As has been taught, we must let the engine run for a bit to get the trans fluid "warm", and run through the gears to check it "correctly". Why is that so? Isn't the amount of fluid in the trans a known amount... 8 quarts, or the Full level, and wouldn't the quantity of fluid be the same whether the fluid is warm or cold? Can't it be checked "cold"? We don't differentiate warm or cold on engine oil checking. It's still a fluid with a known quantity.
Transmission fluid expands as it gets hot. Motor oil does as well, but motor oil isn't serving any kind of hydraulic purpose beyond lubrication. It's not actuating clutches, bands, etc. like transmission fluid does. It's just in there, being thrown about by a pump. The transmission's parts need very specific pressures to operate correctly. Too much or too little, even a half quart, throws those pressures off and will cause problems over the long term. You must run the tranny through all gears to make sure fluid is flowing through all passages in the valve body. If it's not, your fluid level is wrong and can it cause damage or malfunctions.
Do you have any sources for your pressure explanation? The valve body is pressurized by the pump and is regulated by valves. The pan and gear body is at atmospheric pressure (vented), so I can't understand how the fluid level can affect pressures in the valve body (which are measured and controlled by computer).
My understanding is if there isn't enough fluid in the transmission, the solenoids and passages in the valve body won't allow the device they operate to activate properly, which leads to slipping, etc. I could be wrong, but the computer can only compensate for the pressure problems to a point. We only spent a few lectures on the planetary/ automatic transmission subject in my machinery class. The professor hated them. I first figured out my Catalina had low fluid because it would partially engage until I reached some speed and it would grab again.
I mean, if the level drops below the filter, then yeah it'll start sucking air. But the fluid would have to be extremely low.
As long as the filter is submerged, then the pump will get all the fluid it needs, and the pressure will be constant. It operates like a basic sump pump.
A transmission takes a lot more fluid than is critically necessary for operation for cooling capacity, and because you have to account for things like hills.
I have never heard of a transmission "measuring" its own fluid level in order to turn functions on or off. Certainly not one I've ever owned.
I may be mis-describing it. Haha. My understanding is the pump sends pressurized fluid through the valve body to valves that are solenoid activated. They engage bands with the pressure in the valve body when the valves open or close, and that the pressure in the unit varies with engine speed. The bands need to "squeeze" harder to stop gears from moving as the RPMs increase. Not enough pressure or too much pressure due to incorrect fluid levels causes slippage or harsh engagements. My old Pontiac will start showing a delay when it's only a pint low, for example.
Yes, I understand how hydraulic pressure actuates the moving parts. No problem with that.
But I just don't understand how 1 pint lower fluid can contribute to lower line pressure. If I have a pump at the bottom of a well, then the volume of water that is pumped out the top (and as a result, the pressure) , will be the same no matter if the water level in the well is 5 inches or 6 inches, as long as the pump inlet is submerged.
You would actually notice a difference in your example over a good height, the pump needs to put out enough power to overcome the head pressure, which is the weight of the water above the pump. Pumping the same quantity higher or faster means a bigger pump. It may be 50 PSI at the pump, but over some height, it'll eventually go to 0. I don't think that really applies here, though. The 1 pint of fluid in my Catalina must be the difference in partially and fully submerging the inlet. If it's down that little amount, it can suck air, even a tiny bit, causing the pressure problem. Too much fluid can back up in the pan as it's being returned through the cooler lines and overpressurize the system. That should be common sense, though.
Yes I know about head pressure, but I said water level. As in, the pumping height was staying the same and the reservoir level was changing.
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I agree that your Catalina must have a very narrow margin between nominal level, and fluid starvation.
But I don't think too much fluid will over-pressurize, the same way a well pump cannot, even the well becomes full to the brim. It only moves as much fluid as it's designed for. Besides, there are regulators and pressure relief valves for that.
