it’s cold outside now and I was just wondering about my automatic transmission. If you brake fully and apply throttle but the car isn’t moving heat energy is added to the atf right? Would it be beneficial to do this on a cold morning to warmup the atf quicker?
Do not do it!
I have no idea if you are talking about a dump truck or a Yaris. Nonetheless, don't do it. Not only is it dangerous (if your foot slips off the brake), in more modern cars it could cause damage to your engine and/or transmission. The temperature gain would not be worth it.
There really isn't a reason to warm it up, unless you are in extreme sub-zero temps. Letting the engine idle for a couple minutes should do the trick (that is even a hot debate).
If you are experiencing trouble shifting when the trans is cold, it could be an indication of upcoming issues, so get it checked out.
That is not a good idea, nor will it actually help. When you first start a car in cold weather the engine starts to spin the transmission through the torque converter, which pumps the fluid around the tranny and it starts to heat up. It's still spinning and heating up the ATF in park and neutral, putting it into gear does not make it spin or heat up any faster. Revving the engine makes it spin faster and therefore heat up faster.
Holding the car still while you rev in gear makes the engine spin more, but the transmission cannot spin more because you are holding the wheels still, so it puts pressure on the clutch and torque converter which have to absorb the energy. You will prematurely wear or possibly damage these components if you do this a lot, and it won't heat up your ATF any faster to boot! You could also, if your brakes slip, cause an accident.
If your transmission slips when cold it's usually a symptom of something being worn, or your ATF being old. You could also be low in fluid as well. Some auto transmissions are more prone to slipping in cold even if they are in top condition and there's nothing you can do about it.
So, if you have trouble check the fluid level according to your manual, if you still have issues take it to a transmission shop for maintenance or an inspection. Or, if it's fine to drive after a couple minutes idling then just live with it.
You are absolutely correct in park or neutral your converter has no resistance therefore you will rely on the engine to heat up the coolant. Putting it in drive even at idle will heat the vehicle faster in a Mazda 3 or a huge Cat front end loader because the transmission cooler is stacked to the radiator. The heater will provide heat 10 times as fast!!