Automatic Tranny Fluid Level

When you shut down, does the fluid in the radiator cooler drain back, raising the level in the pan?

The 700r4 in the roadster was dry, then the day after a drive had a good sized wet spot. Checked, and found fluid seeping around the dipstick tube.

Today I pulled the dipstick tube and found the o-rings undersized, and the fluid level just even with the dipstick hole in the case. Replaced the o-rings, started and let it warm up, and the level was at mid-point of the OK range where it had been right at the top of the range. I'm thinking it's at it's happy level, and the new o-rings should fix any further drips.

Normally it's fluid from the torque convertor that drains back into the transmission pan.



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Yeah,, What Mike P said.

Thanks, Mikes! I grew up tearing 3-speed sticks apart and never really studied automatics. I had wondered about the converter, but thought of it like a filled bottle standing up in a pan of water, staying full since no air could get in but it makes sense for the converter to drain back, I suppose back through the pump. It also explains why if I start the truck and immediately drop it into reverse (big 10 speed) there's a bit of a sluggish lag for a few seconds as it starts to back - the torque converter is operating with a partial fill, and as it fills it gets more "bite" from the fluid friction.

I'm hoping the pair of slightly larger O-rings seals the filler tube, and I may add 8 or 10 ounces of fluid just to check! Thanks for the input!

The O rings will likely fix the problem.

I have seen situations where if the bracket holding the dipstick tube to the engine is tweaked a bit it will put the tube in a bind (more pressure on one side of the tube than the other) and cause a leak.

I'd advise strongly against over-filling the transmission. Transmission fluid has a pretty good expansion rate. Too much fluid especially if you have a long hard pull, like going up a long steep hill, can cause an overfull transmission to blow excess fluid out the vent. TH400s were pretty notorious for this.



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Originally Posted by Mike P

I'd advise strongly against over-filling the transmission. Transmission fluid has a pretty good expansion rate. Too much fluid especially if you have a long hard pull, like going up a long steep hill, can cause an overfull transmission to blow excess fluid out the vent.

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Thanks, Mike. I was just considering that the "hot/warm" fluid level is below "FULL", about mid-way between ADD and FULL, so adding a cup or so would just bring it closer to "FULL", but that said it's an aftermarket tube (LOKAR), so I can't say that it's dead on with the marks. Probably best to just let it be, with the fluid level just at the top of the tube port on the case when cold, as long as it's happy when hot.

You can do the hot and cold comparison. made easy with your phone / camera! And just like it sounds,pull dipstick when cold and get a picture. Then drive it with the intent of heating the oil. get it hot then snap a picture. Comparing the two pics, does the fluid get to high? or,is there room for it to grow? In an earlier post you mention the oil level was up to the port that the dipstick tube fit into.. I'd say that is high enough, was that hot or cold tho'??? If it was cold then you know the oil is going to grow and raise the level, And I don't think it should go over the seal "permanently".. But by doing the comparo you should be able to gauge where the top oil level will usually be when fully hot.

When it was cold and had not been started the fluid level was just at the bottom of the port that the dipstick/fill tube fits, which I believe includes some amount of fluid that's drained back from the converter so when it's started the fluid level will fall, and then will grow as it warms. I recall in the old days the OEM sticks often had both "Full Hot" and a "Full Cold" marks, as well as the "Add" mark near the bottom. I'm going to run it like that for a bit, and will check the level on the stick after I've driven it enough to get the fluid up to operating temp.
Right now it's down awaiting front sway bar bushings, mailed from TCI in CA today. Hopefully they choose a fast pony, not a mule.

Exactly right! The full hot level and the full cold level. The hard part is knowing how much it "grows". But I think you are right, as long as it is up to the dipstick tube opening, then you know for sure the pan is full.

I’ve always wondered why does transmission fluid “grow” when it gets hot. Surely the oil does not expand that much - does it?

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Originally Posted by Driver50x

I’ve always wondered why does transmission fluid “grow” when it gets hot. Surely the oil does not expand that much - does it?

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Same with any fluid,or most metals even, They expand when heated. How much? That's what I wanted Roger to do for us. Find the range difference between hot & cold. I'd think the oil in the pan runs cooler than what's in the convertor, but depending on load and for how long the load is applied would determine the total growth.

My understanding is the level should be even with top of the pan. (where the gasket sits). This is below the dipstick hole.

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Originally Posted by MNSIU

My understanding is the level should be even with top of the pan. (where the gasket sits). This is below the dipstick hole.

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Thanks! That may be right for the Full Hot condition, but my question was the level after a day (or week) since last running when the fluid has drained back, and where the fluid was coming from. We've established that there are three fluid levels for a given fluid volume. Lowest is "Full Cold" engine running, tranny run through the gears to fill the converter, then in neutral with fluid not warmed. Next is "Full Hot", same drill, but the fluid up to normal operating temp. Highest is after you've parked and walked away, everything has cooled down completely, and some amount of fluid has drained back from the converter. That last condition was when I had a leak, and the reason for my question.