The plug circled is there from the casting process, and is just a plug. That cooler on the side of the trans is used to get the trans up to operating temp fast so as to reduce its internal friction as soon as possible - this is all done to gain fuel economy on the certification cycles.
There is a directional coolant valve controlled by the engine controller, located on the passenger side of the engine compartment on the frame rail next to the alternator. This controls the amount of coolant flow either to the heater core or to the trans cooler. If you're not demanding heat in the cabin, then it directs coolant to the trans oil heat exchanger, else it sends coolant flow to the heater core. During trailer tow it also demands coolant flow to the trans in order to actually absorb heat from the engine since the trans has excess cooling capacity and operates at a low temp during heavy operation.
Most transmissions we've seen both on the Ram and Jeep EcoDiesels operate at 90-100C (195-212F). If you're operating at over 110C (230F) all the time, likely there is a clutch slippage issue in the trans. At 140C the trans depowers and goes into hydraulic default mode (6th gear, converter unlocked). Trying to run the trans cooler will just result in degraded fuel economy. If you want it to do so, just disconnect the coolant lines from the heat exchanger and connect them to one another to bypass it - now it behaves just like the Grand Cherokee.
This from GDE! Thank you Hemi for the link.
Just like CAFE it all boils down to Fuel Economy! We don't care as long as it lasts up until your warranty is up.
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GDE: We don't see a clear advantage or disadvantage to the trans running at 160F or 190F. At colder temps it will have more internal friction, so it might take more effort from the engine to turn it. You also might notice more harsh shifts since, ultimately, they're calibrated at standard operating temps of the trans.
It could be said that the "keep it simple" philosophy applies well. The directional valve already installed is useful for its purpose and in our tuning we've already made further changes to optimize it's function in the name of getting the engine up to temp faster.
If you don't want the benefit of the heater, I would just disconnect the coolant lines from it and connect them together. In this way it behaves like the Grand Cherokee without the heater which operates at the same standard temp as the Ram, just takes longer to get there.
If you want the trans to run permanently at a colder temp you'd have to remove it's thermostatic assembly and let fluid flow to the trans cooler all the time.
If you have the heat on in the cabin, the HVAC module tells the engine controller how much coolant flow to direct to the heater core in order to provide sufficient heat.
With our tune, below 80C of coolant temp you're always sending heat to the heater core regardless if it needs it or not. Afterwards, the balancing of heating requirements kicks in.
The stock tune does the switch-over much sooner, we wanted to prioritize warming up the engine first and everything else afterwards.
