The torque converter
When someone talks about “locking” the automatic tranny, they really mean locking the torque converter. The torque converter is a fluid coupling between the engine and the tranny. When not locked, the fluid (ATF or tranny oil) slips to allow the engine and tranny to turn at different speeds. That slipping causes heat to build up in the fluid. With the engine not working hard, there is very little heat buildup. But when the engine is working hard with the engine turning faster than the transmission, there is lots of slippage and therefore lots of heat buildup.
After the tow vehicle gets up to cruising speed, the torque converter locks the engine and tranny together so there is no fluid coupling slippage and thus not much heat buildup in the fluid. But when climbing a steep grade, the tow vehicle’s computer knows that the engine cannot pull the trailer with that gear ratio, so the tranny needs to downshift to change the gear ratio. So the computer unlocks the torque converter, then causes the tranny to downshift. While the torque converter is unlocked, the fluid coupling slips a lot and thus causes a lot of heat buildup in the tranny fluid. The torque converter remains unlocked until the computer senses that the engine can handle the load without strain.
With an underpowered engine, or with a heavy load that causes the engine to work hard, or even with a powerful engine when climbing a steep grade or going against a strong headwind, the torque converter can remain unlocked a lot.
So increased tranny oil cooling capacity is required to handle the volcano worth of heat the unlocked torque converter adds to the tranny oil.
Here is a pretty good How Stuff Works article on torque converters:
I mentioned that the torque converter lockup is controlled by the vehicle’s computer. By software. So a bright computer programmer can fiddle with the software and allow the driver to lock or unlock the torque converter. Some enthusiasts install a module (computer) that allows them to be able to lock the torque converter. With that mod, you have to be wide awake so you can unlock the torque converter before the engine stalls. But you might be able to keep the torque converter locked longer than the stock computer software “tune” would allow, to prevent the tranny from “hunting” between gears when climbing milder grades.
The above is from a comment to the post here at IRV2.com
the 7.3 already has an exhaust brake modification available that utilizes the existing EBPV valve in the same manner, for less than 100.00 in parts.