damage from overfilling auto transmission

[Donald]

Does anyone have first hand knowledge where overfilling an auto transmission actually caused a problem? What problem does it cause? You do not have the same situation as an engine with piston rods going up and down, possibly causing foaming. I am talking a QT or two extra. Not filling it to the top of the dipstick tube.

 

[apwillard1986]

It is my understanding that it is less a function of overfilling causing physical damage and more a function of it causing a mess if it is overfilled and forced out of vents and such.

 

[LeakySeals]

No first hand because I always let some out. Its a hard one to prove because the effects of an overfill may not be immediate like it would an engine. A transmission is a sealed space. As the fluid expands there is no place to go but out the dipstick tube. That tube alone IMO is not enough to relieve any internal pressure taxing internal seals in different parts of the transmission. So if the question is "can I live with an overfill", I say its a risk not worth taking. Especially a non risk taker like you, Donald

 

[shDK]

The most typical damage is overheating and oil foaming because of to high pressure in the gearhouse.

If the oil is standing high anaugh to reach the cluches they will not be able to move and operate right and cause the cluches to slip. But for that to happen it most be oveefilled a lot.

I repair AT transmissions every day

 

[Gabe]

i thought the overfilling caused a seal to blow.

 

[TechnoLoGs]

I once had a car that, for whatever reason, was supremely low on transission fluid. How it got that way, I do not know.. But, like many GM trannies I have seen, had BLACK fluid. Nice deep BLACK fluid. ....

I do not know how it got so far under, but I had to add about 5 quarts to get it back up to normal. Then I added a quart more or so, over varying methods of how you check the level.

That car ran perfectly after the level was at least full, possibly a quart or a quart and a half or so over. Maybe even two but it did run fine.

This on top of the BLACK fluid.

 

[GreeCguy]

I've always been told that overfilling creates foam which in turn creates tiny air bubbles which for anyone other than Don Ho is a bad thing for trannys.

 

[JHZR2]

My biggest issue with all this is that I find it VERY difficult to get a consistent reading on any ATF dipstick that isnt a straight drop-in to the pan. Any RWD vehicle (previa, S10, Mercedes) that Ive owned and tried to measure always seems to have a different reading on either side of the dipstick, and some inconsistency on the reading even with what is seen.

 

[Quest]

Originally Posted By: shDK
The most typical damage is overheating and oil foaming because of to high pressure in the gearhouse.

If the oil is standing high anaugh to reach the cluches they will not be able to move and operate right and cause the cluches to slip. But for that to happen it most be oveefilled a lot.

I repair AT transmissions every day


BINGO!

Q.

 

[SteveSRT8]

In the old daze we would fill TH350's and 400's until the fluid came up the tube! Then the overflow tube would simply pee out the excess fluid. Many other trans are quite similar.

My NAG1 trans based on the old Mercedes WA580 is actually a "dry sump" design that attempts to keep all fluid out of the rotating assembly, common idea now that economy is more valued.

Wildly platform specific, some slushboxes don't care, others will grenade if overfilled...

 

[LeakySeals]

 

[bmwtechguy]

It's really a similar problem as in an engine. Under normal conditions, the spinning geartrain is not immersed in fluid, but is completely above the fluid level. The exception to this is the final drive or differential part of a transaxle like what is found in a FWD. When the fluid is overfilled enough that the spinning geartrain (clutches, drums, shafts, etc) are whipping up the fluid, and the pump is no longer sucking a fluid but is sucking fluid that is entrained with air bubbles, then no longer is the pumping efficient and hydraulic pressures drop or fluctuate to where clutches and or bands slip, etc. That is where you have the most damage. If overfilled enough, I suppose a seal could actually blow out, especially if the vent at the top gets blocked. In my experience, only a few specific tranmissions over the years have been sensitive to overfill of a quart or less. Most will not have an issue with an extra .5 to a full qt. And even a few do better slightly overfilled in extremely hilly places like Pittsurgh Pa, where I grew up.

 

[SuperDave456]

All I can say is read the owners manual and check to see if you are checking the ATF levels correctly.

