Originally Posted by qwerty

Will that tell you what the dipstick should read?

Originally Posted by shertex

Countless forum members have expressed perplexity when attempting to measure ATF level (including me, at one point). They are told to measure it hot, at operating temperature....say after a good run on the highway for 30 minutes. But then they encounter all sorts of uncertainties, basically revolving around the question of whether it was really hot enough when measured. Can be very frustrating.

It seems to me that the reason the FSM provides the "hot" method is that most people, and most mechanics, will be measuring the level on a warm/hot engine: during a trip, a visit to the mechanic, etc... So it makes sense.

But for DYI'er, there's absolutely no need for this method. The FSM specifies ANOTHER method: measuring at 20-30C (in other words, ambient temp on most days). See p. 1 and 3 here: http://www.w124performance.com/service/w124CD1/Program/Maintenance/My81/2710.pdf

So as long as the engine is cold and it's 68-86F outside, I know I can get an accurate reading every time.

When it's colder (e.g. a cold winter day), I don't know if the measurement would be different since I don't know if there's significant thermal contraction at lower temperatures. Anyone know?

Originally Posted by whunter

Note: A fresh rebuild and/or fluid drain and fill, can give a false dipstick reading in certain under fill conditions = burn out clutch disc's. This is why I recommend measuring exactly how much is removed, and refilling the same volume.

There is significant thermal contraction at lower temperatures.

Checking cold ATF is NOT recommended, due in part to extreme thermal expansion-contraction.

On most transmissions the thermal expansion-contraction is a critical variable due to unique individual unit age/wear factors..

The only ATF level that is relevant is the operating temperature reading.

If there are NO transmission leaks, and the level is correct at operating temperature = the fluid level will remain acceptable (within operating range) for a ridiculous number of miles.

.

Originally Posted by cullennewsom

I found a reference for Dexron III which should be about the same as any other Dexron.
https://docs.google.com/open?id=1KXo09VmkZg_8tLfUkPIRz3M9RudFoZt3-5-UgFarRpxEleo26sIF9AqxzAQ6
Here's one for hydraulic oil
www.hyvair.com/pdf/hydraulicfluid.pdf

There is volumetric expansion which is proportional to temperature. That fact does not necessarily imply that you can't check it cold, but the marks on the dipstick are calibrated for "operating temperature". There is also a question about how to account for torque converter fill, fill of all the other various works of the transmission, and the oil cooler. These factors don't make it impossible, but might give you confusing results based on conditions other than temperature, if you aren't paying careful attention. Another thing you have to worry about is that the transmission is an irregular shaped volume. So if you want to determine where to put your cold calibration tick marks, you need to account for the shape of the transmission. Approximating it as a cube (simplest calculation) might yield bogus results.

Another thing to note is that filling a large reservoir cold might cause an overflow when it heats up, if you accidentally overfill. There are some anecdotal accounts of this happening, burping trans fluid onto a hot exhaust, and causing a fire. Probably in old motor homes with Chevy TH400 & 454.

Here's an explanation of how to calculate the expansion Thermal Expansion - Volumetric or Cubic

Here is a more thorough treatment of the subject. Hydraulic Fluid - Properties

The easiest way, is probably just to make empirical observations based on comparisons of hot vs cold readings, with all other variables fixed*.

* You can't easily fix the amount of entrained air in the fluid, but the difference is probably within the margin of error of measurement precision.

Originally Posted by cullennewsom

There is volumetric expansion which is proportional to temperature. That fact does not necessarily imply that you can't check it cold, but the marks on the dipstick are calibrated for "operating temperature".

Originally Posted by qwerty

And the FSM provides the necessary "adjustment" to those marks for alternative temperature conditions.

That said, hot and cold measurements on my 722.3 never seem to coincide with the FSM specifications.

Originally Posted by shertex

The FSM certainly thinks that measuring cold is fine. It specifies measuring at 20-30C as a legitimate method for measuring the level, giving very exact levels at that temperature for different 722 transmissions, thereby taking account of the thermal contraction. If they recommend it, how is it not recommended??

Originally Posted by shertex

I will also say that I have compared known hot and cold measurements on my cars. That is, in a case where I'm quite confident the tranny is at op temp and reads at the top mark, when I check it the next day cold it reads just where it should read cold.

Originally Posted by shertex

The FSM certainly thinks that measuring cold is fine. It specifies measuring at 20-30C as a legitimate method for measuring the level, giving very exact levels at that temperature for different 722 transmissions, thereby taking account of the thermal contraction. If they recommend it, how is it not recommended??

Originally Posted by Govert

Not exactly. A typical oil change will take place with the automatic transmission oil at 20-30 degrees C, that is why the FSM mentions it. It serves as a guide during oil changes, so that workshops don't have to drive for an hour every time a oil change is done. Prefered oil level check is done with the transmission warm.