The increased load of a trailer causes an increase in the temperature of the automatic transmission fluid. Heat is the worst enemy of an automatic transmission. As the temperature of the fluid increases, the life of the fluid decreases.
It is essential, therefore, that you install an automatic transmission cooler or supplement the one already present.
The cooler, which consists of a multi-tube, finned heat exchanger, is usually installed in front of the radiator or air conditioning compressor, and hooked inline with the transmission cooler tank inlet line. Follow the cooler manufacturer's installation instructions.
Select a cooler of at least adequate capacity, based upon the combined gross weights of the car and trailer.
Cooler manufacturers recommend that you use an aftermarket cooler in addition to the present cooling tank in your radiator.
A transmission cooler can sometimes cause slow or harsh shifting in the transmission during cold weather, until the fluid has a chance to come up to normal operating temperature. Some coolers can be purchased with or retrofitted with a temperature bypass valve which will allow fluid flow through the cooler only when the fluid has reached operating temperature or above.