Someone else posted some very interesting information about transmissions and transmission fluid. I thought it would we good to have here.
Tranny Temp Message on a 3100# trailer normal? - TundraTalk.net - Toyota Tundra Discussion Forum
When you truck is not moving, or moving slow, and you step on gas hard, what is allowing the engine to turn while the truck tires don't move is the torque converter. The torque converter has plates in close tolerance to each other with transmission fluid in between the plates. One plate is spinning very close to a second plate this is not spinning and the transmission fluid in between creates friction which transfers the torque from the spinning plate to the non-spinning plate. This is how a car/truck can be in gear and stopped with the engine running. The constant pull is the torque converter spinning. But the spinning causes a lot of heat. And the faster you turn the engine, the more friction you create, the more heat you create. That is why when you tried to back up with a high engine load and a high load from the trailer, you created enough heat to cause the transmission temp light to come on.
When a truck is up to speed on a road and you let off on the gas, you can often feel the transmission torque converter lock up which is a set of clutches that come together between the plates and lock them in place. When the torque converter locks up, no more heat is generated. When you step on the gas to accelerate or are pulling something heavy, the torque converter unlocks, the engine speed picks up, and you are back to having the plates spin against each other and generate heat. Pulling trailers with a toque converter unlocked generates a lot of heat and is often the cause of the transmission failing.
As your transmission fluid gets old, it looses it's frictional properties and it takes more engine spin to create the same output force. You may find that a transmission fluid change will help some. Or you may notice no difference. But chances are good it will help.
The other thing going on is that the transmission has pressure in it which are used to make it shift. When the transmission is in gear, the solenoids inside the trans direct fluid to first, second, third, etc gear and when the fluid pressure is sent there, a band clamps down around a shaft and that is how the transmission changes gear. If your transmission fluid is bad, or the transmission needs rebuilt, it may not be building enough pressure to hold the band tight enough to keep the shaft from spinning. This is called slipping. Or the band may be just worn out and can't stop the shaft from turning. But one thing is for sure, a transmission does not slip for long. When the friction surface (like a brake pad) wears off, it's done and time for a complete rebuild.
Poor quality transmission fluid (ie, old worn out fluid) and/or a clogged trans filter and/or leaking seals in the transmission can all cause poor pressure and cause slipping.
It is hard to know if the torque convert was just slipping like normal, or if the low gear band was slipping, or both.
But the moral of the story is don't spin your engine hard when your wheels wont turn or you are under heavy load (like a heavy trailer up hill). Jack rabbit starts, racing, flooring it all the time, etc. That is how you wreck a transmission. And the older the transmission, the more you need to baby it.
Burnt transmission oil smell means things have gone very badly and it is time to go to the parts store and look for something that says "miracle" on the bottle and try to poor that in there.