Reading effects and symptoms of of an overfill seems to indicate it can cause pressure problems. Too much can blow out seals, cause lubrication issues, erratic shifts, etc. The people who wrote this mention putting too much transmission fluid in the engine. Not sure if that was an oversight, obviously tranny fluid doesn't go in there, but the rest seems to check out.
https://diycaradvice.com/too-much-transmissionfluid/
This one doesn't have that silly "transmission fluid in the engine" oversight. It also indicates over-pressure is a possibility.
https://mechanicbase.com/transmission/slightly-overfilled-transmission/
I can see how overfilling a transmission a lot would make the fluid start to fill places where it shouldn't be. On mine, the valve body sits right above the pan. If the fluid comes up too high, MAYBE it could start block orifices and stuff.
Then again there's fluid constantly cascading onto it anyway.
These websites like "diycaradvice.com" are unreliable. I don't trust them at all.
It's unclear, but I think your question is about checking transmission fluid level.
Can't it be checked "cold"
No. The transmission must be warmed up, because the operating temperature is a known quantity. If the vehicle has been sitting then the temperature could be anything.
run through the gears
No. Transmission should be idling in 'P'. Running through the gears is only done when the fluid is changed to make sure it fills the valve body properly.
Again, when the vehicle is shut off and cold, the level is undefined.
The fluid level is only defined when the the transmission is operating in park, at a particular temperature, because that's when it has reached steady state.
wouldn't the quantity of fluid be the same whether the fluid is warm or cold?
everything expands with temperature. The fluid, all the parts within the transmission, and the housings that surround it.
https://en.wikipedia.org/wiki/Thermal_expansion
engines contain much less oil. The volume of expansion is insignificant.
Wrong. If you checked your engine oil with the engine running, you'd get a false reading because it was not designed to be done that way. Auto trans dipsticks and level readings are designed to be read at idle and warmed up. That is the facts, Jack.
The ATF and filter were changed about five months ago, and I followed the procedure about putting the car into Park before taking a reading. I suspect you may be right that it was simply overfilled so I'll take the car to another shop for a second look.
Thanks for your help!
Because transmission fluid drains down very slowly. A few hours of cool down will have a higher reading than a transmission that has sat for a week. At operating temperature, all the valves and passages will be filled and the reading more accurate.
what? a car that has sat for a week will read LOWER than one that sat for a few hours?? That doesn't sound right.
Lol. No. I mean the opposite.
Thought so. Although, I don't think the difference would be huge. 100mL at most.
I'm still not convinced why the trans fluid has to be checked when it's warm, and the engine is running. If the trans "holds" 12 qts, and that's what's considered "full" as far as the manufacturer calls it, why would checking it warm vs cold, engine off, make any difference, as to what's moving about in the trans? If the trans is low on fluid (2 qts let's say), then the dipstick (if you have one), will be low, and one would have to add fluid to make it full. Sorry to be contrary!
Here's my problem with our 2005 Scion xA: Although there's no mention in the Owner's Manual on how to check the transmission fluid level, there is a procedure listed in the Service Manual (page AX-82). It says to check the ATF level with the engine and transmission at normal operating temperature. Fine, but the problem is the fluid level on the dipstick goes up far beyond the upper HOT mark--there must be some fluid splashing onto the dipstick so I can't get an accurate reading. What should I do?
There are cold marks on the dipstick, so my question is whether I can use those (with the engine/tranny cold), or since the dipstick can possibly be used to measure fluid levels in another vehicle, they shouldn't be used? I'm guessing maybe neither one because as mentioned above in the thread, checking the fluid level while cold is a bad idea.
There are cold marks on the dipstick, so my question is whether I can use those
No, because "cold" could be anything.
You must use operating temperature because it is a known quantity.
there must be some fluid splashing onto the dipstick
The service manual states the level should be checked in 'P' gear. There should be no splashing.
the fluid level on the dipstick goes up far beyond the upper HOT mark
Sometimes, you need to wipe and dip the dipstick several times before it gives you an accurate reading. Also make sure you check both the front AND back of the stick.
When was it last serviced? Maybe the fluid is worn out. When the defoaming additives wear out, the fluid increases in volume and reads high. Or maybe somebody put in the wrong kind of fluid.
Or maybe, your fluid level is simply too high.