Always check the fluid level
with the engine running (except Honda), the transmission in "park" (except
Dodge products which should be in neutral with the emergency brake applied), and
with the engine at operating temperature. Remove the dipstick and
wipe with a rag. Insert the stick fully and remove. Look at both
sides of the stick to verify the same indication. Repeat the process.
The reason to check both sides of the dipstick is that after the fluid circulates through the transmission, it dumps back into the pan area and will cause an agitation of the fluid. This creates an uneven level and some fluid will "slosh" onto the stick and give a false reading. Some transmissions are worse than others.
Note: If you check the fluid level after the engine has been off for a long time, fluid from the torque converter will drain back into the pan area where the level is measured and give you a false high reading. When the engine is started, the fluid in the pan area is used to fully charge the transmission and torque converter. Also, the difference of fluid temperature will affect the measurement. The fluid volume expands when heated to operating temperature.
Another method of checking the fluid is to turn off the engine and immediately check the level. This will stop the agitation and give an accurate level (no agitation) before the fluid in the torque converter has had a chance to drain back into the pan area which would give a false-high reading.
Tip: If you have added fluid, go through the same procedure, but repeat the process several times before you look for a reading. Some of the added fluid will adhere to the side of the filler tube and can give a false reading.
Tip: If the fluid level is low, you have a leak! Transmissions do not consume fluid. Have the leak diagnosed and repaired to prevent more serious problems.
After you have added fluid, drive the vehicle for a mile or two, then recheck the level. This is especially important in front wheel drive vehicles.
Will overfilling "blow" seals?
In a word, no! The transmission case is vented preventing pressure buildup in normally un-pressurized areas. Severe overfilling can raise the fluid level such that the transmission may lose fluid through the vent or leak from seals that are above the normal fluid level, but the fact remains that the seals that are under pressure and those that are not will not change because of the fluid level.
Will it hurt to overfill the transmission?
In a word, no! Although, it is possible that gross overfilling can cause the fluid to be subjected to moving parts and become aerated which could cause abnormal operation. You may also notice leaks that ordinarily would not occur