Some are much more difficult to read than others.
Running while in park with a hot engine.
Running while in Neutral, parking brake set, with a hot engine.
And with fresh fluid, that is no longer coal black, it can be a real trick to "see" the ATF, as it is very thin when new. I've had to use the "sun at your back, with your sunglasses on" technique to get an accurate reading more than once.

I'm lucky, as the parking space out in front of my house is pretty level.
When I lived in an apt. complex I would drive to Walmart and find a reletively level space to check my ATF level.

I try to check the levels in the same space/location so that there is some consistancy built into the procedure.
Tilting the car a little one way or the other can make a difference, in some cars more than others.

 

[Sunnyinhollister]

I bought a 2005 GMC Sierra that was at least 2 quarts over filled and it never gave me issues before I checked it, nor after the fluid change. My question is if it is overfilled by a pint, will it really matter? Is it that sensitive to an 1/8" of fluid level?

 

[901Memphis]

I've over filled my Taurus before and it came out of the atx vent and onto the exhaust while I was on the highway. That was before I measured what I extracted and just went by the dipstick alone. Funny it drove fine for weeks til I took it out on a long trip that got it up to temperature. It was approximately one quart over filled. Funny thing is it was perfect on the top of the dipstick with shorter warmup periods.

Now I i just do a dipstick siphon I do exactly 4 quarts out with a milk jug an four quarts in then check the level from there. This has worked great so far.

 

[Donald]

Originally Posted By: shDK
The most typical damage is overheating and oil foaming because of to high pressure in the gearhouse.

If the oil is standing high anaugh to reach the cluches they will not be able to move and operate right and cause the cluches to slip. But for that to happen it most be oveefilled a lot.

I repair AT transmissions every day


How many have come in where the damage was done by overfilling?

Some people may overfill to try an fix a failing transmission.

 

[cchase]

Some vehicles have vent tubes that would probably prevent "damage" from occurring, other than a big mess somewhere.

 

[BearZDefect]

I suspect that the result varies significantly between different transmissions. I'll add my experience to those above:

Once after an ATF change I overfilled the transmission in my 1997 i30 (aka Nissan Maxima) by about a quart. The result was that after a very short test drive, the transmission could not reliably engage forward gears. One possibility is enough air got whipped into the fluid that the hydraulics could not apply the pressure that the clutches needed to do their job.

 

[shDK]

We don't see damages caused by overfilling very often. Buy it happens i wouly say a couple of times a month. however to be on the safe side you should drain the bit of oil you overfilled.

What normally kills a AT is buildup of slutch from the cluches that goes to the oil. Therefore they usually die when people don't change oil like they should.

 

[dnewton3]

Overfilling can have effects such as heat as well. Most folks think that "more = better", so they tend to fill things to the top, or more. However, some trannies don't like to be "full".

Case in point where I have specific experience firsthand is the Allison 1000 series behind my Dmax ...
The "correct" method (per some folks I have had direct conversations with who work here in Indy at the Allison center) is to get the tranny up to around 160 degF, and then set the level to the bottom of the "hot" range on the dipstick. This allows for heat expansion to not "overflow" the mechanical operating group in the tranny. As the temps can move safely up to 225 degF or more, then the Allison will not overheat it's fluid and burp out via the vent.

And I proved this to be true. My tranny was running a bit warm (just a bit over 200F) during a long drive westward on I-90. At the campground that evening, after it had cooled to 160 degF, I drained out about two quarts to reset the level at the bottom of the temp range on the stick. Next day, in nearly the very same ambient temps (within a degree or two all day long), at the same travel speeds, same general terrain, the tranny ran about 15 degrees cooler! Now, that is not true of all trannies; only specific to the Ally 1000. But it surely contradicts what many folks think. Most would believe that more fluid would be "better", but it's not always the case. In my situation, being overfilled by two quarts made a big difference.


Originally Posted By: SteveSRT8
Wildly platform specific, some slushboxes don't care, others will grenade if overfilled...

What SRT said can be very much taken to heart; some trannies are nearly indifferent, others are very particular. You have to know your equipment, and be able to adjust accordingly.