Originally Posted by BayouBiker
The integrated radiator transmission coolers are have limited transmission fluid to coolant heat transfer area. If the transmission is generating more heat than can be conducted through that area, the fluid will heat up while the coolant does not until the temperature difference is sufficient to force the heat energy through. As the radiator ages, fouling on the exchange surface makes the issue worse. This is why vehicles designed for heavy towing and hauling have large separate coolers for the transmission. As you learned, manufacturers recommendations are there for a reason.
The heat exchanger system used on the ZF transmissions has proven to work very well. No additional cooling capacity is recommended to be installed.
In the case being cited:
*the vehicle was towing a load within its rated tow capacity
*it appears that the vehicle may have been overloaded, with passengers and cargo, but we don't know weights.
*the vehicle was an older model, and it was reported that the radiator was dirty
*it was reported that nothing failed. A light came on on the dash, which the poster resolved with a piece of gum stuck over the light
*the vehicle was being operated at higher speeds than it may have been capable of, given the older 2.5 l engine model, and steep ascents
*the vehicle experienced very high fuel consumption, suggesting that it was working hard
*the replacement vehicle was a newer and larger BMW SUV, with the same heat exchanger design. It was reported to work very well
Lots to learn here, including loading to rated weights, maintaining a vehicle and cooling system, and so on. I wouldn't put cooling system design in that list.