As a car is being used, it is good preventive maintenance practice to check the quality and quantity of the transmission fluid at intervals.
The quantity check is done by monitoring the fill level of the transmission oil.
Also, transmission fluids degrade over time. At a point, it becomes necessary to remove old fluid and put in fresh fluid. The process of filling requires transmission level check.
Transmission oils expand as their temperature increases. Therefore, the level at one temperature say 30 degrees Celsius will not be same as the level at 100 degrees Celsius. This behavior is evident if you monitor the oil level on the transmission oil dipstick when the transmission is cold and when it is hot (for transmissions that have dipstick). For transmissions that do not use dipstick, there is special procedure used in checking the fill level. This type (as in the later) is referred to as a sealed transmission. We have a step by step guide on how to check transmission fluid level.
Should fluid of a transmission with dipstick be checked when it is cold or hot?
In this case, the fluid level can be checked when cold and when hot. There is cold and hot marks on the transmission oil dipstick. It is good to check it at both cold and hot temperatures.
We have a detailed article on how to use the dipstick to read oil level.
Should fluid of a sealed transmission be checked when it is cold or hot?
For this type of transmission, cold and hot level marks are not used. The transmission oil level is rather checked within a temperature window given by the manufacturer. Some transmissions use 40 degrees Celsius to 45 degrees Celsius. Some other transmissions use a different temperature range. If the fluid level is checked at below a temperature that is below this given window, there will be a wrong inference of an under fill and vice versa. To monitor the temperature, a scan tool or a handheld infrared thermometer is required. Take a look at detailed information on checking transmission oil temperature.