Can someone please explain to me why it is that the engine has to be running to check tranny fluid? I get that the engine turns the main shaft of the tranny, and this is circulating the fluid, buy why is this necessary? I'm just asking because I'm thinking about marking my dipstick with the engine off and using that as a reference.

 

With the engine running the front pump fills the torque converter (if needed) and servos and valve body with fluid.
With engine off fluid may drain back in varying amounts depending on how long it has been not running and condition of check valves.

 

pump also fills the trans cooler in the front which has lines that hold fluid so all of the above and the trans cooler and the lines contain fluid so they have to be full before you will get an acurate reading on the dipstick. the dipstick should read high when the engine is not running

 

Okay. So, theoretically speaking, if one was to make positively sure that the tranny fluid was at the correct level, he could mark the dipstick w/ the engine off and use that as a reference. Theoretically...

 

Provided the level was checked exactly the same length of time after the engine had been shut off on every check and the level was recalibrated after maybe every 10000mls it may be possible.

 

I used to check new cars (Chevies) during make-ready with the engine off, I just knew from experience where the level should be. But this was on brand-new transmissions which theoritically would have very little bleed-down. I don't do it on older vehicles because of the variables mentioned above.