temperature - ATF level change vs oil change

[edwardh1]

what causes oil level to be somewhat stable on dipstick with temp change, compared to atf which really changes- what causes this?

and what causes atf ok range on the dipstick to be so small?

 

[FazerZ]

Modern automatics do funny things compared to engines.

1) Baffles / sumps

Some automatic transmissions/transaxles have temperature-controlled baffling present to trap fluid inside certain areas of the housing. GM likes to do this on many of their side-pan designs. This can cause a change in apparent level at different temperatures.

2) Torque converter drainback

This is less of an issue when everything in the unit is working normally and tight, however as miles rack up the torque converter hydraulic circuits can begin to leak off fluid when the vehicle is off and increase the sump ATF level. This is all changed within a bit of start-up time as the converter is immediately charged with oil as soon as the vehicle is started.

3) Thermostatic cooler control

Introduced here back in the 2000s, some transmissions (looking at you Ford) have a thermostatically controlled hydraulics which restrict the amount of ATF going to the cooler until operating temperature is reached. The idea was to protect the transmission at extremely low operating temperatures where the lube circuit may not operate properly (gelling) and roast the gearset due to lack of lube flow. The thermostatic controlled valving loops the lube circuit inside the transmission or at the cooler lines prior to the cooler and keeps the flow going until the unit needs additional cooling.

It's best to check the fluid according to the manufacturers' recommendations and instruction. If you don't have this info, almost all the instructions will state to check at operating temperature.

Different sump/pan designs may account for the narrow operating range on some dipsticks. In reality, the operating range of some automatics is quite large due to the factors explained above. ATF churn and excessive foaming due to overfilling (high sump levels) can cause low hydraulic pressure and fluid venting which can damage the unit. It's really dependent on the family or unit to know how much range you have available. Some units allow for excessive fluid without issue and some units due to design will not tolerate much if any overfill.

 

[barryh]

I doubt if it's anything much to do with differences in coefficient of expansion. My guess would be it's that Archimedes chap in the bath.

The oil sump is just that, a sump where as the transmission has the gubbins immersed in the ATF such that the cross section of ATF at the surface is smaller relative to overall volume and the level will therefore rise further on expansion. Mercury thermometers work on the same principle though with a greater effect.