done a couple of flush and fills with hi-quality synthetic fluid ( never mind what Honda/Acura says ). I use Amsoil fully synthetic ( no one else, not even Mobil 1, is fully synthetic anymore ) for both tranny and engine.