ATF pan drain/fill - Hot or cold?

[flatlandtacoma]

Would it be preferable to do a ~3 quart refresh of the fluid after driving with everything warmed up, or cold like overnight?

The way I see it - Hot might have more junk in suspension to drain out and might drain marginally faster. Cold might get more fluid out as more has drained down into the pan (at least my Tacoma's dipstick reads a lot higher after sitting overnight than with the engine running.)

My Haynes manual warns to do it cold, but it is written for a complete pan drop and filter change which could be pretty dangerous hot.

Any opinions?
Ken

 

[flatlandtacoma]

Oops forgot to add, 1999 Toyota Tacoma A340E Transmission.

 

[grm386wa]

Personally, Id let the car sit for several hours before I dropped the pan. Let the fluid flow from the tourqe converter and into the pan.


WHenever I do a syphon of my transmission fluid, I get about 2 quarts hot, or about 7 quarts cold.
I'll take 7 quarts over 2 anyday.

If I was worried about "debris" being suspended in hot fluid, Id do a syphon while hot....refill....then syphon while cold.....and repeat process in about a week.

 

[Steelers]

Ditto. I get 4 instead of 2 out when I syphon my 4R70W after it sits for an hour or so. Its simply easer when the fluid cools down somewhat. That pump and tubing gets real hot otherwise.

 

[flatlandtacoma]

The pan has a drain plug so no siphoning or pan drop involved.

 

[grm386wa]

I realize you are going through the pan. However, the same theory applies. If the car has been sitting for a few hours, the fluid from the torque converter will drain into the pan. Hopefully, with a cold transmission, you will drain more fluid.

 

[unDummy]

Just do the drain/refill hot.

Not all TC's drain into the pan.

And, if you want to drain more ATF, just grab lunch while its draining.

 

[b_rubenstein]

quote:

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Originally posted by The Critic:
Some GM units have a thermostatic valve that will open when its hot, so you'll be able to drain more fluid while the unit is cold. Not sure if this is the case with Toyota units though.

---

Those thermostatic valves generally control flow to the ATF cooler. Its purpose is to get the ATF fluid up to operating temp as fast as possible. It will have no effect on how much fluid is in the pan, or drain out of the pan.

 

[psudaytona]

The amount of fluid in my pan nearly doubles when its shut down for over an hour. I would drain it cold, besides transmission fluid goes EVERYWHERE usually when you drop a pan, and if its burning hot, its gonna make the rest of your day unpleasant.

 

[The Critic]

Some GM units have a thermostatic valve that will open when its hot, so you'll be able to drain more fluid while the unit is cold. Not sure if this is the case with Toyota units though.

 

[b_rubenstein]

The reason for draining the fluid when hot/warm is that if there varnish in the trans it will be dissolved in the fluid. Once the fluid is cold it will reform on the internal metal surfaces.

Now, if the trans doesn't have a drain plug, dropping a pan full of hot fluid is a real safety hazzard: I wouldn't do it. Since on my car it's easy enough to pull a hose off the ATF cooler, I do full fluid exchanges and than can be done with warm/hot fluid.

 

[Ray H]

Varnish doesn fall out of solution when it cools because it doesn't go into solution when the lube is warmed to operating temperature. (Sure wish it did, though - wouldn't THAT be a problem solver!) I just drain it hot and let things sit for several hours. What's gonna drain down by waiting until everything is cool will do so my way, too, with the advantage the lion's share of suspended crud will drain without settling in the bottom of the case as the fluid cools and that may or may not be as effectively swept out by cooled fluid.