Automatic transmission fluid change questions
 

Why do we need to get it to a certain temp to do fluid leveling? I’d just start the car and get under and fill until it leaks out of the overflow? Still learning. Cheers

Because fluid expands as its temperature increases.

Yeah but why does Toyota say between like 40 and 50 celsius to do it? Tedious process

Wouldn’t you want the temps to be higher than that?

Incredibly easy with a scangauge to read exact temp.

Probably they deemed that the highest “safe” temperature to not risk getting burned. also if doing it from cold it’s way quicker vs having to let it warm up to a higher temperature while idling.

No idea. If it was simply expansion of the oil, it would have been simple to alter the overflow tube to a suitable cold fluid height. The oil is going to expand the same % every time.

Could it be that it really is that simple but that they don’t want a leaky fill plug to drain the fluid below what’s nominally needed?

It’s not that complicated.

They specify a temp because of fluid expansion.

They chose 109-117 because this is still hot enough to allow almost anyone regardless of outside temp the ability to do this procedure, especially since you need the engine to be running and cycle gears to make sure fluid is all picked up. So this temp buys you a little time. It is also the hottest temp they probably deemed reasonably safe as you wouldn’t want much hotter than 120f to be dripping onto you. Also since you will be warming up the trans if it was say 170F that would take forever to get to idling already takes long enough driving. So really on all accounts 109-117 makes the most logical and practical sense.