I want to add a better cooling system on my transmission (2009 5r44e). Obviously it's not that complicated to add an auxiliary cooler spliced into the return line coming off the radiator. My question is, why bother keeping the radiator cooler hooked up? The thing is a joke and obviously heated by the engine coolant so doesn't it make more sense to simply plug the ports in the radiator and then run the cooler lines directly to the aftermarket cooler mounted in front of the radiator? How big of a cooler should I get if I want to do it this way? I'm currently looking at a 12x11" plate and fin style. Your thoughts?

 

I agree anytime you can afford to bypass the internal radiator transcooler go for it. i recommend doing this just to make sure trans fluid and coolant will never have a chance to mix toghter if a very rare but possible failure of the built in cooler ever happens.

As far as what size cooler to get I would not know, I would contact a company like B&M and ask their experts then use that data to shop around for a cooler that suits your needs and driving style.

 

The trans cooler serves another purpose, it heats up trans fluid in cold weather.
If you live in the south then not an issue but in cold climates the trans cooler helps to maintain minimum heat levels in the transmission.

Lower rad temp should run about 170-180degF with coolant flowing, best operating trans fluid temp is 190-200degF.

You don't have to plug trans cooler connections, once fluid is drained out.